3.1 General concepts

Modeling of aerosol particle sources and sinks is of the utmost importance because these processes largely control the spatiotemporal distributions of aerosol particle concentrations and size distributions. In addition, in polluted environments, uncertainties are dominated by emissions, whereas in remote regions transport and aerosol processes control the uncertainty. For a given source strength, sinks also control the atmospheric residence times of aerosol particles, which is in turn a key indicator of long-range transport of aerosol species. A good representation of aerosol particle sources and sinks is particularly important to determine the overall analysis and forecast of particle mass, surface area, and number concentrations in regions with few observations for data assimilation. A discrepancy in aerosol sources and/or sink processes can cause a systematic drift in aerosol particle concentrations and AOD over the forecast range in a forecasting system with data assimilation. This is because often the data assimilation corrects for the bias in sources and/or sinks. This correction is often not retained in the subsequent forecast integration due to the fact that the model does not represent the emission and removal processes adequately. For this reason, it is useful to also set user requirements for source and sink observations of aerosol particles. Efforts to formulate aerosol data assimilation with emissions fluxes as well as or instead of mixing ratios as a control variable might have a role to play in correcting these forecast drifts, although such observations would remain important constraints in such a framework.

It is appropriate to differentiate sources of aerosols and aerosol precursors that are directly emitted by human activities from those (natural or anthropogenic) emissions that depend on natural processes. User requirements for directly emitted anthropogenic emissions can be articulated around the following criteria: accuracy, spatial resolution, temporal resolution, speciation, aerosol size distribution, and hygroscopicity. User requirements for emissions that depend on meteorological processes also include requirements for key meteorological and environmental quantities that control such emissions, for example winds and surface conditions or any other parameters that may lead to aerosol formation.

3.2 User requirements for desert mineral dust emissions

For a reliable prediction of mineral dust aerosol, sufficiently accurate knowledge of both the emitting soil and the deflating winds is needed. Both aspects suffer from insufficient observational constraints, creating a large challenge for quantitative emissions predictions. Important source regions globally include the Sahara–Sahel, southwest Asia–Middle East, Taklimakan–Gobi deserts of China, Australia, and the southwest United States–adjacent Mexico (Prospero et al., 2002). However, larger source regions show substantial fine structure and throughout the world there are also many individual sources such as in Patagonia, the Arctic plains, and countless dry or drying lake beds. Estimating dust emissions sources can also be performed with satellite data (for examples see Di Tomaso et al., 2017; Escribano et al., 2016, 2017; Huneeus et al., 2012; Schutgens et al., 2012; Yumimoto and Takemura, 2013).

Dust models typically employ maps of dust source functions (e.g., Ginoux et al., 2012; Zender et al., 2003) because soil properties in arid and hyper-arid regions from global inventories are insufficient to provide consistent soil texture information. This includes aspects such as soil particle size distribution and binding energies but also the existence of roughness elements and soil moisture content that impact on mobilization thresholds. See Darmenova et al. (2009) for a comprehensive review. This severely limits the level of complexity that can be put into models representing the physical processes of dust emissions (e.g., Kok et al., 2014; Marticorena and Bergametti, 1995; Shao, 2001). In order to obtain a better understanding of the involved uncertainties, an update to the objective comparison of different dust source inventories by Cakmur et al. (2006) would be desirable and could be extended to take into account uncertainties in the dust emissions parameterization itself.

In addition to that, dust emissions is further complicated by suppressing influences of soil moisture (Fécan et al., 1998) and vegetation cover, including brown vegetation from a previous rainy period (Kergoat et al., 2017), which can vary on relatively small time and spatial scales. This is particularly acute in the semiarid Sahel with its seasonal vegetation, also creating large variations in surface roughness (Cowie et al., 2013). There is currently a debate as to what extent the mineralogy of emitted dust particles should be taken into account, as this would alter its interactions with both radiation (Journet et al., 2014) and cloud microphysics (Nickovic et al., 2016). While certainly this is an interesting field of research, the former aspect is probably more relevant on longer timescales, and the latter is not even considered in most current dust prediction models.

Surface wind speeds, particularly peak gusts, are also poorly represented in many meteorological models (Knippertz and Todd, 2012) and this induces errors in both dust emissions and subsequent transport (Menut et al., 2015). Indeed, given the strong nonlinearity in dust production to wind, the gusts may dominate the nature of dust production (e.g., Reid et al., 2008). This may be particularly true for northern Africa but many aspects apply to other source regions around the world, too. For example, many models create too much vertical mixing in the stable nighttime planetary boundary layer (PBL) over arid areas, leading to an underestimation of nocturnal low-level jets and a too flat diurnal cycle in surface winds (Fiedler et al., 2013; Largeron et al., 2015; Roberts et al., 2017). This is partly related to an underestimation of turbulent dust emissions during the day (Klose and Shao, 2012). Another substantial problem is the lack of dust generation related to cold pools (haboobs) associated with moist convection over the Sahel and Sahara (and many other desert areas in Asia, Australia, and America), a process largely absent in models with parameterized convection (Marsham et al., 2011; Heinold et al., 2013; Pantillon et al., 2015, 2016). This leads to even reanalyses missing the summertime maximum in dust-generating winds in the central Sahara (Cuevas et al., 2015; Roberts et al., 2017).

It is challenging to improve model representation of dust generation due to an enormous lack of observations from key source regions. The logistically difficult and politically unstable Saharan and Middle East regions have large areas void of any ground stations. What is required to better understand and specify the meteorology of dust production is a much denser network of stations that observe standard meteorological parameters such as wind, temperature, humidity, and pressure, ideally located in some of the main source regions. Given the large diurnal cycle and the short lifetime of some dust-raising mechanisms, particularly moist convection, an hourly or better time resolution would be desirable (Bergametti et al., 2017; Cowie et al., 2015). A first step in creating such a network was undertaken during the recent Fennec project, which deployed stations in 2011 (Hobby et al., 2013), but the deployed stations could not be maintained beyond 2013 (Roberts et al., 2017), and thus do not provide continuous monitoring or a long climatology, but they have demonstrated that (i) reporting the sub-3 min variance in winds is generally unimportant, but resolving the diurnal cycle is critical; (ii) there are substantial biases even in analyzed winds, which miss the summertime wind maximum in the central Sahara; and (iii) it is important to evaluate dust uplift together with model winds, and observational records of this relationship are invaluable (Roberts et al., 2018).

