Aerosol Optical Depth Trends Research Articles

A 30-Year Climatology (1990–2020) of Aerosol Optical Depth and Total Column Water Vapor and Ozone over Texas

Abstract A 30-yr time series (4 February 1990–4 February 2020) of aerosol optical depth (AOD) of the atmosphere, total precipitable water (TPW), and total column ozone has been conducted in central Texas using simple, highly stable instruments. All three parameters in this ongoing measurement series exhibited robust annual cycles. They also responded to many atmospheric events, including the historic volcanic eruption of Mount Pinatubo (1991), a record El Niño (1998), an unprecedented biomass smoke event (1998), and La Niña that caused the driest drought in recorded Texas history (2011). Reduced air pollution caused mean AOD to decline from 0.175 to 0.14. The AOD trend measured for 30 years by a light-emitting diode (LED) sun photometer, the first of its kind, parallels the trend from 20 years of measurements by a modified Microtops II. While TPW responded to El Niño–Southern Oscillation conditions, TPW exhibited no trend over the 30 years. The TPW data compare favorably with 4.5 years of simultaneous measurements by a nearby NOAA GPS (r2 = 0.78). The 30 years of ozone measurements compare favorably with those from a series of NASA ozone satellites (r2 = 0.78). In 2016, 194 comparisons of Microtops II and world standard ozone instrument Dobson 83 at the Mauna Loa Observatory agreed within 1.9% (r2 = 0.81). The paper concludes by observing that students and citizen scientists can collect scientifically useful atmospheric data with simple sun photometers that use one or more LEDs as spectrally selective photodiodes.

Multi-spatiotemporal patterns of aerosol optical depth and influencing factors during 2000–2020 from two spatial perspectives: The entire Yellow River Basin region and its urban agglomerations

Analyzing on the patterns and causes of atmosphere pollution in Yellow River Basin region (YRB) is critical for its construction of ecological civilization and high-quality development. Aerosol optical depth (AOD), an important optical property of aerosol that characterizes the attenuation of light by aerosols, is an indicator of regional atmospheric pollution. Based on multi-source data, we investigated spatiotemporal patterns of AOD and its influencing factors over YRB during 2000–2020 from two spatial perspectives: the entire region and its urban agglomerations. The spatial patterns and trends of AOD and its spatial response to several controls was quantitatively analyzed using Theil-Sen Slope, Mann-Kendall test and spatial principal component analysis, and its periodicity and spatial differences was finely described by extreme-point symmetric mode decomposition. The multi-year average AOD are higher in the middle and lower reaches of YRB than that in the upper reaches and significant seasonal differences of AOD spatial pattern are observed in YRB. Spatial patterns of AOD in YRB are mainly regulated by meteorology, topography and socio-economy, various factors and their combination exert different impacts on AOD. Despite the high contribution of meteorological factors in most urban agglomerations, their acting forces on AOD are not consistent. In terms of spatial trend, the areas where AOD decrease (94.54%) are much larger than the areas where AOD increase. For temporal trend, AOD exhibit a nonlinear downward trend throughout YRB, and its periodicity come from interaction of multi-time-scale components: inter-annual oscillations (imf5) and seasonal oscillations: seasonal periodic fluctuation (imf3), dust weather and man-made agricultural activities caused volatility (imf2) and fluctuation caused abrupt event (imf1), seasonal oscillations dominate the AOD changes. Those are of great significance for improving our understanding of the spationtemporal patterns and its controls of AOD on YRB and its urban agglomerations.

Evaluation and comparison of CMIP6 models and MERRA-2 reanalysis AOD against Satellite observations from 2000 to 2014 over China

