Aerosol Optical Depth Peak Research Articles

Land Use and Land Cover Influence on Sentinel-2 Aerosol Optical Depth below City Scales over Beijing

Atmospheric aerosols can impact human health, necessitating the understanding of their distribution determinants, especially in urban areas. The study discusses the relationships between five major land cover types and aerosol optical depth (AOD) within a city combining the high-resolution satellite-derived AOD products (derived from Sentinel-2) and land cover products (60 m and 100 m, respectively) for Beijing and its surroundings from 2017 to 2019. Contribution analysis is performed to quantitatively evaluate the influences of land cover on regional AOD over the study area. Patterns of aerosol distribution remarkably vary in time and space. Statistics of seasonal average AOD peak in spring and then progressively decline from summer through autumn to winter. High AOD values coincide with a low normalized difference vegetation index (NDVI) and a high normalized difference built-up index (NDBI). Urban and built-up land is a major contributor to regional AOD in the study area, especially in spring; forest and grassland always reduce AOD. Anthropogenic activities have a non-negligible influence on AOD and can even reverse the contribution of a land cover type to aerosols. Insights of the study promote the comprehension of the impacts of land cover on aerosols and air pollution and contribute to the planning of land use within a city.

Aerosol Climatology Over South and Southeast Asia: Aerosol Types, Vertical Profile, and Source Fields

AbstractAerosol climatology during typical haze dominating period over South and Southeast Asia was explored using several Earth‐Observing A‐Train satellite products retrieved in between 2010 and 2020. Comparatively high aerosol optical depth (AOD) with dominance of fine and UV‐absorbing aerosols are noted across the Indo‐Gangetic plain, South Asia (IGP; AOD: 0.58; UVAI: 0.74) against weak UV‐absorbing fine aerosols over Southeast Asia (SEA; AOD: 0.26; UVAI: 0.07). Among inland IGP sites, decadal mean AOD resembles in Lahore (0.72 ± 0.45), Delhi (0.81 ± 0.46), Kanpur (0.84 ± 0.42), and Varanasi (0.78 ± 0.45); all exhibiting bimodal AOD distribution with a first peak in early November followed by a second in early January. In contrast, except mainland site Chiang Mai, all SEA maritime cities resemble in having typical September‐October AOD peak, with the presence of fine and UV‐neutral aerosols. Urban hotspots across IGP and SEA (except Dhaka, Chiang Mai) denote a spatially consistent minor increasing trend in AOD (0.2–1.8 × 10−2 year−1) while increase in UVAI is more prominent over upper IGP. Dust aerosols dominate only in Karachi (46%) against strong UV‐absorbing smoke aerosols over rest of the IGP (71–91%), and UV‐neutral smoke aerosols across SEA (84–92%). Vertical stratification of aerosol types is noted across IGP as in lower atmosphere (<4 km) polluted dust/urban aerosols remain abundant, with gradual decrease in dust aerosols from upper to lower IGP and consequent increase in smoke aerosols. At upper atmosphere (>4 km), however, dust aerosols clearly dominate. Over SEA, smoke are the most abundant aerosols across the atmospheric column followed by polluted dust. No evidence of intracontinental transport of aerosols from IGP to Southeast Asia or vice versa is, however, noted.

The impact of moderately absorbing aerosol on surface sensible, latent, and net radiative fluxes during the summer of 2015 in Central Europe

The impact of moderately absorbing aerosols on the energy budget over Central Europe is discussed, based on experimental observations and numerical simulations obtained for the summer of 2015. Aerosol events, defined as aerosol optical depth (AOD) at 500 nm greater than 0.15, especially in August, are mostly attributed to transport of biomass burning (BB) from Eastern Europe. Shortwave (SW) aerosol radiative forcings (ARF) at the surface and the top of the atmosphere (TOA) are estimated from ensembles of ten and eight observational and model-based approaches, respectively. Different measuring methods, including unmanned aviation system (UAS) and ground-based measurements, radiative transfer models, including MOTRAN and Fu-Liou, and parameterisations of aerosol optical properties regarding full vertical profiles, columnar and surface properties, are used in these approaches. The mean ARF is −15.9 ± 2.1 W/m2, -9.1 ± 1.4 W/m2, and 7.0 ± 1.0 W/m2, respectively, for the Earth's surface, TOA, and atmosphere under clear conditions for June–August 2015. During an aerosol event with AOD peak of about 0.6 at 500 nm, the daily mean surface, TOA, and atmosphere ARF are around −30, −18, and 13 W/m2, respectively. The mean ARF differences between all methods are about 4.0 W/m2 for the surface and about 2.3 W/m2 for the TOA, which correspond to 23% of ensemble means. Aerosols are also shown to have a significant impact on observed surface sensible and latent heat fluxes for the study period. Flux sensitivity to AOD for a solar zenith angle of 45° is −70 ± 41 W/m2/τ500, -112 ± 56 W/m2/τ500, and -119 ± 19 W/m2/τ500, respectively, for sensible, latent, and net SW and longwave (LW) radiation flux. When averaged over day time, sensitivities of sensible heat, latent heat fluxes, and net radiation fluxes to AOD are reduced by about 50%, 20%, and 70%, respectively.