The lack of observations in combination with the difficult-to-represent meteorology also leads to substantial deviations among different analysis products, even on continental scales (Roberts et al., 2015), creating substantial differences in dust emissions (e.g., Menut, 2008). However, the fine-scale nature of dust emissions prevents large scale observations from providing constraint on what a “correct” dust source function is; rather available observations provide only a gross tuning parameter (Khade et al., 2013). Particularly the depth of the Saharan heat low, which is crucial for the large-scale circulation over northern Africa and thus a dominating factor for dust generation, can vary substantially among different analyses or model simulations with different resolution (Marsham et al., 2011). A much denser network of high-quality pressure and wind observations is needed to better constrain models in this regard. Pressure measurements have the advantage of being less affected by local conditions (e.g., topographic circulations, inhomogeneities in roughness) than wind measurements and have – through data assimilation – a far greater impact on the analyzed heat low, which in turn controls the model winds. However, direct wind measurements over under-observed source regions would also be highly desirable.

In addition, our knowledge of the amount and the size distribution of the emitted mineral dust particles is limited. Significant diversity exists among measurement methods for airborne dust (Reid et al., 2003), with aerodynamic and inversion methods being generally in agreement (Reid et al., 2008), and with optical particle counters showing larger sizes. This leaves mass as one of strongest constraints on the system. Investment is required in instrumentation that can accurately characterize coarse and giant aerosol particles. A network of ground stations is subsequently required that in addition to standard meteorology measures mineral dust emissions, ideally including mass or number size distributions of emitted particles. Ideally such stations should be complemented with information about the state of the soil (texture, soil moisture, vegetation, mineralogy). Some such efforts were made during recent field campaigns such as Fennec (Marsham et al., 2013), the Bodélé Dust Experiment (BoDEx) (Washington et al., 2006), and the Japanese Australian Dust Experiment (JADE) (Ishizuka et al., 2008) just to name a few examples. Longer-term monitoring stations, however, are very rare, with the African Monsoon Multidisciplinary Analyses (AMMA) Sahelian Dust Transect (SDT) being a notable exception (Bergametti et al., 2017; Marticorena et al., 2010). Worth mentioning are also the CV-DUST project (Pio et al., 2014) and the Cape Verde Atmospheric Observatory (CVAO) with its long-term dust record (Fomba et al., 2014). An extension of such activities to more remote source areas would be highly desirable.

Given the relative lack of in situ data, a continued reliance on remote sensing is anticipated in coming years, but a number of challenges remain. First, obscuration of dust by cloud (Kocha et al., 2013) is likely a problem that cannot be solved. Second, much summertime dust is emitted at night (Marsham et al., 2013) but most current products are daytime only, requiring better information from wavelengths other than visible ones. Infrared products from geostationary satellites are being developed but still have biases related to atmospheric moisture and uncertainties from the dust optical properties (Banks et al., 2013, 2018). These would need to be further improved and provided in NRT for data assimilation, but have been useful for source detection (Schepanski et al., 2007). Newly developed dust optical depth products such as those from infrared high-spectral sensors (e.g., Infrared Atmospheric Sounding Interferometer (IASI); Capelle et al., 2014; Klüser et al., 2012; Peyridieu et al., 2013) or those produced with the Generalized Retrieval of Aerosol and Surface Properties (GRASP) algorithm (Chen et al., 2018) are promising but have more limited space–time coverage. In addition, location of AERONET stations closer to source regions (as discussed in Li et al., 2016) would allow evaluation of models and satellite retrievals near the source (e.g., the short-term deployment during the Fennec field campaign; Banks et al., 2013), and retrievals from such observations should in future account for particles with diameters exceeding 30 µm (Ryder et al., 2013).

Lidar technique advancements that have occurred in the last decade allow better insight into the desert dust distribution in the atmospheric columns today. With respect to the conventional passive remote-sensing techniques, lidar measurements provide optical properties of atmospheric aerosol as a function of the altitude. This implies that aerosol layers can be identified and characterized in terms of optical properties by lidar measurements. Different lidar techniques exist with different levels of accuracy, but their added value for desert dust observations in measurement campaigns, long-term measurements, and model evaluation is widely demonstrated (see Ansmann et al., 2017; Mona et al., 2012, for more details). In addition, depolarization measurement capability allows reliable identification of nonspherical particle presence and therefore reliable information on the contribution of the desert particle to the aerosol backscatter and extinction coefficient as a function of the altitude. Desert dust profiles provided by CALIPSO at a global level since 2006 improved our knowledge of the desert dust distribution in the atmospheric column and of the transport mechanisms and impacts worldwide (e.g., Yu et al., 2015). At a ground-based level, lidar networks like EARLINET, MPLNET, and AD-Net are improving more and more in terms of methodologies and observational capability, also fostering the link and synergy with more operational communities like the ceilometer one. This said, characterization of the the near-surface environment is problematic, with attenuation being an issue for space and airborne lidars and overlap corrections for lidars at the surface looking upwards. Regardless, the advancements in lidar observations are going to improve the overall knowledge of the desert dust vertical distribution, in particular close to the source regions, through satellite measurements (CALIPSO, the Cloud-Aerosol Transport System (CATS), to a limited extent ESA Doppler wind lidar ALADIN on Aeolus, and to a fuller extent ATLID on EarthCARE) and low-cost automatic systems like ceilometers.

Finally, the dust-focused satellite data should be complemented by improved spaceborne assessments of soil moisture, vegetation cover (green and brown), and soil mineralogy to better characterize varying conditions in source regions (Kergoat et al., 2017). For soil mineralogy, airborne and spaceborne spectroscopic mapping (such as DLR EnMAP and upcoming NASA-EMIT missions) provides a new resource to determine the relative abundance of the key dust source minerals with sufficient detail and coverage, but this resource has been virtually unexplored in the context of dust modeling.

3.3 User requirements for marine aerosol particle emissions

Sea spray provides the largest mass flux of any aerosol type (Andreae and Rosenfeld, 2008) and sea salt aerosol dominates the total aerosol loading over the remote oceans (Haywood et al., 1999). There are few long-term measurement sites of marine aerosol, all restricted to islands or coastal sites (e.g., MAN, https://aeronet.gsfc.nasa.gov/new_web/maritime_aerosol_network.html, last access: 18 July 2018). The source of sea spray aerosol is strongly dependent upon environmental conditions, primarily the local surface wind speed, but also on wave state (Norris et al., 2013b), water temperature, salinity, and the presence of surfactants (de Leeuw et al., 2011). Biological material in the surface water can contribute to a significant organic component in the sea spray aerosol, increasingly so with decreasing particle size (de Leeuw et al., 2011). Most models, however, use simple source functions formulated in terms of wind speed only; the most widely used is that of Monahan et al. (1986), which is often applied well beyond the range of conditions from which it was derived and for which it is valid (Spada et al., 2013). Jaeglé et al. (2011) found discrepancies between modeled and observed marine aerosol concentrations correlated with sea surface temperature; significant improvement in agreement was found when the model sea spray source function was modified to include a temperature dependence. This result is consistent with a number of laboratory studies which show an increase in coarse-mode aerosol production with increasing water temperature (e.g., Mårtensson et al., 2003; Salter et al., 2014a; Sellegri et al., 2006; Woolf et al., 1987). Indeed, there appears to be a number of physical and biological effects that can strongly perturb the bubble–aerosol production relationship (Keene et al., 2017).