Rapid industrialization and urbanization along with a growing population are contributing significantly to air pollution in China. Evaluation of long-term aerosol optical depth (AOD) data from models and reanalysis, can greatly promote understanding of spatiotemporal variations in air pollution in China. To do this, AOD (550 nm) values from 2000 to 2014 were obtained from the Coupled Model Inter-comparison Project (CIMP6), the second version of Modern-Era Retrospective analysis for Research, and Applications (MERRA-2), and the Moderate Resolution Imaging Spectroradiometer (MODIS; flying on the Terra satellite) combined Dark Target and Deep Blue (DTB) aerosol product. We used the Terra-MODIS DTB AOD (hereafter MODIS DTB AOD) as a standard to evaluate CMIP6 Ensemble AOD (hereafter CMIP6 AOD) and MERRA-2 reanalysis AOD (hereafter MERRA-2 AOD). Results show better correlations and smaller errors between MERRA-2 and MODIS DTB AOD, than between CMIP6 and MODIS DTB AOD, in most regions of China, at both annual and seasonal scales. However, significant under- and over-estimations in the MERRA-2 and CMIP6 AOD were also observed relative to MODIS DTB AOD. The long-term (2000–2014) MODIS DTB AOD distributions show the highest AOD over the North China Plain (0.71) followed by Central China (0.69), Yangtse River Delta (0.67), Sichuan Basin (0.64), and Pearl River Delta (0.54) regions. The lowest AOD values were recorded over the Tibetan Plateau (0.13 ± 0.01) followed by Qinghai (0.19 ± 0.03) and the Gobi Desert (0.21 ± 0.03). Large amounts of sand and dust particles emitted from natural sources (the Taklamakan and Gobi Deserts) may result in higher AOD in spring compared to summer, autumn, and winter. Trends were also calculated for 2000–2005, for 2006–2010 (when China introduced strict air pollution control policies during the 11th Five Year Plan or FYP), and for 2011–2014 (during the 12th FYP). An increasing trend in MODIS DTB AOD was observed throughout the country during 2000–2014. The uncontrolled industrialization, urbanization, and rapid economic development that mostly occurred from 2000 to 2005 probably contributed to the overall increase in AOD. Finally, China's air pollution control policies helped to reduce AOD in most regions of the country; this was more evident during the 12th FYP period (2011–2014) than during the 11th FYP period (2006–2010). Therefore this study strongly advises the authority to retain or extend these policies in the future for improving air quality.

A global-scale analysis of the MISR Level-3 aerosol optical depth (AOD) product: Comparison with multi-platform AOD data sources

This study analyses the applicability of the recently released Level-3 (L3) daily and monthly aerosol optical depth (AOD) products (version F15_0032) over land retrieved from the Multiangle Imaging Spectroradiometer (MISR) instrument. For this purpose, daily AOD data from 427 Aerosol Robotic Network (AERONET) sites worldwide during 2001–2018 and 39 China Aerosol Remote Sensing Network (CARSNET) sites across China during 2002–2014 were selected for comparison. Also, MISR-based size-segregated AODs were collected to compare with the coarse- and fine-mode AODs retrieved from AERONET to reveal the particles size modes that mainly contribute to the offset of MISR total AOD relative to observations. By comparing with five other monthly AOD datasets, including two MODIS (Moderate resolution Imaging Spectroradiometer) retrieval products, a multi-satellite merged product, and two aerosol reanalysis, MERRA-2 (Modern-Era Retrospective Analysis for Research and Applications) and CAMS (Copernicus Atmosphere Monitoring Service), we then explored the applicability of MISR for characterizing the climatology, interannual variations, and long-term (2003–2017) trends in regional aerosol loadings over the 12 regions of interest. Overall, about 80% (60%) of the daily AOD values retrieved by MISR fall within expected error bounds of ± [0.05 + 0.2 × AOD] (±[0.03 + 0.1 × AOD]), with a high correlation (R = 0.87). Our comparison results show that although the V23 algorithm addresses several issues in L2 AOD retrieval relative to the previous version, the L3 data aggregated from L2 data tends to on average overestimate low AOD values and underestimate high AOD values. These offset relative to observations are mainly attributed to the overestimation of coarse-mode AOD and the underestimation of fine-mode AOD by MISR. Intercomparison of 2003–2017 AOD trends from multiple data sources indicate that MISR can capture well the increases occurring in South Asia, as well as the decreases occurring in Eastern China, Eastern United States, and Western Europe. Our study confirmed that MISR L3 product performs reliably at regional scales, particularly in typically polluted areas, but caution is still needed when applied to areas where sparse sampling is encountered.

