Annual Average Aerosol Optical Depth Research Articles

The aerosol optical properties and PM2.5 components over the world's largest industrial zone in Tangshan, North China

To achieve an in-depth understanding of the aerosol optical properties in the highly-industrial region of Tangshan, we provided systematic aerosol optical properties analysis in this largest industrial zone for the first time. The aerosol optical datasets (2013.05–2015.04) and chemical component data of PM2.5 (2014–2015) obtained from the Tangshan site of the campaign on atmospheric aerosol research (CARE-China) network were analyzed. The results showed that the Tangshan region was seriously affected by fine-mode industrial aerosols all year, which would promote the accumulation of pollutants and influence the atmospheric circulation through changing the vertical temperature gradient. The annual average aerosol optical depth (AOD) and Ångstrӧm exponent (α) were 0.80±0.26 and 1.05±0.10, respectively. The aerosol optical properties revealed significant seasonal characteristics. The maximum seasonal average AOD (1.03±0.62) and α (1.12±0.19) accompanied the highest seasonal secondary inorganic aerosol concentrations (SIA: SO42−,NO3−,NH4+), 53.33μg/m3, occurred in summer, and this phenomenon was attributed to the photochemical reactions favored by the high temperature and humidity. During the spring, frequent dust events led to the maximum Ca2+ concentration of 6.57μg/m3 and the lowest seasonal α of 0.98±0.31. Coal was used for generating heat in winter, resulting in the highest levels of pollutant emissions (Cl−, Elemental carbon (EC) and organic carbon (OC)). The aerosol type classifications showed that the industrial aerosols were the main controls in the summer and fall, representing 56%–58% of the total aerosols. While for spring and winter, mixed aerosols represented 53%–54% of the total aerosols. Hygroscopic growth effect of aerosols existed all year, which could enhance the negative radiative forcing and eventually cool the earth-atmosphere system. The classification Wing for Tangshan data showed high AOD values (>0.70) were mainly clustered in the fine-mode growth wing and coarse-mode. Of these values, summer showed a dense AOD distribution and the highest magnitude of fine-mode AODs, with a corresponding fine particle fraction of η~90% and an Rf ~0.20μm. In addition, spring showed the most coarse-mode aerosols.

Optical properties and source analysis of aerosols over a desert area in Dunhuang, Northwest china

Aerosol observational data for 2012 obtained from Dunhuang Station of CARE-China (Campaign on Atmospheric Aerosol Research Network of China) were analyzed to achieve in-depth knowledge of aerosol optical properties over Dunhuang region. The results showed that the annual average aerosol optical depth (AOD) at 500 nm was 0. 32 ± 0. 06,and the Angstrom exponent (α) was 0. 73 ± 0. 27. Aerosol optical properties revealed significant seasonal characteristics. Frequent sandstorms in MAM (March–April–May) resulted in the seasonal maximum AOD,0. 41 ± 0. 04,and a relatively smaller α value,0. 44 ± 0. 04. The tourism seasons,JJA (June–July–August) and SON (September–October–November) coincide with serious emissions of small anthropogenic aerosols. While in DJF (December–January–February),the composition of the atmosphere was a mixture of dust particles and polluted aerosols released by domestic heating; the average AOD and α were 0. 29 ± 0. 02 and 0. 66 ± 0. 17,respectively. Different air masses exhibited different degrees of influence on the aerosol concentration over Dunhuang in different seasons. During MAM,ranges of AOD (0.11–1.18) and α (0.06–0.82) were the largest under the dust influence of northwest-short-distance air mass in the four trajectories. Urban aerosols transported by northwest-short-distance air mass accounted for a very large proportion in JJA and the mixed aerosols observed in SON were mainly conveyed by air masses from the west. In DJF,the similar ranges of AOD and α under the three air mass demonstrated the analogous diffusion effects on regional pollutants over Dunhuang.