Extensive in situ measurement of aerosol particles within the marine atmospheric boundary layer is unlikely to be viable. Satellite remote-sensing approaches offer the possibility of estimating both ambient aerosol loading and the source flux of marine aerosol. Passive measurement of reflected solar radiation can provide AOD (Remer et al., 2005) and some information on both size and vertical distribution (Kokhanovsky, 2013). Active remote sensing can provide much better vertical resolution, and if multiple wavelengths are used, size distributions can be inferred. Both passive and active techniques suffer, however, from the fact that aerosol retrievals are only possible under cloud-free conditions. Moreover, complicating matters is that there is more diversity in individual size measurements of sea spray than any other aerosol species (Reid et al., 2006).

The source of sea spray aerosol is breaking waves and the bursting of bubbles generated by them. Many source functions, including that of Monahan et al. (1986), scale a production flux of sea spray per unit area whitecap – integrated over its lifetime – by a whitecap fraction parameterized as a function of wind speed. There remains, however, an order of magnitude uncertainty in the parameterization of the whitecap fraction, and there is increasing evidence that neither the production of aerosol per unit area whitecap nor the lifetime of a whitecap are independent of the scale of wave breaking or other water properties (Norris et al., 2013a; Callaghan, 2013; Spada et al., 2013; Salter et al., 2014b, 2015). Recent work on satellite retrievals of the whitecaps (Anguelova and Gaiser, 2011, 2013; Anguelova and Webster, 2006) shows significant promise as a means of providing this driving parameter for sea spray source functions and implicitly accounting for the wide range of important controlling factors in addition to wind speed (Salisbury et al., 2013, 2014). It might also ultimately allow a source function to be specified directly in terms of the satellite measurements. While such an approach would provide near global coverage, the temporal sampling interval is dependent on satellite orbit.

The combination of satellite-based estimates of both aerosol loading and source flux offers the optimum means of constraining operational model representation of marine aerosol. Future progress depends on improvements to, and validation of, the retrievals and on improved estimates of the dependence of sea spray production on wave breaking and water properties. Measurements at very high wind speeds are also required to better constrain the parameterized source functions under extreme conditions, when sea spray production is greatest, for example during hurricanes or tropical storms.

3.4 User requirements for anthropogenic and biogenic aerosol emissions

What is generally perceived as anthropogenic air pollution is in fact a result of complex and poorly understood photochemical processing as well as emissions from point and area sources. Often, anthropogenic emissions are taken to be those associated with domestic, industrial, and mobile sources. However, agricultural emissions, including fertilizers and open maintenance burning, are inconsistently included in the terms biogenic and anthropogenic, respectively. This ambiguity can be initially handled by accepting that, from an aerosol point of view, it is all a single class of processes and anthropogenic and biogenic emissions follow similar processing in models. Gridded emissions inventories are commonly generated for primary particles (e.g., primary organic matter, POM; and black carbon, BC). Sulfates, nitrates, other inorganics, secondary organic aerosol (SOA), and BC are supplemented by emissions of key gases important for secondary aerosol particle production (e.g., SO₂, NO_x, ammonia, isoprene, alkenes, aromatics, terpenes). These inventories are the result of large-scale land classification maps, fuel inventories, and transportation corridor databases. Individual source classifications vary by study author but often include power production, heavy industry/smelting, biofuels, mobile sources, road dust, agricultural field emissions, agricultural–domestic stack and burn piles, and plant emissions of species such as isoprene and terpenes. We classify larger open biomass burning, including agriculture field burning, as distinct.

Aerosol particle sources are usually prescribed from compiled emissions inventories. Despite the efforts put into emissions inventories by the community and continuous progress, there remain inherent difficulties in producing accurate inventories. This is for a number of reasons such as the large variety of point and diffuse sources, uncertainties in emissions factors, unknown or unaccounted for sources, and the model emissions approach that is applied (López-Aparicio et al., 2017). Among emissions uncertainties, there is even a hierarchy of errors. While point and area sources are less uncertain year after year thanks to satellite data, emissions factors remain uncertain due to the impossibility of measuring them in realistic conditions and due to their strong dependence on the environment. Moreover, satellite-based inventories may miss small sources as is the case for smoke inventories in agricultural burning regions.

Since the error in emissions inventories automatically translates into a similar or even larger error in concentrations, a user requirement on emissions uncertainties might be tempting. However it should be kept in mind that uncertainties and biases in emissions are difficult to estimate and reducing the error to a single number might not be possible. Aerosol source inversion techniques (e.g., Escribano et al., 2017; Huneeus et al., 2012) have made some progress but are not yet at a stage at which they can constrain emissions inventories to better than the user requirement. Such studies can nevertheless point to regional problems in emissions inventories.

One ideally requires emissions inventories that have a resolution as good as the model resolution. For global modeling systems, this amounts to a spatial resolution and sampling of typically 50 km, although of course many benefits in modeling aerosol transport and deposition may be gained by running NWP at a high resolution, even if sources are not known at that resolution. As computing power increases, it is relatively easy to increase model resolution. Sub-grid-scale information in emissions inventories can be used to post-process and downscale, at least statistically, the simulated model concentrations (Wang et al., 2014). New methods based, for example, on population density as a proxy are also being used (Mailler et al., 2017). For these reasons, it is appropriate that global emissions inventories always aim for spatial resolution and sampling that are higher than those of models at a given time (i.e., we recommend a minimum of ∼ 10 km resolution given the current state of play). Even higher resolutions (<1 km) are required for regional and urban air quality models given that the typical scale for emissions is very small (e.g., the width of a road for surface traffic).

Temporal distribution of emissions inventories can be critical as emissions inventories need to sample the diurnal, weekly, and seasonal cycles in emissions. Since some aerosol data products are only available for the daytime (e.g., AOD retrieved in the visible part of the electromagnetic spectrum), it is important to deal with the diurnal cycle in emissions so as not to introduce biases in the simulated quantities. As modeling improves, it may become necessary to move from static gridded inventories to include feedbacks with societal (e.g., public holidays, agricultural practices) or meteorological (e.g., influence of cold spells on emissions from heating–biofuel systems or dry spells–wind on stack burning) conditions. Biogenic emissions from plants also have a strong dependency on temperature and water stress.

Aerosol particle speciation in global aerosol models should be reflected in global emissions inventories with a minimum of aerosol precursors such as SO₂, NH₃, and NO_x and primary aerosol particles such as elemental carbon or BC and POM. Industrial dust and fly ash are often left out but can be important in some regions (such as China) and should be included in user requirements. Requirements for speciation for volatile organic compounds (VOCs) are more difficult to set out because it is unclear what level of complexity is required in global aerosol models whose aim is to reproduce mass or number concentrations or optical thickness due to SOA. We argue here that speciation of VOCs is directly related to the complexity of the aerosol scheme considered and is more difficult to link to user requirements. This is further complicated by SOA production likely being a product of joint anthropogenic emissions. At the minimum bulk seasonal emissions of key classes of reactive VOCs are required (e.g., alkenes, aromatics, isoprene, terpenes).