Tibetan Plateau driven impact of Taklimakan dust on northern rainfall

In our previous study (Li et al., 2019a), using observational data, we found some evidences of Taklimakan dust effects on the convective clouds over the Tibetan Plateau (TP) and then causing heavy rainfall in the downstream region. Here, a simulation study is performed to quantify the effects of Taklimakan dust on the cloud properties over the TP and downstream rainfall using the Spectral Radiation-Transport Model for Aerosol Species (SPRINTARS) coupled to the Nonhydrostatic Icosahedral Atmospheric Model (NICAM). The results indicate that the dusts in Taklimakan desert can be lifted to the northern slope of the TP. The dust aerosols transported to the TP can diminish the cloud effective radius (CER), but they can increase the cloud optical depth (COD), liquid water path (LWP) and ice water path (IWP) over the TP. The maximum impact of the dusts on the CER lags behind the peak of aerosol optical depth (AOD). Meanwhile, the dusts show a more profound impact on the ice-cloud optical depth (ICOD) than liquid-cloud optical depth (LCOD), which implies a more significant impact on the ice particles than liquid drops in convective clouds over the TP. Overall, due to the indirect effects of Taklimakan dust aerosols, the clouds are developed further over the TP. The eastward movement of the clouds polluted by Taklimakan dust can delay the occurrence of heavy rainfall for 12 h but intensify the rainfall over the northern area. This modeling study verified and quantified the indirect effect of Taklimakan dusts on the cloud properties over the TP and further impacts on the rainfall over the northern area. • Effects of Taklimakan dust on the cloud and rainfall surrounding the TP are simulated. • Taklimakan dust can decrease the CER but increase the COD, LWP and IWP over the TP. • Eastward movement of the dust-polluted clouds over the TP can intensify the rainfall over the northern area.

Spatiotemporal analysis of ground and satellite-based aerosol for air quality assessment in the Southeast Asia region

Satellite observations for regional air quality assessment rely on comprehensive spatial coverage, and daily monitoring with reliable, cloud-free data quality. We investigated spatiotemporal variation and data quality of two global satellite Aerosol Optical Depth (AOD) products derived from MODIS and VIIRS imagery. AOD is considered an essential atmospheric parameter strongly related to ground Particulate Matter (PM) in Southeast Asia (SEA). We analyze seasonal variation, urban/rural area influence, and biomass burning effects on atmospheric pollution. Validation indicated a strong relationship between AERONET ground AOD and both MODIS AOD (R2 = 0.81) and VIIRS AOD (R2 = 0.68). The monthly variation of satellite AOD and AERONET AOD reflects two seasonal trends of air quality separately for mainland countries including Myanmar, Laos, Cambodia, Thailand, Vietnam, and Taiwan, Hong Kong, and for maritime countries consisting of Indonesia, Philippines, Malaysia, Brunei, Singapore, and Timor Leste. The mainland SEA has a pattern of monthly AOD variation in which AODs peak in March/April, decreasing during wet season from May–September, and increasing to the second peak in October. However, in maritime SEA, AOD concentration peaks in October. The three countries with the highest annual satellite AODs are Singapore, Hong Kong, and Vietnam. High urban population proportions in Singapore (40.7%) and Hong Kong (21.6%) were associated with high AOD concentrations as expected. AOD values in SEA urban areas were a factor of 1.4 higher than in rural areas, with respective averages of 0.477 and 0.336. The AOD values varied proportionately to the frequency of biomass burning in which both active fires and AOD peak in March/April and September/October. Peak AOD in September/October in some countries could be related to pollutant transport of Indonesia forest fires. This study analyzed satellite aerosol product quality in relation to AERONET in SEA countries and highlighted framework of air quality assessment over a large, complicated region.