Long-Term Variability of Dust Events in Southwestern Iran and Its Relationship with the Drought

Dust storms represent a major environmental challenge in the Middle East. The southwest part of Iran is highly affected by dust events transported from neighboring desert regions, mostly from the Iraqi plains and Saudi Arabia, as well as from local dust storms. This study analyzes the spatio-temporal distribution of dust days at five meteorological stations located in southwestern Iran covering a period of 22 years (from 1997 to 2018). Dust codes (06, 07, 30 to 35) from meteorological observations are analyzed at each station, indicating that 84% of the dust events are not of local origin. The average number of dust days maximizes in June and July (188 and 193, respectively), while the dust activity weakens after August. The dust events exhibit large inter-annual variability, with statistically significant increasing trends in all of five stations. Spatial distributions of the aerosol optical depth (AOD), dust loading, and surface dust concentrations from a moderate resolution imaging spectroradiometer (MODIS) and Modern-Era Retrospective analysis for Research and Applications (MERRA-2) retrievals reveal high dust accumulation over southwest Iran and surrounding regions. Furthermore, the spatial distribution of the (MODIS)-AOD trend (%) over southwest Iran indicates a large spatial heterogeneity during 2000–2018 with trends ranging mostly between −9% and 9% (not statistically significant). 2009 was the most active dust year, followed by 2011 and 2008, due to prolonged drought conditions in the fertile crescent and the enhanced dust emissions in the Iraqi plains during this period. In these years, the AOD was much higher than the 19-year average (2000 to 2018), while July 2009 was the dustiest month with about 25–30 dust days in each station. The years with highest dust activity were associated with less precipitation, negative anomalies of the vegetation health index (VHI) and normalized difference vegetation index (NDVI) over the Iraqi plains and southwest Iran, and favorable meteorological dynamics triggering stronger winds.

Trends in physical, optical and chemical columnar aerosol characteristics and radiative effects over South and East Asia: Satellite and ground-based observations

Recent satellite observations of atmospheric aerosol loading over Asia indicate a dipole pattern in the aerosol optical depth (AOD) with a substantial decrease in AOD over East Asia and persistent increase in AOD over South Asia, the two global hotspots of aerosol emissions. Aerosol emissions over Asia are also changing rapidly. However, the evolution of physical, optical and chemical columnar aerosol characteristics, and their radiative effects over time, and the resultant impacts of such evolving trends on climate and other associated risks are not yet properly quantified, and used in climate impact assessments. In order to do so, we closely examine, in addition to satellite observations, for the first time, high-quality, ca. two-decade long ground-based observations since 2001 of aerosols and their radiative effects from several locations in the Indo-Gangetic Plain (IGP) in South Asia and the North China Plain (NCP) in East Asia. A clear divergence in the trends in AODs is evident between the IGP and the NCP. The single scattering albedo (SSA) is increasing, and the absorption AOD due to carbonaceous aerosols (AAODCA) is decreasing over both regions, confirming that aerosols are becoming more scattering in nature. The trends in observed aerosol content (AOD) and composition (SSA) are statistically significant over Kanpur in the IGP and Beijing in the NCP, two locations with longest ground-based records. The aerosol radiative forcing of atmosphere (ARFATM) and resultant atmospheric heating rate (HR) are decreasing over both regions. However, current regionally coherent and high annual HR of 0.5–1.0 K day−1 has severe implications to climate, hydrological cycle, and cryosphere over Asia and beyond. These results based on high-quality observations over a large spatial domain are of great significance and are crucial for modelling and quantifying aerosol-climate interactions.

Insights into recent aerosol trends over Asia from observations and CMIP6 simulations

Aerosols continue to contribute the largest uncertainty in climate change. Over Asia, a global aerosol hotspot, spatial patterns of aerosol emissions are changing mainly because of changes in anthropogenic emissions, producing a dipole in atmospheric aerosol loading between East (decrease in emissions) and South Asia (increase in emissions). The resultant aerosol radiative effects are expected to be different as compared to the last decades of the 20th century because of this emerging Asian aerosol dipole. The projection and assessments of radiative and climate impacts of aerosols rely on simulating accurately the aerosol properties, thus, making it imperative that current climate models involved in climate assessments including the Intergovernmental Panel on Climate Change Assessment Reports, simulate well the magnitude and trends in changing aerosol properties. For the first time, in this study we analyze trends in aerosol properties over Asia from satellite and ground-based observations, and simulations from climate models in Coupled Model Intercomparison Project Phase 6 (CMIP6) experiment with state-of-the-art treatment of aerosol chemistry, physics and meteorology. The results reveal large inter-model differences in model estimates, and discrepancies between model simulations and observations as most models are not able to capture the recent observed magnitudes and trends in aerosol optical depth (AOD) and single scattering albedo (SSA) over Asia. The absolute and the relative (percent) trends (positive and/or negative) in AOD are significantly higher than the trends in SSA. The aerosol-induced effective radiative forcing within the atmosphere simulated with three CMIP6 models show a positive (increasing) trend over Asia. A positive trend in atmospheric heating due to aerosols in model simulations is consistent with model simulated trends in AOD (positive) and SSA (negative). These results on model-observations comparison need to be taken into account while examining the projected/expected future climate impacts due to aerosols, and potential value of various mitigation measures, in particular on regional and decadal climate change in Asia which is largely uncertain. Main findingOur analysis of satellite and ground-based observations, and simulations from climate models in CMIP6 experiment with state-of-the-art treatment of aerosol chemistry, physics and meteorology reveal large difference in model calculations, and most models are not able to capture the recent observed trends in aerosol optical depth and single scattering albedo over Asia during 2000-2018.