Aerosol particle properties, such as size and composition, play an important role in determining the aerosol particle radiative efficiency and the ability to serve as cloud condensation nuclei as well as a role in health-related impacts. User requirements for aerosol particle mass or number size distributions translate into user requirements for aerosol particle size resolution at the emissions points. Such user requirements can be expressed in several ways, i.e., in PM₁₀, PM_2.5, and PM₁ emissions rates, or in combined requirements for aerosol particle mass and number emissions rates for typical aerosol size ranges. Historically, the focus has been first on PM₁₀, then PM_2.5, and lastly on PM₁ for both health impacts and the particulate matter connection to cloud formation. The concept itself of particulate matter at a given size cutoff is directly linked to the availability of sampling inlets, but with more current and future instruments we can expect to have complete information on the aerosol size distribution.

3.5 User requirements for open biomass burning aerosol emissions

Biomass burning emissions represent a highly temporally and spatially variable source of aerosols to the atmosphere and reliable and timely estimates are a key input to air quality and atmospheric composition forecasts. Here we define open biomass burning emissions as emissions by fire consuming open vegetation in fields, grasslands, or forests. Biofuel or stacked agricultural burning are included in the anthropogenic and biogenic emissions and not considered as open biomass burning. Within the International Global Atmospheric Chemistry (IGAC) project, there is at present about half a dozen advanced global aerosol models that include emissions from vegetation fires that could be included in a multi-model ensemble forecast. Only half of the models participating in IGAC are currently operational; the others remain in the research and development stage (for some, only the vegetation fire component is not operational).

Several real-time smoke forecasting products exist and are related to satellite-based active fire hot spot or burn area databases. The most established global aerosol forecasts are represented in the International Cooperative for Aerosol Prediction (ICAP). Four models include dedicated smoke treatment: CAMS (ECMWF and partners), MASINGAR (MRI-JMA), GEOS-5 (NASA), and NAAPS (U.S. Navy), for which the first two use emissions from the Global Fire Assimilation System (GFAS; Di Giuseppe et al., 2017, 2018; Kaiser et al., 2012), the second from a similar Quick Fire Emissions Dataset (QFED; Darmenov and da Silva, 2013) based on fire radiative power (FRP), and the last from the hot-spot-based Fire Locating and Modeling of Burning Emissions (FLAMBE) system (Reid et al., 2009a). Currently most models scale biomass burning emissions to reach acceptable values of biomass burning aerosol optical thickness close to observations (MODIS or AERONET). This scaling factor ranges from 1.7 for the Met Office Unified Model limited area model configuration over South America that was used for the South American Biomass Burning Analysis (SAMBBA) campaign (Kolusu et al., 2015) to 1.8–4.5 for GEOS-5 (Colarco et al., 2011) and 3.4 for CAMS (Kaiser et al., 2012). In CAM5 (Tosca et al., 2013), regional scaling factors are used (Lynch et al., 2016). In small-fire regions, the required factors can be much larger (Petrenko et al., 2017). The need for these correcting factors arises from both possible underestimation of the biomass burning aerosol emissions and model biases.

Emissions of aerosols, and other pollutants, associated with open biomass burning are estimated using emissions factors which convert between the mass of fuel consumed (derived from FRP or burnt-area observations) and the species of interest via the carbon content of the fuel (e.g., Akagi et al., 2011; Andreae and Merlet, 2001; Kaiser et al., 2012). These emissions factors are typically calculated using laboratory or field campaign measurements of smoke constituents, which, while providing accurate measurements, may not be fully representative of all biomass burning and smoke conditions. In particular, large uncertainties, and missing observations, persist in emissions factors for different fuel types (e.g., peat), fire conditions (smoldering vs. flaming), and smoke processing scenarios (e.g., in the presence of clouds, daytime vs. nighttime conditions) following Akagi et al. (2011), for example. Increased and more extensive in situ measurements of different fire types would provide the data required to improve emissions factors currently used in the operational models. Incorporating meteorological parameters (French et al., 2004, 2011), such as surface temperature, humidity, and soil moisture, which could be carried out in NRT in the operational models, will also be beneficial in adapting otherwise static emissions factors to particular environmental conditions. A special case is also provided by peat fires, which for their extent and intensity are an important contributor to global carbon emissions, especially during events in Indonesia related to the El Niño–Southern Oscillation (for example, see the dedicated section in the BAMS State of the Climate 2015; Benedetti et al., 2016, or Huijnen et al., 2016). The remotely detectable signal from peat fires is relatively small and the proportionality to biomass burnt is less certain for these fires than for aboveground fires. Also, the emissions factors vary for individual fires so that estimates on a small scale have a limited accuracy. Observations that would help in better constraining the fire emissions factors would be of great usefulness.

There are other ways in which fire emissions can be estimated globally, for example from smoke observations or from burnt-area estimations. These two alternative approaches could not be used in a real-time operational framework and have limitations themselves. The uncertainties in emissions estimates from smoke observations are still large due to variable and relatively poorly known optical properties of aerosols, poorly characterized errors of the used atmospheric chemistry and transport models, and noise in the satellite observations. For burnt-area products, uncertainties arise mostly from small fires remaining undetected in the burnt-area observations and large uncertainties in the estimates of the rather variable input of available fuel load and combustion completeness. For peat fires in particular, the burn depth is not constrained with global observations.

An increase in the number and coverage of observations will certainly improve biomass burning emissions estimates. Currently fire products from sensors on low orbit (MODIS; Visible Infrared Imaging Radiometer Suite, VIIRS) and geostationary satellites (Spinning Enhanced Visible and Infrared Imager, SEVIRI; GOES; Himawari-8) are available. To estimate emissions, observation gaps may occur due to cloud cover or when satellite observations are not available, and the consistent merging of FRP from different satellites is still an open research topic because their values are often very different and globally biased. However, combining the high temporal resolution of the geostationary products, which would greatly help in accounting for the usually strong diurnal cycle of fire emissions, and the higher precision and global reach of low-Earth-orbiting products is an important objective. Future satellite observations might help in reducing the discrepancy between low-Earth-orbiting and geostationary products.

To support the assessment of fire impacts, measurements of the combustion species (aerosols and reactive and greenhouse gases) are needed. There are several stations that can support verifications of haze forecast, but their number is very limited and some existing stations do not share data in a timely manner. There is also a network of ground-based observations, including GAW stations and other global networks (e.g., AERONET). Lidar networks can also help to identify plume heights.

