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Air quality in mega cities is one of the major concerns due to serious health issues and its indirect impact to the climate. Among mega cities, Beijing city is considered as one of the densely populated cities with extremely poor air quality. The meteorological parameters (wind, surface temperature, air temperature and relative humidity) control the dynamics and dispersion of air pollution. China National Environmental Monitoring Centre (CNEMC) started air pollution index (API) as of 2000 to evaluate air quality, but over the years, it was felt that the air quality is not well represented by API. Recently, the Ministry of Environmental Protection (MEP) of the People's Republic of China (PRC) started using a new index "air quality index (AQI)" from January 2013. We have compared API and AQI with three different MODIS (MODIS - Moderate Resolution Imaging SpectroRadiometer, onboard the Terra/Aqua satellites) AOD (aerosol optical depth) products for ten months, January-October, 2013. The correlation between AQI and Aqua Deep Blue AOD was found to be reasonably good as compared with API, mainly due to inclusion of PM2.5 in the calculation of AQI. In addition, for every month, the correlation coefficient between AQI and Aqua Deep Blue AOD was found to be relatively higher in the month of February to May. According to the monthly average distribution of precipitation, temperature, and PM10, the air quality in the months of June-September was better as compared to those in the months of February-May. AQI and Aqua Deep Blue AOD show highly polluted days associated with dust event, representing true air quality of Beijing., (Copyright © 2013 Elsevier B.V. All rights reserved.)

Aerosols play important roles in modulations of cloud properties and hydrological cycle by decreasing the size of cloud droplets with the increase of aerosols under the condition of fixed liquid water path, which is known as the first aerosol indirect effect or Twomey-effect or microphysical effect. Using high-quality aerosol data from surface observations and statistically decoupling the influence of meteorological factors, we show that highly loaded aerosols can counter this microphysical effect through the radiative effect to result both the decrease and increase of cloud droplet size depending on liquid water path in water clouds. The radiative effect due to increased aerosols reduces the moisture content, but increases the atmospheric stability at higher altitudes, generating conditions favorable for cloud top entrainment and cloud droplet coalescence. Such radiatively driven cloud droplet coalescence process is relatively stronger in thicker clouds to counter relatively weaker microphysical effect, resulting the increase of cloud droplet size with the increase of aerosol loading; and vice-versa in thinner clouds. Overall, the study suggests the prevalence of both negative and positive relationships between cloud droplet size and aerosol loading in highly polluted regions. [ABSTRACT FROM AUTHOR]

Satellite aerosol data provide good temporal sampling and superior spatial coverage relative to ground-based stations, especially over the Indian subcontinent. We have used ground-based Kanpur Aerosol Robotic Network (AERONET) station aerosol optical depth (AOD), located in the central part of the Indo-Gangetic (IG) basin, to validate Multiangle Imaging SpectroRadiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS) Terra level 3 AOD products for the 2001–2005 summer and winter seasons. MISR has been found to perform better than MODIS during both seasons, which we attribute to its unique design (viewing and spectral capability), despite its lower revisit frequency compared to MODIS. In both seasons, the MISR level 3 product distinctly shows a large gradient of AOD along north–south (N–S) and west–east (W–E) transects over the IG basin, compared to the southern part of the Indian subcontinent. An increase in satellite-derived aerosol loading over the 2000–2005 time period has been found over major cities located in the IG basin; this increase is also reflected in ground-based air quality data of the Central Pollution Control Board (CPCB). High MISR AOD during the winter season can be attributed primarily to anthropogenic activities, as major dust storms take place during summer (April–June). We have computed the spatial correlation between MISR and MODIS AOD during the 2000–2005 winter (November–January) seasons, when the effect of desert dust is smallest and the anthropogenic component over the IG basin is largest. The average pixel count with MISR-MODIS correlation coefficient >0. 6 during the winter season has been found to be higher over the IG basin and India (35. 6% and 32. 2% of pixels, respectively) compared to the world (20. 4% of pixels). Since the MODIS AOD retrieval algorithm works best for dense and dark vegetative regions, whereas the MISR retrieval is generally less sensitive to surface type, we attribute this result to the relatively high vegetation cover (using MISR Normalized Difference Vegetation Index as a proxy) found in the IG basin and over India. [Copyright &y& Elsevier]