Impacts of Aerosol Loading in the Hindu Kush Himalayan Region Based on MERRA-2 Reanalysis Data

The impacts of climate change have severely affected geosphere, biosphere and cryosphere ecosystems in the Hindu Kush Himalayan (HKH) region. The impact has been accelerating further during the last few decades due to rapid increase in anthropogenic activities such as modernization, industrialization and urbanization, along with energy demands. In view of this, the present work attempts to examine aerosol optical depth (AOD) over the HKH region using the long-term homogeneous MERRA-2 reanalysis data from January, 1980 to December, 2020. The AOD trends are examined statistically with student’s t-test (t). Due to a vast landmass, fragile topography and harsh climatic conditions, we categorized the HKH region into three sub-regions, namely, the northwestern and Karakoram (HKH1), the Central (HKH2) and the southeastern Himalaya and Tibetan Plateau (HKH3). Among the sub-regions, the significant enhancement of AOD is observed at several potential sites in the HKH2 region, namely, Pokhara, Nainital, Shimla and Dehradun by 55.75 × 10−4 ± 3.76 × 10−4, 53.15 × 10−4 ± 3.94 × 10−4, 51.53 × 10−4 ± 4.99 × 10−4 and 39.16 × 10−4 ± 4.08 × 10−4 AOD year−1 (550 nm), respectively, with correlation coefficients (Rs) of 0.86 to 0.93. However, at a sub-regional scale, HKH1, HKH2 and HKH3 exhibit 23.33 × 10−4 ± 2.28 × 10−4, 32.20 × 10−4 ± 2.58 × 10−4 and 9.48 × 10−4 ± 1.21 × 10−4 AOD year−1, respectively. The estimated trends are statistically significant (t > 7.0) with R from 0.81 to 0.91. Seasonally, the present study also shows strong positive AOD trends at several potential sites located in the HKH2 region, such as Pokhara, Nainital, Shimla and Dehradun, with minimum 19.81 × 10−4 ± 3.38 × 10−4 to maximum 72.95 × 10−4 ± 4.89 × 10−4 AOD year−1 with statistical significance. In addition, there are also increasing AOD trends at all the high-altitude background sites in all seasons.

Analysis of Aerosol Optical Depth from Sun Photometer at Shouxian, China

We use two cloud screening methods—the clustering method and the multiplet method—to process the measurements of a sun photometer from March 2020 to April 2021 in Shouxian. The aerosol optical depth (AOD) and Angström parameters α and β are retrieved; variation characteristics and single scattering albedo are studied. The results show that: (1) The fitting coefficient of AOD retrieved by the two methods is 0.921, and the changing trend is consistent. The clustering method has fewer effective data points and days, reducing the overall average of AOD by 0.0542 (500 nm). (2) Diurnal variation of AOD can be divided into flat type, convex type, and concave type. Concave type and convex type occurred the most frequently, whereas flat type the least. (3) During observation, the overall average of AOD is 0.48, which is relatively high. Among them, AOD had a winter maximum (0.70), autumn and spring next (0.54 and 0.40), and a summer minimum (0.26). The variation trend of AOD and β is highly consistent, and the monthly mean of α is between 0.69 and 1.61, concerning mainly continental and urban aerosols. (4) Compared with others, the single scattering albedo in Shouxian is higher, reflecting strong scattering and weak aerosol absorption.