Fire emissions occur most of the time in the PBL. However, for some large fires, estimated at roughly 15 % of all fires, fire emissions are released in the free troposphere above the PBL (Martin et al., 2012; Sofiev et al., 2012; Val Martin et al., 2010). In some extreme cases, fire emissions can even reach the upper troposphere–lower stratosphere region (Fromm et al., 2006). The height in the atmosphere at which this occurs is often referred to as the injection height. An observational dataset of injection heights exists through the MISR Plume Height Project (MPHP; Nelson et al., 2013), based on a combination of MISR smoke aerosol and MODIS thermal anomaly products. This dataset has recently been updated and extended to produce the MPHP2 dataset. These observations have been very useful in calibrating and/or evaluating global biomass burning emissions injection height datasets (Sofiev et al., 2013). Satellite products that can provide, in NRT, an estimate of this injection height would greatly help in accurately forecasting large biomass burning events. Another factor of uncertainty, to a lesser extent, is the shape of the vertical injection profile. In this case, profiling observations would be required (see also discussion on lidar observations in Sect. 3.2).

In this section we have highlighted some of the challenging aspects related to the estimation of emissions from biomass burning. In addition, extensive work in drawing user requirements has recently been performed by the Interdisciplinary Biomass Burning Initiative (IBBI) and GAW SAG-APP. A draft of Regional Vegetation Fire and Smoke Pollution Warning and Advisory System (VFSP-WAS): Concept Note and Expert Recommendations was written, which forms the basis of user requirements for biomass burning aerosols (WMO GAW Report No. 235, available at http://www.wmo.int/pages/prog/arep/gaw/documents, last access: 18 July 2018).

3.6 User requirements for removal processes

Wet and dry deposition and sedimentation are important removal processes that control the prediction of atmospheric aerosol distribution. However, the aerosol deposition fluxes themselves may become important NAAP forecast products, for example to forecast the soiling of solar panels.

The removal processes are modeled as a function of available meteorological variables describing boundary layer mixing. Wet deposition requires information about the occurrence of convection, precipitation, and fog. Dry deposition modeling requires information related to particle size, shape, density, and hygroscopicity. It also needs information about the state of the land surface and the vegetation, in particular for soluble aerosols. NAAP takes these meteorological variables from the underlying operational NWP models. It should be noted that improving the forecast of precipitation remains a major challenge for the NWP. Inaccuracies of the precipitation forecast directly influence the quality of the aerosol forecasts. Improving the surface information can be achieved by better linking NAAP to advanced land-surface modeling and by updating to the most recent land-use datasets.

Observations of wet deposition fluxes are available from acid deposition networks. These observations could be used to evaluate wet deposition of soluble aerosols such as sulfate, nitrates, and ammonia. For this purpose, the observations need to be made available in a timely manner and at a temporal resolution suited for NAAP evaluation, which is often higher than the frequency (i.e., annual means) required for impact monitoring. Finally, the observed wet deposition fluxes are often strongly influenced by local processes, which makes it necessary to filter the observation in such a way that they are representative of the scale resolved by the NAAP models.

While some observations of wet deposition are made routinely, fewer observations of dry deposition are available. Uncertainties in deposition contribute substantially to the insufficient constraints, in particular of the mineral dust mass budget in atmospheric mineral dust models. Currently, there are very few stations measuring dust deposition, both in the vicinity of and far from source regions (e.g., Bergametti and Fôret, 2014). CARAGA (Fu et al., 2017; Laurent et al., 2015), which is a network of automatic deposition collectors installed throughout the western Mediterranean Basin, samples mass flux of atmospheric insoluble deposition weekly. This initial effort is focusing to constrain regional models of dust simulation but the potentiality of these low-cost and automatic instruments can also be used in remote and isolated regions such as the Sahara. Recently, Yu et al. (2015) tried to infer dust deposition by combining MODIS and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data. Their CALIOP-based multiyear mean estimate of dust deposition matches better with estimates from in situ measurements and model simulations than previous satellite-based estimates.

Measuring deposition fluxes is still a scientific challenge. Therefore, the different sites often use different instruments and observational protocols (e.g., procedure to minimize contamination by local sources), which limits the comparability among observations. It is therefore desirable to enhance or develop standards for deposition measurements and to encourage continuous operation.

The Workshop on Measurement-Model Fusion for Global Total Atmospheric Deposition (MMF-GTAD), recently organized by the GAW Scientific Advisory Group for Total Atmospheric Deposition (SAG-TAD), explored the feasibility and methodology of producing, on a routine retrospective basis, global maps of atmospheric gas and aerosol concentrations as well as wet, dry, and total deposition. In particular they reviewed the current state of global measurements (ground based and satellite), chemical transport modeling (global and hemispheric), and measurement–model fusion and mapping techniques (see GAW Report 234, at https://www.wmo.int/pages/prog/arep/gaw/documents, last access: 18 July 2018).

4.1 General concepts

Data assimilation systems have become an important component of aerosol prediction for both gas- and particulate-phase species. Several global and regional models currently provide analysis of gases and/or aerosol particles. Currently five centers routinely assimilate aerosol data into their models, ECMWF CAMS, FNMOC–NRL, GMAO, JMA, and the UK Met Office. As an example among others, the CAMS system incorporates retrieved observations of ozone, CO, SO₂, NO₂, and AOD in its analysis in order to provide initial conditions for the prediction of these species. The assimilated data are currently based on retrieved products. Bayesian, statistical, or empirical methods are usually applied, depending on the complexity of the instruments and the observation characteristics.

Direct assimilation of atmospheric clear-sky radiances in the UV, visible, and near-infrared wavelengths, where the aerosol signal is strongest, is being considered as a future step, which would allow a seamless assimilation of data from different satellite instruments. This has been shown possible in a study by (Weaver et al., 2007) but it has not been pursued in operational contexts as of now. The use of current and potentially future thermal infrared instruments has also been researched for infrared-sensitive species such as dust in relation to dust, sea surface temperature (SST), water vapor, and temperature assimilation (Bogdanoff et al., 2015; Merchant et al., 2006; Quan et al., 2013; Schmit et al., 2009). There is a delicate trade-off between the complexity and rapidity of the radiative transfer code, particularly in the shortwave part of the spectrum, as well as between model complexity and skill. Complexity may be required for accurately simulating clear-sky aerosol radiances in cases of low and high aerosol loads while rapidity is required in an operational context. Consideration of polarization might also be necessary for the shorter wavelengths, thus further increasing the complexity and hence the computational cost of the radiative transfer calculations. The optimality of assimilating retrieved aerosol products versus radiances and the choice of a suitable algorithm or method for fast radiative transfer at short wavelengths are still being debated. On the one hand, direct radiance assimilation avoids the problem in diversity between the model and the retrieval assumptions (aerosol type, refractive index, meteorological parameters, etc.); on the other hand, the complexity of the observations might complicate or even prevent the implementation of radiance assimilation, especially for advanced sensors such as multi-angle instruments or polarimeters. In the end, the most pragmatic approach prevails in an operational context; hence the assimilation currently depends heavily on the availability of good-quality retrieval products with reliable uncertainty estimates.