Observations of aerosol color ratio and depolarization ratio over Wuhan

The aerosol color ratio, depolarization ratio and aerosol optical depth (AOD) were measured by a two-wavelength-depolarization lidar at Wuhan, China during the period from May 2015–July 2016. The annual average AOD at Wuhan was about 0.33 during the period 2015–2016. The seasonal average AOD is small (0.26 ± 0.25) during the winter (December–February) season and large (0.4 ± 0.1) during the summer (June–August) season. The monthly average color ratio is small (0.23 ± 0.09) in January and large (0.76 ± 0.21) in August with an annual average value 0.54. The maximum monthly mean depolarization ratio (0.2 ± 0.07) occurred in the month of October, while the minimum (0.06 ± 0.02) occurred in the month of September, and the annual mean depolarization ratio was about 0.17. An analysis of temporal variations of color ratio and depolarization ratio suggests the presence of coarse and non-spherical particles during the autumn. The aerosol color ratio between 0.3 and 2.0 km was large (0.65), suggesting a large number of coarse particles in this range. The vertical distribution of the depolarization ratio is uniform. Finally, the spatial aerosol distribution under different weather conditions and its relationship with the color ratio is investigated in detail. The color ratio value of 0.74 could be used as a threshold for distinguishing polluted weather from clean weather. The aerosol optical and physical properties are investigated to provide a comprehensive understanding of aerosol radiative forcing and environmental problems in this region.

Variations in Aerosol Optical Depth over Three Northeastern Provinces of China, in 2003-2014

Based on the MODIS-Aqua aerosol optical depth (AOD) products from 2003 to 2014, Nighttime Lights Time data from DMSP satellites and basic meteorological data, the AOD spatial distributions of interannual and seasonal variations over three northeastern provinces of China(Liaoning, Jilin, Heilongjiang) were analyzed. It was found that there was a northeast-southwest area of high annual average AOD composed of Dalian, Shenyang, Changchun, Harbin and other cities, the 12-year average AOD value was 0.4-0.8. The low AOD occurred in the eastern and northern areas of the three northeastern provinces of China, where the forest-covering rate was high, and the 12-year average AOD value was less than 0.3. The seasonal variations of annual average AOD showed an increasing trend from spring to summer, then decreased in autumn and increased again in winter. The interannual variations of AOD over three northeastern provinces of China showed a decreasing trend in most areas, but the increasing trend occurred in the northeast-southwest region with the axis formed by Shenyang, Changchun and Harbin, revealing the polarization in recent 10 years over three northeastern provinces of China. In addition, spatial distribution of annual average AOD over three northeastern provinces of China in the years of strong and weak Western North Pacific Summer Monsoon was studied. Affected by the surface wind field, annual average AOD in weak monsoon years was higher than that in strong monsoon years.

Spatial distribution and temporal variation of aerosol optical depth in the Sichuan basin, China, the recent ten years

The applicability of the MODIS (Moderate Resolution Imaging Spectroradiometer) aerosol optical depth (AOD) product was verified using CE-318 sun photometric data for the Sichuan area. The results show that MODIS_3K AOD agrees well with the AOD derived from the CE-318 sun photometer, with a high correlation coefficient (R2 = 0.91). And we used the MODIS AOD product analyzed the spatial distribution and temporal variation of the annual, seasonal and monthly distribution of AOD in the Sichuan area over the last ten years (2006–2015). In the Sichuan area, the mean change of AOD over the 10-year period showed a significant decreasing trend, reaching its maximum in 2010 and its minimum in 2015. Considering only the first five years, the lowest value was reached in 2008 and the overall trend was a standard symmetrical “V” type. Over the later five years, the annual average AOD value showed an overall decreasing trend. The monthly averaged AOD indicated a “double peaks” fluctuation trend; the two peaks appeared in March and August, and AOD changes were relatively small between September and January. Overall, the average value of AOD reached its highest values in spring, and its lowest values in autumn. In the city of Chengdu, the monthly and seasonal changes in the trend were essentially the same as those of the whole Sichuan area; However, the Chengdu AOD monthly and seasonal average change rate was higher than that of Sichuan as a whole. This shows that Chengdu is still the main contributor to the enhanced AOD value for the entire Sichuan region. The spatial distribution of AOD values indicates an “east high, west low” situation across the Sichuan area and Chengdu is one of three cities with the highest AOD values. However, the spatial variation of AOD trend value indicated an “east low, west high” the spatial distribution across the Sichuan region, opposite to the spatial distribution. Moreover, in the past 10 years, the overall AOD changes in Sichuan showed a downward trend, especially in Chengdu, suggesting that environmental remediation measures have achieved initial improvements. But, average AOD of some parts of western Sichuan is increasing, indicating that other regions in Sichuan should strengthen their management and remediation of haze weather.