Long-Term Trends and Impact of SARS-CoV-2 COVID-19 Lockdown on the Primary Productivity of the North Indian Ocean

COrona VIrus Disease (COVID) 2019 pandemic forced most countries to go into complete lockdown and India went on complete lockdown from 24th March 2020 to 8th June 2020. To understand the possible implications of lockdown, we analyze the long-term distribution of Net Primary Productivity (NPP) in the North Indian Ocean (NIO) and the factors that influence NPP directly and indirectly, for the period 2003–2019 and 2020 separately. There exists a seasonal cycle in the relationship between Aerosol Optical Depth (AOD), Chlorophyll-a (Chl-a) and NPP in agreement with the seasonal transport of aerosols and dust into these oceanic regions. In Arabian Sea (AS), the highest Chl-a (0.58 mg/m3), NPP (696.57 mg/C/m2/day) and AOD (0.39) are observed in June, July, August, and September (JJAS). Similarly, maximum Chl-a (0.48 mg/m3) and NPP (486.39 mg/C/m2/day) are found in JJAS and AOD (0.27) in March, April, and May (MAM) in Bay of Bengal. The interannual variability of Chl-a and NPP with wind speed and Sea Surface Temperature (SST) is also examined, where the former has a positive and the latter has a negative feedback to NPP. The interannual variability of NPP reveals a decreasing trend in NPP, which is interlinked with the increasing trend in SST and AOD. The analysis of wind, SST, Chl-a, and AOD for the pre-lockdown, lockdown, and post lockdown periods of 2020 is employed to understand the impact of COVID-19 lockdown on NPP. The assessment shows the reduction in AOD, decreased wind speeds, increased SST and reduced NPP during the lockdown period as compared to the pre-lockdown, post-lockdown and climatology. This analysis is expected to help to understand the impact of aerosols on the ocean biogeochemistry, nutrient cycles in the ocean biogeochemical models, and to study the effects of climate change on ocean ecosystems.

Long term trend in aerosol direct radiative effects over Indian Ocean region from multi- satellite observations

ABSTRACT Long-term (2000–2020) trend in aerosol optical depth (AOD) and the associated radiative effect over Indian Ocean region (Arabian Sea and Bay of Bengal) during winter time south Asian outflow is investigated using multi-satellite data. Analysis revealed a statistically significant increasing trend of ~0.01 year−1 over the study regions. This study revealed that aerosols are confined within 2 km from ocean surface, and the trend in columnar AOD is significantly modulated by aerosol loading at higher altitudes. Although there has been no significant change in aerosol direct radiative forcing efficiency, the significant enhancement in AOD over the study region has induced an increase in cooling at the top of the atmosphere by ~0.1 to 0.2 W m−2 year−1.

A comparative study of EOF and NMF analysis on downward trend of AOD over China from 2011 to 2019

In recent decades China has experienced high-level PM2.5 pollution and then visible air quality improvement. To understand the air quality change from the perspective of aerosol optical depth (AOD), we adopted two statistical methods of Empirical Orthogonal Functions (EOF) and Non-negative Matrix Factorization (NMF) to AOD retrieved by MODIS over China and surrounding areas. Results showed that EOF and NMF identified the important factors influencing AOD over China from different angles: natural dusts controlled the seasonal variation with contribution of 42.4%, and anthropogenic emissions have larger contribution to AOD magnitude. To better observe the interannual variation of different sources, we removed seasonal cycles from original data and conducted EOF analysis on AOD monthly anomalies. Results showed that aerosols from anthropogenic sources had the greatest contribution (27%) to AOD anomaly variation and took an obvious downward trend, and natural dust was the second largest contributor with contribution of 17%. In the areas surrounding China, the eastward aerosol transport due to prevailing westerlies in spring significantly influenced the AOD variation over West Pacific with the largest contribution of 21%, whereas the aerosol transport from BTH region in winter had relative greater impact on the AOD magnitude. After removing seasonal cycles, biomass burning in South Asia became the most important influencing factor on AOD anomalies with contribution of 10%, as its interannual variability was largely affected by El Niño. Aerosol transport from BTH was the second largest contributor with contribution of 8% and showed a decreasing trend. This study showed that the downward trend of AOD over China since 2011 was dominated by aerosols from anthropogenic sources, which in a way confirmed the effectiveness of air pollution control policies.