Emissions are not part of the analyzed fields but are specified either from established emissions inventories (an extensive intercomparison of a selection of these inventories is given in Granier et al., 2011), as a dynamic boundary condition from satellite observations as is the case for the emissions of biomass burning aerosols, CO, and other species from wild fires (e.g., GFAS, Kaiser et al., 2012; FLAMBE, Reid et al., 2009b; QFED, Petrenko et al., 2012), or computed in the model for some natural aerosol emissions (as sea spray or mineral dust). Estimation of emissions through data assimilation will be the next step for global models. This has already been successfully tried in regional models (e.g., Elbern et al., 2007; Khade et al., 2013) and in off-line or online global models (i.e., Di Tomaso et al., 2017; Escribano et al., 2017; Huneeus et al., 2012).

The most common approach is the adjustment of initial conditions in a manner similar to meteorological data assimilation used in NWP. Optimal interpolation, variational approaches (3D- and 4D-Var), EnKF, or hybrid techniques combining the advantages of both variational and EnKF techniques are all applicable and have been used at various operational centers in various setups (Benedetti et al., 2009; Di Tomaso et al., 2017; Rubin et al., 2016, 2017; Sekiyama et al., 2010; Zhang et al., 2008). Research is still ongoing for the optimal definition of the background error covariance matrices for aerosols, including errors deriving from the misspecification of the emissions. Hybrid 4D-Var–EnKF systems could be used to this end. Independently of the specific assimilation framework, assimilation is a key data-hungry application.

4.2 User requirements for data assimilation

In the past, the aerosol prediction and assimilation community had to use data that were being made available, and not necessarily aimed at the needs of this community. Aerosol products were often provided with climate applications in mind and made available as daily means or monthly averages. While the needs of the operational community are largely similar to those of the climate research community, the timeliness requirements are different. By the year 2000, operational aerosol data products such as from the Advanced Very High Resolution Radiometer (AVHRR) had seen some use in NRT data assimilation, but data quality and delivery time were problematic. A breakthrough was made after the launch of Terra with the creation of the joint NASA–NOAA “bent pipe” program followed some years later by the NASA LANCE servers (Michael et al., 2010), providing many NRT composition, weather, and surface products at ∼3 h latency (such as AIRS, MISR, MLS, MODIS, MOPPIT, or OMI). Expedited products (∼24 h) are also available, for example, from the CALIPSO program. Future aerosol-related lidar missions such as EarthCARE and Aeolus, are now establishing best-effort NRT data delivery, following the example of LANCE expedited products. This has also been made possible by the fruitful collaboration between modeling community and data provider, in an effort to make an optimal use of the resources and provide the best service to the end users.

As discussed previously, most aerosol assimilation systems at the moment rely on products such as AOD, rather than raw measurements such as satellite radiances. However, the tendency in the future will likely be towards the use of satellite radiances, either raw or aggregated and possibly cloud-cleared, for consistency with the current approach in NWP. This represents a challenge for both the model developers and the data providers and might also involve joint development of observation operators. The last point is particularly true considering that there is a fundamental inconsistency between simulated and observed variables. The prognostic variables in the model are the mass and number concentrations of the individual species, whereas the observed variables are mostly optical properties. Converting from one to the other necessitates assumptions and, consequently, is a source of error which has to be mitigated.

Ground-based instruments such as AOD from the AERONET program, are generated in a timely and consistent manner that makes them a candidate for assimilation as well. In the case of lidar measurements, aerosol backscatter, attenuated backscatter, or extinction are all candidate variables and desired for assimilation. The lidar community (e.g., EARLINET, MPLNET, NEIS) is currently establishing protocols for data posting and format as part of the GALION program.

Some general recommendations related to data assimilation observational requirements are outlined below.

5.1 General concepts

Model development is cyclically intertwined with evaluation and validation efforts. Identified model deficiencies in the evaluation processes set the requirements for development and subsequent evaluation, thus repeating the cycle. NWP has well-established evaluation protocols of prediction products, whereas similar procedures for aerosol forecasting are still being defined (Benedetti et al., 2011; Reid et al., 2011). The less developed verification system for NAAP compared to NWP, together with NAAP's additional degrees of freedom, dearth of suitable observations that inform specific model physics shortcomings, and overall lack of standardized evaluation processes are key limiting factors to the advancement of operational aerosol prediction. One major difference between NWP and NAAP verification is that NWP often relies heavily on verification of a forecast system against its own (or another) analysis. This approach seems much less suitable for NAAP for which the models have far more degrees of freedom and the observational constraint on the analysis is much weaker.

For operational forecasting purposes, we distinguish between two different evaluations: operational model evaluation that is conducted as soon as observations of the forecast period are made available and benchmark testing in which the model's performance in simulating a given event or longer time period (e.g., seasonal or annual cycle) is examined in depth. Operational evaluation, sometimes referred to as verification, is generally part of the operational forecasting process and is therefore performed on a regular basis in NRT, whereas benchmark evaluation can made any time after the forecast period, and observations that were not available for the NRT evaluation can be included. These two evaluations are complementary, while the operational evaluation allows the quantification of the confidence and predictive accuracy of the model products and quickly identifies problems which may arise in the forecast. Benchmark testing identifies weaknesses of individual models and provides an assessment of model performance and uncertainties. This is in turn useful information for the forecast users and helps set requirements for long-term model development.

It is not within the scope of this paper to list and describe the requirements for an extensive model evaluation associated with model developments. This could involve various aspects of the aerosol life cycle such as aerosol–cloud interactions, heterogeneous chemistry, removal processes, etc. Each one of these aspects would require a large and specific set of observations. In the present section, we will focus on the evaluation conducted as part of the implementation of an operational forecast. For operational purposes, it is important that these observations are delivered timely and on a regular basis, to ensure the possibility of a routine evaluation. As pointed out in Sect. 2, in addition to aerosol measurements it is also important to include meteorological and chemical observations in the model evaluation process to complement and understand the resulting aerosol predicted fields. Moreover, taking into account that there are operational forecasting systems with data assimilation, it is important to include independent observational datasets (not used during the data assimilation process) in the model evaluation.