Impact of dust and smoke mixing on column-integrated aerosol properties from observations during a severe wildfire episode over Valencia (Spain)

The most destructive wildfire experienced in Spain since 2004 occurred close to Valencia in summer 2012. A total of 48.500ha were affected by two wildfires, which were mostly active during 29–30 June. The fresh smoke plume was detected at the Burjassot measurement station simultaneously to a severe dust episode. We propose an empirical method to evaluate the dust and smoke mixing and its impact on the microphysical and optical properties. For this, we combine direct-sun measurements with a Cimel CE-318 sun-photometer with an inversion methodology, and the Mie theory to derive the column-integrated size distribution, single scattering albedo (SSA) and asymmetry parameter (g). The mixing of dust and smoke greatly increased the aerosol load and modified the background aerosol properties. Mineral dust increased the aerosol optical depth (AOD) up to 1, while the smoke plume caused an extreme AOD peak of 8. The size distribution of the mixture was bimodal, with a fine and coarse modes dominated by the smoke particles and mineral dust, respectively. The SSA and g for the dust-smoke mixture show a marked sensitivity on the smoke mixing-ratio, mainly at longer wavelengths. Mineral dust and smoke share a similar SSA at 440nm (~0.90), but with opposite spectral dependency. A small dust contribution to the total AOD substantially affects the SSA of the mixture, and also SSA at 1020nm increases from 0.87 to 0.95. This leads to a different spectral behaviour of SSA that changes from positive (smoke plume) to negative (dust), depending on the dust and smoke mixing-ratio.

Seasonal and spatial variations of global aerosol optical depth: multi-year modelling with GEOS-Chem-APM and comparisons with multiple-platform observations

Recent AeroCom phase II experiments showed a large diversity in aerosol optical depth (AOD) among 16 detailed global aerosol models, which contributes to the large uncertainty in the predicted aerosol radiative forcing. The GEOS-Chem-APM, a global size-resolved aerosol model, can be considered as a representative AeroCom II model. In this study, multi-year AOD data (2004–2012) from ground-based Aerosol Robotic Network (AERONET) measurements and Moderate Resolution Imaging Spectroradiometer (MODIS), Multi-angle Imaging SpectroRadiometer (MISR) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite retrievals are used to evaluate the performance of GEOS-Chem-APM in capturing observed seasonal and spatial AOD variations. Compared to the observations, the modelled AOD is overall good over land, but quite low over ocean possibly due to low sea salt emission in the model and/or higher AOD in satellite retrievals, specifically MODIS and MISR. We chose 72 AERONET sites having at least 36 months data available and representative of high spatial domain to compare with the model and satellite data. Comparisons in various representative regions show that the model overall agrees well in the major anthropogenic emission regions, such as Europe, East Asia and North America. Relative to the observations, the modelled AOD is systematically lower in biomass burning regions such as South Africa and South America possibly due to uncertainties in emission inventory, but slightly higher in North Africa likely associated with stronger dust emissions in the model. The model is able to capture the realistic seasonal cycle in all regions, including the peak of AOD in major dust events months and biomass burning seasons. The simulated inter-annual variability is overall consistent with the observations, which is distinctly shown in South Africa and South America with strong inter-annual variability compared to other regions.

Climatological simulations of ozone and atmospheric aerosols in the Greater Cairo region

An integrated chemistry-climate model (RegCM4-CHEM) simulates present-day climate, ozone and tropospheric aerosols over Egypt with a focus on Greater Cairo (GC) region. The densley populated GC region is known for its severe air quality issues driven by high levels of anthropogenic pollution in conjuction with natural sources such as dust and agricultural burning events. We find that current global emission inventories underestimate key pollutants such as nitrogen oxides and anthropogenic aerosol species. In the GC region, average-ground-based NO2 observations of 40-60 ppb are substantially higher than modeled estimates (5-10 ppb), likely due to model grid resolution, improper boundary layer representation, and poor emissions inventories. Observed ozone concentrations range from 35 ppb (winter) to 80 ppb (summer). The model reproduces the seasonal cycle fairly well, but modeled summer ozone is understimated by approximately 15 ppb and exhibits little interannual variability. For aerosols, springtime dust events dominate the seasonal aerosol cycle. The chemistry-climate model captures the springtime peak aerosol optical depth (AOD) of 0.7-1 but is slightly greater than satellite-derived AOD. Observed AOD decreases in the summer and increases again in the fall due to agricultural burning events in the Nile Delta, yet the model underestimates this fall observed AOD peak, asmore » standard emissions inventories underestimate this burning and the resulting aerosol emissions. Our comparison of modeled gas and particulate phase atmospheric chemistry in the GC region indicates that improved emissions inventories of mobile sources and other anthropogenic activities are needed to improve air quality simulations in this region.« less