Spatiotemporal Distribution of Aerosol Optical Depth Based on Landsat Data in the Hinterland of the Guanzhong Basin and Its Relationship with Urbanization

Aerosol optical depth (AOD) is one of the most fundamental optical properties of aerosols that characterizes the attenuation of light by aerosols and is an indicator of regional air pollution. Based on the blue band surface reflectance database from the MOD09A1 products for the period 2000-2019 and the ASTER spectral database, AOD was estimated from Landsat TM/OLI data using the deep blue algorithm (DB). Multi-year average/annual average and seasonal AOD values for the period 2000-2019 were then calculated to analyze the spatial characteristics and temporal variations of AOD using the empirical orthogonal function method (EOF). Furthermore, the impacts of urbanization on the spatio-temporal distribution of AOD were analyzed. The obtained results are summarized as follows:① The multi-year average AOD spatial distribution in the hinterland of the Guanzhong Basin was significantly affected by topography and human activities, with higher AOD values and variationsin areas of low altitude and high-intensity human activities compared to the surrounding mountains. Thus, changes in AOD in the study area are mainly affected by anthropogenic factors. AOD also showed significant seasonal variations, whereby spring (0.34) > summer (0.33) > autumn (0.23) > winter (0.12), and the largest regional differences occurred in summer; ② The annual average AOD (from 2000-2019) showed the trend of "increase-decrease-increase", and reached a maximum in 2005, with the high AOD area gradually moving to the south. The distribution of AOD values in spring and summer was relatively discrete, while it is in a low-value agglomeration state in winter; ③ Three main AOD spatial distribution modes were identified based on the EOF, which had cumulative contribution rate of 83.0%. The spatial distribution trend of AOD showed regional consistency, with feature vectors consistent with the altitude, thus reflecting the difference of AOD at different altitudes. Taking the Qinling Mountains as the dividing line, the AOD presented the "north-south" pattern, AOD showed a "north-south" pattern, reflecting the uniqueness of the regional development in the Guanzhong Basin compared to the southern Qinling Mountains. The "southeast-northwest" distribution pattern indicated that the AOD presented a reverse change trend between urban and non-urban; and ④ The results of correlation analysis between the AOD and urbanization revealed a positive correlation with permanent population density (R2=0.707, P<0.05), impervious surface density (R2=0.377, P<0.05), and industrial POI density (R2=0.727, P<0.5). These results are significant for improving the monitoring of air quality in the Guanzhong Basin and for the construction of an urban ecological environment.

Temporal evolution of aerosols and their extreme events in polluted Asian regions during Terra's 20-year observations

Aerosol pollution is an acute environmental issue in developing countries. Asia has been experiencing rapid changes in anthropogenic aerosols during the past two decades due to fast growth in population and economy. It is still an open question how aerosol loadings, represented by aerosol optical depth (AOD), have evolved in this century, particularly during the past decade when China and India implemented a clean air act aiming to improve air quality. Based on Terra aerosol retrievals and aerosol reanalysis, a change point of AOD trend is detected at 2010 in East China versus a persistent increasing AOD trend in the Indian subcontinent with no detectable change point from 2000 to 2019. In East China, positive AOD trend (+0.11 ± 0.022 decade−1) is confirmed from 2000 to 2010 (hereinafter the former period) yet negative trend (−0.26 ± 0.027 decade−1) is identified from 2011 to 2019 (hereinafter the later period). In the Indian subcontinent, persistent positive trend (+0.04 ± 0.001) is detected from 2000 to 2019 (hereinafter the whole period). All of these trends are attributed mainly to changes in sulfate aerosols. Further analysis of the aerosol pollution extreme events (APEE; defined as daily AOD over the long-term local 90th AOD percentile) manifest a positive trend (+0.16 ± 0.029 decade−1) of the APEEs' magnitude in East China during the former period yet a negative trend (−0.11 ± 0.020 decade−1) during the latter period; the Indian subcontinent demonstrates a positive trend (+0.02 ± 0.004 decade−1) during the whole period due to increasing sulfate aerosols. The APEEs have become more frequent (+3.5 ± 0.53 day month−1 decade−1) in East China during the former period yet less frequent (−3.6 ± 0.39 day month−1 decade−1) during the latter period; in the Indian subcontinent, more frequent APEEs (+1.1 ± 0.25 day month−1 decade−1) have been detected during the whole period. Consistent with the AOD trends, clear-sky radiation in East China shows a negative trend at the surface (−3.2 ± 0.67 W m−2 decade−1), a positive trend in the atmosphere (+1.4 ± 0.68 decade−1), and a negative trend at the top of the atmosphere (−1.8 ± 0.43 decade−1) during the former period, respectively; opposite trends with much larger magnitude are seen during the latter period. In the Indian subcontinent, the clear-sky radiation trends during the whole period are −1.4 ± 0.38, +1.7 ± 0.31, and + 0.5 ± 0.16 W m−2 decade−1 at the surface, in the atmosphere, and at the top of the atmosphere, respectively. Comparison of radiation trends at clear-sky and all-sky conditions suggests that absorbing aerosols dominate the radiation budget in the atmosphere and the aerosol reanalysis of the Modern-Era Retrospective Analysis for Research ans Applications version 2 (MERRA-2) might overestimate the radiation response to clouds. This study provides an up-to-date analysis of the long-term trends in aerosols and their extreme events and radiation in two of the world's heavily polluted regions and the results have important implications for assessment of the environmental and climatic impacts of the ongoing clean air acts in Asia.