Referring to the requirements of observations as outlined in the introduction (i.e., ease of access and consistency, uncertainty, and speed of delivery), globally consistent and available datasets such as for AOD from AERONET or NASA satellites by default currently drive the evaluation process and consequently model development. AERONET's ability to provide high accuracy of fine- and coarse-mode AOD data over the globe with typical preliminary data availability within 6–24 h makes it a favored metric variable (Sessions et al., 2015). Likewise, the maturity, coverage, speed, and ease of access of MODIS aerosol retrievals makes MODIS AOD retrievals the dominant satellite verification product (as discussed in Sect. 4 favored for data assimilation as well). This dominance of AOD to some degree is because of the exclusion of perhaps more applicable baseline variables not meeting the noted observational requirement, such as PM_2.5 and PM₁₀ or aerosol vertical distribution. As discussed in Sect. 3, additional evaluation variables related to model microphysics (chemical composition, absorption, size, full solar and infrared radiative properties, etc.) are only sporadically available and are rarely collected simultaneously.

Satellite remote sensing is the most convenient tool for providing global aerosol spatial and temporal distributions. There has been a series of significant satellite system developments to aid NAAP evaluation and development. In the 1990s, the community in earnest utilized AVHRR and TOMS. In 2000, with the launch of the NASA Terra satellite with MODIS, MISR in 2002 with Aqua and the formation of the A-train (with CALIPSO, CloudSat, OMI, and PARASOL), and European sensors such as IASI all led to significant advancement in observations and model evaluation. Such polar satellites are at relatively low altitudes (between 500 and 800 km), covering a global domain at high spatial resolution with a consistent sensor. However, the polar sensors provide few measurements per day over the same point. Conversely, geostationary satellites are situated at a set point over the Equator, at 36 000 km height, and provide measurements over a given disk. For instance, Meteosat second generation (MSG), which hosts the SEVIRI radiometer, is an essential tool for NRT monitoring in Europe and Africa. Next-generation geostationary satellite sensors, such as Advanced Himawari Imager (AHI) on Himawari-8, Advanced Baseline Imager (ABI) on GOES-16 and GOES-17, and eventually Flexible Combined Imager (FCI) on Meteosat Third Generation (MTG), will likewise result in a jump in the community's ability to characterize aerosol fields from space with much needed temporal resolution.

Along with satellite sensor development, algorithm development has also changed to take advantage of previously gained or new sensor capabilities. While AOD is still currently the dominant metric, many satellite products, both passive and active, are useful and indeed used by the community, including various forms of UV aerosol indices, significant event products, infrared dust products, and scale height–vertical distribution. Each product has its particular strength or weakness projected on the core remote-sensing challenges of lower boundary conditions (e.g., surface properties), cloud mask, aerosol microphysics, and ultimately system noise characteristics, each of which leads to tractable biases in all of these products (see for example Campbell et al., 2012; Hyer et al., 2011; Reid et al., 2013; Shi et al., 2013a, b; Toth et al., 2018; Zhang and Reid, 2006). This has led to the development of new products. For example, the MODIS dark target method fails over bright surfaces due to insufficient contrast. To compensate, the MODIS deep blue algorithm was developed to take advantage of lower albedo in blue wavelengths of arid regions (Hsu et al., 2004). Most recently, day–night band imagery on VIIRS has led to new developments in nighttime aerosol product development, further expanding temporal coverage into the night (Fu et al., 2018; Johnson et al., 2013; McHardy et al., 2015; Wang et al., 2016).

Since the atmospheric residence time of aerosol particles in the troposphere is relatively short (from hours to ∼ 1 week, depending on species-specific physical processes and meteorological conditions) and the footprint area of a single station may be limited, there is a need for ground-based observation networks with sufficient density and representativeness of stations. These are for direct evaluation of the models as well as verifying the satellite products used. A description of the current and future needs for the observing system has been provided in Laj et al. (2009). Clearly, all analyses point to the need for improving geographical coverage of measuring stations. Point measurements are biased towards populated areas as in Europe and the United States (see Fig. 1). Data collected from commercial aircraft can provide invaluable observations for model evaluation (e.g., In-service Aircraft for a Global Observing System, IAGOS; http://www.iagos.org/, last access: 18 July 2018). At the moment, however, this is not established for operational aerosol applications. Moreover, due to the spatial and temporal skewness of the distribution of data collected from aircraft (often more dense close to airports), some care needs to be put into assimilating them into operational systems.

Figure 1Map of surface stations currently included in GAWSIS. Figure courtesy of MeteoSwiss.

Various ground-based observational systems are in operation to monitor aerosol properties in the atmosphere (GAW Report 2016) that can be policy-driven, science-driven, or both. Their organizational structure may vary. Among the main contributors to the aerosol observing system are

• near-surface concentration measurements, including the European Monitoring and Evaluation Programme (EMEP; http://www.emep.int, last access: 18 July 2018), the Interagency Monitoring of Protected Visual Environments (IMPROVE) program (http://vista.cira.colostate.edu/improve/, last access: 18 July 2018), and the African Monsoon Multidisciplinary Analysis (AMMA) Sahelian Dust Transect (Marticorena et al., 2010).

• aerosol variables at near surface provided by the European Research Infrastructure ACTRIS (Aerosol, Clouds, and Trace Gases Research InfraStructure, https://www.actris.eu/, last access: 18 July 2018) and NOAA ESRL's Global Monitoring Division with its specific program on aerosol monitoring (https://www.esrl.noaa.gov/gmd/aero/, last access: 18 July 2018).

• vertical profiles, including the European Aerosol Research Lidar Network (EARLINET; http://www.earlinet.org, last access: 18 July 2018), which is part of ACTRIS; the Asian Dust and Aerosol Lidar Observation Network (AD-Net; http://www-lidar.nies.go.jp/AD-Net/, last access: 18 July 2018); the Latin America Lidar Network (LALINET, http://lalinet.org/, last access: 18 July 2018); and the NASA Micro-Pulse Lidar Network (MPLNET, https://mplnet.gsfc.nasa.gov/, last access: 18 July 2018). There is a current effort to unify a subset of data from all of these networks as part of GALION.

• AOD, including the global Aerosol RObotic NETwork (AERONET; https://aeronet.gsfc.nasa.gov/, last access: 18 July 2018) (Holben et al., 1998), the Global Atmospheric Watch Precision Filter Radiometer (GAW-PFR) Network (Wehrli, 2008), and the Sky Radiometer Network (SKYNET; http://www.skynet-isdc.org/, last access: 18 July 2018) (Takamura et al., 2004). Details on homogeneity of AOD from different networks can be found in Kim et al. (2008).

• wet deposition, including EMEP, the Acid Deposition Monitoring Network in East Asia (EANET, http://www.eanet.asia/, last access: 18 July 2018), the National Atmospheric Deposition Program (NADP, http://nadp.sws.uiuc.edu/, last access: 18 July 2018), and the Canadian Air and Precipitation Monitoring Network (CAPMoN) in North America.

All these networks (except AERONET, AMMA, and EMEP) are GAW contributing networks. These ground-based station networks cover different types of regions documenting variability in aerosol properties: clean and polluted continental, marine, Arctic, dust, biomass burning, and free troposphere. While global GAW stations are expected to measure as many of the key variables as possible, the approximately 300 GAW regional stations generally carry out a smaller set of observations.