Investigating the coupling between phytoplankton biomass, aerosol optical depth and sea-ice cover in the Greenland Sea

We investigate the relationship between satellite-derived time series for microalgal biomass, measured using remotely sensed chlorophyll-a (CHL: mgm−3), aerosol optical depth (AOD) and sea ice cover (ICE) in the Greenland Sea (10° W–10° E, 65–80° N) over the decadal period 2003–2012. Zonal averages for all variables were computed in 5-degree latitude bands. Unlike other regions of the Arctic Ocean, the marginal ice zone in the Greenland Sea is confined to north of 75° N. The CHL time series is characterized by high interannual variability, especially in the northern marginal sea ice zone (MIZ) where variability in sea ice extent is a likely factor. The ten-year climatology shows that CHL increases from March, reaching a seasonal peak in May in the southern sector and in June in the northern sectors. The climatological peak of AOD is achieved in April in all latitude bands, about a month before the peak in CHL. This suggests that the Arctic aerosol burden is strongly affected by continental sources in early spring.Interestingly, a summer increase in AOD (which succeeds the CHL maximum) is seen in some years. Sea ice extent in early spring is less than 40% in the northern sector in all years. There is considerable interannual variability in both the onset of melt and the extent of sea ice loss during summer, with the minimum summer sea ice extent decreasing to almost zero in five of the years. Cross-correlation analyses of the three times series identify a statistically significant relation between CHL and AOD in six of the years in the southern sector, but no correlation between CHL and ICE was found in the northern sector. There is, however, a significant correlation between AOD and ICE time series in the northern sector in four years. High AOD values registered in early spring are most likely of anthropogenic origin, however, peaks later in summer coincident with the phytoplankton bloom and high emissions of biogenic aerosol precursors such as dimethylsulfide and other primary aerosols of marine origin, suggest the summer atmospheric aerosol burden is likely influenced by biogenic emissions.

Analysis of aerosol characteristics and their relationships with meteorological parameters over Anhui province in China

The characteristics of Aerosol Optical Depth (AOD) and Ångström exponent are analyzed and AODs are compared with CE318 measurements from 2001 to 2009 over Anhui province in China, with an evaluation of potential contributing factors, including regional burning emissions and meteorology. The result indicates that MODIS AODs (Collection 5, C005) are in good agreement with those from CE318 with correlation coefficient of above 0.80 and RMS of below 0.21 in Anhui. Most of MODIS cases fall in the range of ∆τ=±0.05±0.20τ at four AERONET sites. Aerosol optical properties show a notable spatial–temporal distribution feature in Anhui. The values of lower AOD and higher Ångström exponent both appear at southern mountain area. We document seasonal variations in patterns of AOD and Ångström exponent in Anhui, for which peak values respectively occur in June and August. Time series plots for AOD and fire pixel counts are produced to provide a better understanding of influence of burning emission for aerosol. Agricultural residue burning occurs seriously in June, which leads to appearance of AOD peak values. The results between AODs and meteorological parameters show good relationships at Fuyang. Wind speed and wind direction can strongly influence aerosol optical depth and its size, meanwhile, relative humidity and visibility respectively have positive and negative correlation with AODs. Mixed layer depths and AODs have similar monthly variation trend. We use HYSPLIT model for trajectory analysis to reveal the origins of air masses for different levels AODs and Ångström exponent, with the aim of discriminating contributions of different sources.

Spatial and temporal variations of AOD over land at the global scale

Spatial and temporal variations of aerosol optical depth (AOD) over land at the global scale were investigated using the MODIS C005 data set from July 2002 to June 2007. Global average AOD, AODc (coarse mode AOD) and AODf (fine mode AOD) were mapped and analysed. It was found that AOD in Asia and Africa is nearly 2 times higher than that in South America, Europe and North America. A high level of η (fine model fraction (FMF) of AOD) was recorded where AOD is relatively low. Meanwhile, low η (0.05–0.3) was observed where AOD is extremely high. It suggests that coarse particles contribute much more than fine particles to global AOD over land. High AOD level in Nigeria and north India is dominated by coarse particles due to dust events. The average AOD in China (∼0.7) is almost 50% higher than that in India (∼0.55) and nearly 2–3 times higher than that in North America and Western Europe (0.2–0.3), due to spring dust in the west and intensive anthropogenic activities in the eastern part of China. Overall, the average AOD in the USA was limited in a relatively low level. At the same time, AOD in the western part of the USA (AODc = 0.28 and AODf = 0.06) is much higher than that in the east (AODc = 0.06 and AODf = 0.17). The annual cycle of global mean AOD showed a sinusoidal shape, where peaks occur during June–August and the lowest AODs appear in November–January. The peak month, peak value and possible sources for six major continents are Asia (April, AODpeak = 0.31, dust; June–July, AODpeak = 0.27, secondary aerosol), Africa (March, AODpeak = 0.28, dust; August, AODpeak = 0.28, wildfires), South America (September, AODpeak = 0.31, wildfires), North America (May–July, AODpeak = 0.18, dust and wildfires), Europe (March–April, AODpeak = 0.18, secondary aerosols), Oceania (December–January, AODpeak = 0.09, secondary aerosols). The inter-annual variation of AOD was insignificant at the global scale. However, the inter-annual variations at the regional scale were substantial. The abnormal peak of AOD during summer 2003 was probably due to dry and hot weather and intensive wildfires.