Variability, predictability, and uncertainty in global aerosols inferred from gap-filled satellite observations and an econometric modeling approach

Time series analyses and stochastic modeling assessments of aerosols are critical for climate change and human health studies. However, the precise characterization of the aerosol optical depth (AOD), its variability, trends, and predictability, and its associated uncertainty at the global scale is largely unexplored. In addition, gaps in satellite-retrieved AODs across space and time remain an obstacle to accurately revealing aerosol properties. This study uses MODIS Collection 6 AOD retrievals for a time series analysis and modeling of global aerosols from 2003 to 2015. Random forest (RF) regression is first applied to replace the missing data in the satellite AOD retrievals. AOD variations and trends are then investigated and future values are predicted using a Mann–Kendall analysis approach and an autoregressive integrated moving average (ARIMA) model, respectively. The results indicate that the developed RF model enhances the AOD data coverage significantly, with the root mean square error and mean absolute error statistical metrics well below 0.13 and 0.08, respectively. High AOD loadings are found over East, South, and Southwest Asia, West and Central Africa, and northern South America. Prominent discrepancies are shown in aerosol variations and trends likely because of dust emissions, biomass burning, fossil fuel combustion, and socioeconomic practices, which has significant implications for climate systems and mitigation policy-making. The ARIMA model delineates AOD features with clear annual and seasonal variations and with high accuracy over most regions. The performance of the model is jointly impacted by the data quality and data values. Overall, our study suggests the feasibility and applicability of the RF model in reconstructing area-scale satellite missing AOD retrievals, as well as the ability of the stochastic ARIMA model to accurately depict and forecast AOD profiles. The globally simulated and predicted aerosols will improve evaluations of aerosol effects for climate and epidemiological studies.

Spatiotemporal variations and long term trends analysis of aerosol optical depth over the United Arab Emirates

This research provides a comprehensive long term time series analysis of the spatiotemporal variability of aerosol optical depth (AOD) over the United Arab Emirates (UAE) using MODIS (Moderate-resolution imaging spectroradiometer) MAIAC (Multi-Angle Implementation of Atmospheric Correction) AOD (aerosol optical depth) product for the period 2003–2018. The UAE is located in one of the driest places on earth, the Arabian Peninsula, with two large extended desert regions that form major atmospheric dust sources in the world. The atmospheric dust emitted by these vast deserts is transported locally and regionally and it is the primary major contributor to the overall aerosol optical depth in this area. The investigation of the spatiotemporal trends of AOD over the UAE showed a significant obvious annual seasonal variability. Results showed a significant increase in AOD during summer due to high wind speed, resulting in scattered events of small to large scale haboobs within the study area, high temperature, and high relative humidity. The increased humidity levels are due to increased temperatures in the summer season leading to significant evaporation from the neighbouring water bodies (mainly the Arabian Gulf and the Sea of Oman) leading to a significant increase in water vapour levels in the atmosphere. Summar AOD values during the study period ranged between 0.42 – 0.73. The second highest AOD was observed in spring followed by winter. Spring AOD values ranged between 0.33-0.58 and winter values ranged between 0.25 – 0.41. The spatial distributions of AOD showed higher values over the coastal and desert areas. Coastal areas portrayed higher values as well likely due to the high levels of evaporation and increased anthropogenic activities such as coastal urban developments and automobile pollution. Time series analysis of the multitemporal dataset confirmed the SARIMA forecast model as the best fit which can be used to provide an accurate and reliable monthly forecast for the AOD concentrations in the UAE.