Column-integrated aerosol optical properties and direct radiative forcing based on sun photometer measurements at a semi-arid rural site in Northeast China

Ground and satellite remote sensing measurements have revealed heavy aerosol loading in China; however, aerosol optical properties and direct radiative forcing in Northeast China – important in climate modeling and remote sensing – have not been widely studied. We studied four years of continuous sun photometer measurements at Tongyu, a typical semi-arid rural site in Northeast China, to better understand column-integrated aerosol optical properties and direct radiative forcing. The annual average aerosol optical depth (AOD) at 500nm was 0.20±0.26; the Ångström exponent (AE) between 440 and 870nm was 1.37±0.64; and the single scattering albedo (SSA) at 440nm was 0.91±0.05. The AOD at this rural site was a quarter of that observed in the polluted North China Plain and Yangtze River Delta regions. Anthropogenic fine-mode particles were the dominant contributor to AOD. The AOD and AE showed generally opposite seasonal variation patterns. Relatively higher AOD values in summer (0.26±0.27) and spring (0.24±0.30) were likely related to long-range transportation of anthropogenic aerosols from southern industrial regions in summer, and the increased contribution of dust events in spring. The minimum AOD (0.16±0.22) was concurrent with the maximum AE (1.75±0.76), observed in winter. On average, the absorption AOD (AAOD) at 440nm and its absorption Ångström exponent (AAE) between 440 and 870nm were 0.06±0.03 and 1.04±0.43, respectively. The mean AAE was considerably higher than 1 in autumn and winter, indicating that brown carbon from biomass burning contributed greatly to aerosol absorption. The AAE was lower than 1 in summer and spring, related to the coating of black carbon particles. Large negative aerosol direct radiative forcing was estimated at the bottom of the atmosphere, with relatively lower values estimated at the top of the atmosphere; the means were −26.28 and −9.42Wm−2, respectively. This resulted in a strong cooling effect on the surface, but warming in the atmosphere, potentially impacting the regional climate.

Multi-decadal aerosol variations from 1980 to 2009: a perspective from observations and a global model

Abstract. Aerosol variations and trends over different land and ocean regions from 1980 to 2009 are analyzed with the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model and observations from multiple satellite sensors and available ground-based networks. Excluding time periods with large volcanic influence, aerosol optical depth (AOD) and surface concentration over polluted land regions generally vary with anthropogenic emissions, but the magnitude of this association can be dampened by the presence of natural aerosols, especially dust. Over the 30-year period in this study, the largest reduction in aerosol levels occurs over Europe, where AOD has decreased by 40–60% on average and surface sulfate concentrations have declined by a factor of up to 3–4. In contrast, East Asia and South Asia show AOD increases, but the relatively high level of dust aerosols in Asia reduces the correlation between AOD and pollutant emission trends. Over major dust source regions, model analysis indicates that the change of dust emissions over the Sahara and Sahel has been predominantly driven by the change of near-surface wind speed, but over Central Asia it has been largely influenced by the change of the surface wetness. The decreasing dust trend in the North African dust outflow region of the tropical North Atlantic and the receptor sites of Barbados and Miami is closely associated with an increase of the sea surface temperature in the North Atlantic. This temperature increase may drive the decrease of the wind velocity over North Africa, which reduces the dust emission, and the increase of precipitation over the tropical North Atlantic, which enhances dust removal during transport. Despite significant trends over some major continental source regions, the model-calculated global annual average AOD shows little change over land and ocean in the past three decades, because opposite trends in different land regions cancel each other out in the global average, and changes over large open oceans are negligible. This highlights the necessity for regional-scale assessment of aerosols and their climate impacts, as global-scale average values can obscure important regional changes.

Uncertainties in global aerosol simulations: Assessment using three meteorological data sets