Aerosol analysis and forecast in the European Centre for Medium-Range Weather Forecasts Integrated Forecast System: 3. Evaluation by means of case studies

[1] A near real-time system for assimilation and forecasts of aerosols, greenhouse and trace gases, extending the ECMWF Integrated Forecasting System (IFS), has been developed in the framework of the Global and regional Earth-system Monitoring using Satellite and in-situ data (GEMS) project. The GEMS aerosol modeling system is novel as it is the first aerosol model fully coupled to a numerical weather prediction model with data assimilation. A reanalysis of the period 2003–2009 has been carried out with the same system. During its development phase, the aerosol system was first run for the time period January 2003 to December 2004 and included sea salt, desert dust, organic matter, black carbon, and sulfate aerosols. In the analysis, Moderate Resolution Imaging Spectroradiometer (MODIS) total aerosol optical depth (AOD) at 550 nm over ocean and land (except over bright surfaces) was assimilated. This work evaluates the performance of the aerosol system by means of case studies. The case studies include (1) the summer heat wave in Europe in August 2003, characterized by forest fire aerosol and conditions of high temperatures and stagnation, favoring photochemistry and secondary aerosol formation, (2) a large Saharan dust event in March 2004, and (3) periods of high and low sea salt aerosol production. During the heat wave period in 2003, the linear correlation coefficients between modeled and observed AOD (550 nm) and between modeled and observed PM2.5 mass concentrations are 0.82 and 0.71, respectively, for all investigated sites together. The AOD is slightly and the PM2.5 mass concentration is clearly overestimated by the aerosol model during this period. The simulated sulfate mass concentration is significantly correlated with observations but is distinctly overestimated. The horizontal and vertical locations of the main features of the aerosol distribution during the Saharan dust outbreak are generally well captured, as well as the timing of the AOD peaks. The aerosol model simulates winter sea salt AOD reasonably well, however, showing a general overestimation. Summer sea salt events show a better agreement. Overall, the assimilation of MODIS AOD data improves the subsequent aerosol predictions when compared with observations, in particular concerning the correlation and AOD peak values. The assimilation is less effective in correcting a positive (PM2.5, sulfate mass concentration, Angstrom exponent) or negative (desert dust plume AOD) model bias.

Correlations between the satellite‐derived seasonal cycles of phytoplankton biomass and aerosol optical depth in the Southern Ocean: Evidence for the influence of sea ice

The relationship between the production of dimethylsulfide (DMS) in the upper ocean and atmospheric sulfate aerosols has been confirmed through local shipboard measurements, and global modeling studies alike. In order to examine whether such a connection may be recoverable in the satellite record, we have analyzed the correlation between mean surface chlorophyll (CHL) and aerosol optical depth (AOD) in the Southern Ocean, where the marine atmosphere is relatively remote from anthropogenic and continental influences. We carried out the analysis in 5‐degree zonal bands between 50°S and 70°S, for the period (1997–2004), and in smaller meridional sectors in the Eastern Antarctic, Ross and Weddell seas. Seasonality is moderate to strong in both CHL and AOD signatures throughout the study regions. Coherence in the CHL and AOD time series is strong in the band between 50°S and 60°S, however this synchrony is absent in the sea‐ice zone (SIZ) south of 60°S. Marked interannual variability in CHL occurs south of 60°S, presumably related to variability in sea‐ice production during the previous winter. We find a clear latitudinal difference in the cross correlation between CHL and AOD, with the AOD peak preceding the CHL bloom by up to 6 weeks in the SIZ. This suggests that substantial trace gas emissions (aerosol precursors) are being produced over the SIZ in spring (October–December) as sea ice melts. This hypothesis is supported by field data that record extremely high levels of sulfur species in sea ice, surface seawater, and the overlying atmosphere during ice melt.