The most widely used network is AERONET, providing measurements of AOD as well as aerosol size distribution and real and complex indices of refraction for high-AOD periods with over 600 sites around the world. Most stations report in NRT and products are used at several centers for both routine and retrospective validation. Future developments for AERONET will include nighttime lunar photometry, much desired by the modeling community. While recognizing the current efforts, there is still the need to secure long-term funding for ground-based stations and to further develop infrastructure and data protocols in order to fully support forecasting aerosol activities. Much effort has also been dedicated to standardizing protocols and formats to ensure quality assurance, traceability, data quality, and data accessibility of aerosol observations from both ground-based and space-based sensors. However, this is still a challenge.

Most of the routine measurements (e.g., hourly or daily basis) are conducted as part of regional networks and infrastructures and their usage for operational evaluation is limited to the configuration of the model used in the forecast. Depending on the model resolution, global models do not always capture the spatial variability in the individual stations, particularly for mountain sites and urban sites where the influence of unresolved topography and/or local emissions sources are dominant factors for the aerosol distribution. Station data are sometimes selected if representative of background conditions or they may be aggregated. For that reason, for model evaluation, it is mandatory to provide additional information on the observation site with a correct classification based on its spatial representation (regional or global) and its localization (environment types and emissions types). In addition, detailed information on the model individual aerosol components has become more important in the last years. Speciated information on dust, biomass burning, and anthropogenic aerosol particles is needed for several socioeconomic applications (e.g., solar energy and air quality). However, the presence of different types of aerosols mixed in the measurement points should introduce errors in the comparison between individual aerosol model outputs and observations.

5.2 User requirements for operational evaluation

Operational evaluation is specific for models used operationally. It involves operational online verification of model output, plausibility checks and quality control. As in the case of data assimilation (see Sect. 4), highly temporally resolved data are needed. The operational evaluation is an assessment of how the forecast behaves relative to observations that are in NRT (i.e., available within 24 to 48 h since the forecast run), allowing the modeling group and the end users to have a quick overview of the quality of the forecast. Note that the timeliness requirement is less stringent than for assimilation. At present the most used product to evaluate aerosol model outputs in NRT are surface and/or satellite-based atmospheric-column-integrated variables such as AOD (at a reference wavelength of 550 nm). Only recently have products like aerosol size distribution, aerosol scattering, or absorption coefficients become available in NRT from a limited number of stations. However, the ability of a model to reproduce AOD at a station or even a region may not always be a good indicator of its performance to reproduce surface concentration or vertical aerosol distribution (Huneeus et al., 2016; Reid et al., 2017), even though these model variables are clearly interconnected. Therefore, and in the absence of emissions and deposition routine observations, model evaluation should combine atmospheric-column-integrated variables with vertical profiles (extinction coefficient at a reference wavelength at 550 nm to provide information about the height and thickness of the aerosol layer), and surface measurements (such as PM₁₀, PM_2.5, and PM₁). They also provide an evaluation of the aerosol size distribution on surface level. For the atmospheric column, the AE (which provides aerosol size information) and the separation of AOD into fine-mode and coarse-mode contributions can be used to evaluate the aerosol size distribution.

Additionally, since datasets of weather surface records have better spatial and temporal coverage, observations of horizontal visibility included in meteorological reports are used as an alternative way to monitor aerosol events in NRT and to qualitatively evaluate the aerosol forecasts. In addition, key meteorological variables such as surface winds (linked to emissions of natural aerosols) and precipitation (linked to wet deposition) should be considered.

5.3 User requirements for benchmark testing

Benchmark testing examines individual processes and input drivers that may affect model performance and requires detailed atmospheric measurements that are not, typically, routinely available and can provide better quality control. In addition to those variables considered in the operational evaluation, benchmark testing is expected to include as many of the key variables as possible. Comprehensive measurements of aerosol size distributions, chemical composition, and optical properties are needed. Such observations should ideally be collocated with detailed meteorological information and vertical distribution (e.g., lidars and radiosondes). Routine long-term measurements of aerosol size distributions, chemical composition, and optical properties in operational ground-based networks are urgently needed for model evaluation. Budget constraints are removing some of the very few sites still in existence. Measurements should include the following: mass concentrations of chemical components (soot, organics, ammonia, sulfate, nitrate, mineral dust, and sea salt), number concentrations (of PM₁, PM_2.5, and PM₁₀), and size distribution (if possible resolved by chemical species). Evaluating whether relevant emissions and feedback processes are treated accurately by a model is challenging, although data assimilation can provide valuable information (Pope et al., 2016). In addition to key meteorological parameters associated with aerosol emissions (e.g., surface winds and soil moisture), the effects of aerosols on radiation and clouds, for example, depend on the physical and chemical properties of the aerosols. Evaluating direct and semi-direct aerosol effects on aerosol absorption properties requires aerosol optical properties in addition to AOD such as AAOD, particle depolarization (relative to aerosol speciation), altitude distribution (relative to clouds), radiation observations such as solar irradiance (downward and net shortwave radiation, downward longwave radiation, and outgoing longwave radiation), and solar surface albedo. Evaluating indirect aerosol effects on clouds and precipitation is even more challenging and it would require additional detailed observations of cloud properties such as cloud optical depth, cloud droplet number concentrations, or cloud-top height and thickness (used to evaluate aerosol and deep or shallow convective cloud interactions). For benchmark testing, there is also a need for co-located and simultaneous meteorology and chemistry measurements at locations carefully selected to ensure spatial representativeness. To fully understand processes, more sites with co-located observations of visibility, cloud, radiation, vertical profiles of temperature, relative humidity, and winds and aerosol properties would be highly desirable. Precipitation and deposition observations are also extremely relevant for benchmarking. Innovative designs for global measurement systems (existing technological platforms such as commercial aircraft, cell phones, cars, etc.) should be further exploited. Such a task should fit the mandate of international organizations such as WMO and EUMETNET (see GAW Report 226 on Coupled Chemistry-Meteorology/Climate Modelling, available from WMO).

5.4 Format and accessibility

As in the case of data assimilation (see Sect. 4), observations used in the model evaluation have to be compatible with the model output fields. In this sense, it would be desirable to work on the establishment of formats and common protocols for data harmonization and exchange. This is the main objective of the World Data Centers. At present, there are six GAW World Data Centers (WDCs), each responsible for archiving one or more GAW measurement parameters or measurement types. They are operated and maintained by their individual host institutions. They collect, document, and archive atmospheric measurements and the associated metadata from measurement stations worldwide and make these data freely available to the scientific community. In some cases, GAW WDCs also provide additional products including data analyses, maps of data distributions, and data summaries. However, each GAW WDC treats their database independently even if different communities are providing the same aerosol parameter. This fact can introduce some discrepancies into the definition of one parameter, creating problems for the model-to-observation comparisons.