Winter AOD trend changes over the Eastern Mediterranean and Middle East region

AbstractThe present study documents the winter aerosol optical depth (AOD) trends over the Eastern Mediterranean and Middle East (EMME) region using MERRA‐2 and moderate‐resolution imaging spectroradiometer (MODIS) collection 6.1 data. A significant upward AOD trend was identified during the years 2000–2010, whereas the AOD followed a significant downward trend during the years 2010–2017. Our analysis indicates that aeolian dust is the main contributor to AOD changes. The winter AOD changes are related to meteorological factors over the EMME region. During the early period (2000–2010), a significant decrease in sea level pressure induced dry and hot southeasterly winds blowing from the desert regions in the Middle East toward the EMME, which increased the temperature and reduced the relative humidity, thus enhancing evaporation and promoting soil drying. In contrast, during the late period (2010–2017), a significant increase in sea level pressure, accompanied by an increase in the North Atlantic Oscillation (NAO) index, induced northwesterly winds from higher latitudes, which decreased the temperature and increased the relative humidity, reducing dust mobilization in the EMME, in particular, in Iraq and Egypt. This shows to what extent AOD trends in the EMME region are controlled by changing meteorological weather conditions.

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 (&lt;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 (&gt;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.

Aerosol Trends during the Dusty Season over Iran

This study assessed the aerosol climatology over Iran, based on the monthly data of aerosol optical depth (AOD) derived from the reanalysis-based Modern Era Retrospective Analysis for Research and Applications (MERRA-2) and the satellite-based Moderate Resolution Imaging Spectroradiometer (MODIS). In addition, sea level pressure, wind speed, temperature, relative humidity, precipitation, and soil moisture from the ERA5 reanalysis dataset were applied to investigate the climate-related effects on temporal AOD changes. Our analysis identified positive and negative AOD trends during 2000–2010 and 2010–2018, respectively, which are likely linked to aeolian dust changes. The dust-driven AOD trends were supported by changes in the Ångström exponent (AE) and fine mode fraction (FMF) of aerosols over Iran. During the early period (2000–2010), results of AOD-meteorology correlation analyses suggest reduced soil moisture, leading to increased dust emissions, whereas our results suggest that during the later period (2010–2018) an increase of soil moisture led to decreased AOD levels. Soil moisture appears to be a key factor in dust mobilization in the region, notably in southwestern Iran, being influenced by adjacent mineral dust sources. These phenomena were affected by large-scale sea level pressure transformations and the associated meteorology in the preceding winter seasons. Using a multiple linear regression model, AOD variability was linked to various meteorological factors in different regions. Our results suggest that climatic variations strongly affect the dust cycle, with a strong dependence on wintertime conditions in the region.

Interdecadal Changes in Aerosol Optical Depth over Pakistan Based on the MERRA-2 Reanalysis Data during 1980–2018

The spatiotemporal evolution and trends in aerosol optical depth (AOD) over environmentally distinct regions in Pakistan are investigated for the period 1980–2018. The AOD data for this period was obtained from the Modern-era retrospective analysis for research and applications, version 2 (MERRA-2) reanalysis atmospheric products, together with the Moderate-resolution imaging spectroradiometer (MODIS) retrievals. The climatology of AODMERRA-2 is analyzed in three different contexts: the entire study domain (Pakistan), six regions within the domain, and 12 cities chosen from the entire study domain. The time-series analysis of the MODIS and MERRA-2 AOD data shows similar patterns in individual cities. The AOD and its seasonality vary strongly across Pakistan, with the lowest (0.05 ± 0.04) and highest (0.40 ± 0.06) in the autumn and summer seasons over the desert and the coastal regions, respectively. During the study period, the annual AOD trend increased between 0.002 and 0.012 year−1. The increase of AOD is attributed to an increase in population and emissions from natural and/or anthropogenic sources. A general increase in the annual AOD over the central to lower Indus Basin is ascribed to the large contribution of dust particles from the desert. During winter and spring, a significant decrease in the AOD was observed in the northern regions of Pakistan. The MERRA-2 and MODIS trends (2002–2018) were compared, and the results show visible differences between the AOD datasets due to theuseof different versions and collection methods. Overall, the present study provides insight into the regional differences of AOD and its trends with the pronounced seasonal behavior across Pakistan.