Current global aerosol models use different physical and chemical schemes and parameters, different meteorological fields, and often different emission sources. Since the physical and chemical parameterization schemes are often tuned to obtain results that are consistent with observations, it is difficult to assess the true uncertainty due to meteorology alone. Under the framework of the NASA global modeling initiative (GMI), the differences and uncertainties in aerosol simulations (for sulfate, organic carbon, black carbon, dust, and sea salt) solely due to different meteorological fields are analyzed and quantified. Three meteorological data sets available from the NASA Goddard Data Assimilation Office (DAO) general circulation model (GCM), the Goddard Institute for Space Studies (GISS) GCM, version II' and the NASA Goddard Global Modeling and Assimilation Office (GMAO), finite‐volume GCM (FVGCM) are used to drive the same aerosol model. The global sulfate and mineral dust burdens with FVGCM fields are 40% and 20% less than those with DAO and GISS fields, respectively, due to its larger precipitation. Meanwhile, the sea salt burden predicted with FVGCM fields is 56% and 43% higher than those with DAO and GISS, respectively, due to its stronger convection especially over the Southern Hemispheric Ocean. Sulfate concentrations at the surface in the Northern Hemisphere extratropics and in the middle to upper troposphere differ by a factor of 3 between the three meteorological data sets. The agreement between model calculated and observed aerosol concentrations in the surface source regions is similar for all three meteorological data sets. Away from the source regions, however, the comparisons with observations differ greatly for DAO, FVGCM, and GISS, and the performance of the model using different meteorological data sets varies depending on the site and the compared species. Sensitivity simulations with the NASA GEOS‐4 assimilated fields show that the interannual variability of aerosol concentrations can be higher than a factor of 2 depending on the location and season, which is generally, however, smaller than the differences due to using different meteorological data sets. Global annual average aerosol optical depth at 550 nm is 0.120–0.131 for the three meteorological data sets. However, the contributions from different aerosol components to this total optical depth differ significantly, which reflects differences in the aerosol spatial distributions. The global annual average anthropogenic and all‐sky aerosol direct forcing at the top‐of‐the atmosphere is estimated to be −0.75, −0.35, and −0.40 W m−2 for DAO, FVGCM, and GISS fields, respectively. Regional differences can be much larger (by a factor of 4–5) in the tropics over the ocean and in the polar regions.

Aerosol optical depths and direct radiative perturbations by species and source type

We have used the Laboratoire de Météorologie Dynamique General Circulation Model (LMDZT GCM) to estimate the relative contributions of different aerosol source types (i.e., fossil fuels, biomass burning, and “natural”) and aerosol species to the aerosol optical depth (AOD) and direct aerosol radiative perturbation (DARP) at the top‐of‐atmosphere. The largest estimated contribution to the global annual average AOD (0.12 at 550 nm) is from natural (58%), followed by fossil fuel (26%), and biomass burning (16%) sources. The global annual mean all‐sky DARP in the shortwave (SW) spectrum by sulfate, black carbon (BC), organic matter (OM), dust, and sea salt are −0.62, +0.55, −0.33, −0.28, and −0.30 Wm−2, respectively. The all‐sky DARP in the longwave spectrum (LW) is not negligible and is a bit less than half of the SW DARP. The net (i.e., SW+LW) DARP distribution is predominantly negative with patches of positive values over the dust source regions, and off the west coasts of Southern Africa and South and North America. For dust aerosols the SW effect is partially offset by LW greenhouse effect.

Estimates of global multicomponent aerosol optical depth and direct radiative perturbation in the Laboratoire de Météorologie Dynamique general circulation model

The global cycle of multicomponent aerosols including sulfate, black carbon (BC), organic matter (OM), mineral dust, and sea salt is simulated in the Laboratoire de Météorologie Dynamique general circulation model (LMDZT GCM). The seasonal open biomass burning emissions for simulation years 2000–2001 are scaled from climatological emissions in proportion to satellite detected fire counts. The emissions of dust and sea salt are parameterized online in the model. The comparison of model‐predicted monthly mean aerosol optical depth (AOD) at 500 nm with Aerosol Robotic Network (AERONET) shows good agreement with a correlation coefficient of 0.57(N = 1324) and 76% of data points falling within a factor of 2 deviation. The correlation coefficient for daily mean values drops to 0.49 (N = 23,680). The absorption AOD (τa at 670 nm) estimated in the model is poorly correlated with measurements (r = 0.27, N = 349). It is biased low by 24% as compared to AERONET. The model reproduces the prominent features in the monthly mean AOD retrievals from Moderate Resolution Imaging Spectroradiometer (MODIS). The agreement between the model and MODIS is better over source and outflow regions (i.e., within a factor of 2). There is an underestimation of the model by up to a factor of 3 to 5 over some remote oceans. The largest contribution to global annual average AOD (0.12 at 550 nm) is from sulfate (0.043 or 35%), followed by sea salt (0.027 or 23%), dust (0.026 or 22%), OM (0.021 or 17%), and BC (0.004 or 3%). The atmospheric aerosol absorption is predominantly contributed by BC and is about 3% of the total AOD. The globally and annually averaged shortwave (SW) direct aerosol radiative perturbation (DARP) in clear‐sky conditions is −2.17 Wm−2 and is about a factor of 2 larger than in all‐sky conditions (−1.04 Wm−2). The net DARP (SW + LW) by all aerosols is −1.46 and −0.59 Wm−2 in clear‐ and all‐sky conditions, respectively. Use of realistic, less absorbing in SW, optical properties for dust results in negative forcing over the dust‐dominated regions.