• Reduced snow cover was observed over the eastern Himalaya region due to dust particles deposition. • Dust aerosols that travel over the ocean increase RH and WV over the area where the dust arrives. • Strong mixing of fine and coarse particles presents during crop burning coincided with Diwali. Dust storms during the pre-monsoon season (April–June) and crop residual burning events during the post-monsoon season (mid-October–mid-November) are common every year over the Indo-Gangetic plains (IGP). In this paper, we have compared dust storm and crop burning aerosols characteristics for the years 2016, 2017 and 2018 using ground and satellite data. CALIPSO data show that dust layers extended from the ground upto an altitude of about 5 km and the smoke from crop burning upto the height of 2 km. Characteristics of dust and crop burning aerosols show pronounced difference based on Kanpur AERONET data. Dominance of coarse particles (0.6–15 μm) during dust storms (pre-monsoon season), while fine particles (0.05–0.6 μm) dominate during crop residual burning. The spectral variations of single scattering albedo (SSA) during dusty days and crop burning days show low and high fractions of anthropogenic aerosols. We have also observed the impact of dust particles on Himalayan snow (cover, albedo and reflectance) and meteorological parameters (relative humidity and water vapor) on the surface and lower atmosphere using MODIS data and AIRS data at different pressure levels. Pronounced aerosols behaviors of the crop residual burning event coincided with Diwali festival on 30–31 October 2016 were observed. Our detailed analysis combining ground and satellite observations provides better understanding of aerosol optical and microphysical properties of dust storms and crop residual burnings. The results will be valuable in monitoring surrounding environment, identifying the emission source and dynamics of dust storms and crop burning emissions over India, especially in the IGP. [ABSTRACT FROM AUTHOR]

Natural and anthropogenic aerosols over the Kingdom of Saudi Arabia (KSA) play a major role in affecting the regional radiation budget. The long-term variability of these aerosols' physical and optical parameters, including aerosol optical depth (AOD) and Ångström exponent (α), were measured at a location near central KSA using the Solar Village (SV) AERONET (Aerosol Robotic Network) station during the period December 1999–January 2013. The AERONET measurements show an overall increase in AOD on an annual basis. This upward trend is mainly attributed to a prolonged increase in the monthly/seasonal mean AOD during March–June and during August–September. In contrast, lower AOD values were observed during November–December. This can be attributed to a low frequency of dust outbreaks and higher precipitation rates. An overall, weak declining trend in α was observed, except during the summer. The spring and summer seasons experienced a pronounced increase in the number of coarse particles (~2 µm) during April 2006–January 2013 as compared to December 1999–March 2006, suggesting an increase in natural aerosol loadings. Using the HYSPLIT model, it was found that the March 2009 dust storm contributed to the mixing of long-transported dust with anthropogenic local emissions near the SV. The results suggest that extensive industrial activity contributed to the increase of anthropogenic emissions over KSA during the period April 2006–January 2013. [ABSTRACT FROM AUTHOR]

The northeastern parts of China, including Beijing city, the capital of China, were hit by an intense dust storm on 15 April 2015. The present paper discusses aerosol and meteorological parameters associated with this dust storm event. The back trajectory clearly shows that the dust originated from Inner Mongolia, the border of China, and Mongolia regions. Pronounced changes in aerosol and meteorological parameters along the dust track were observed. High aerosol optical depth (AOD) with low Ångström exponent (AE) are characteristics of coarse-mode dominated dust particles in the wavelength range 440–870 nm during the dusty day. During dust storm, dominance of coarse aerosol concentrations is observed in the aerosol size distribution (ASD). The single scattering albedo (SSA) retrieved from AERONET station shows increase with higher wavelength on the dusty day, and is found to be higher compared to the days prior to and after the dust event, supported with high values of the real part and decrease in the imaginary part of the refractive index (RI). With regard to meteorological parameters, during the dusty day, CO volume mixing ratio (COVMR) is observed to decrease, from the surface up to mid-altitude, compared with the non-dusty days due to strong winds. O3 volume mixing ratio (O3VMR) enhances at the increasing altitudes (at the low-pressure levels), and decreases near the surface at the pressure levels 500–925 hPa during the dust event, compared with the non-dusty periods. An increase in the H2O mass mixing ratio (H2OMMR) is observed during dusty periods at the higher altitudes equivalent to the pressure levels 500 and 700 hPa. The mid-altitude relative humidity (RH) is observed to decrease at the pressure levels 700 and 925 hPa during sand storm days. With the onset of the dust storm event, the RH reduces at the surface level. [ABSTRACT FROM AUTHOR]

Abstract: Column-integrated aerosol black carbon (BC) concentration, [BC] has been retrieved over Kanpur (an industrial city in the Gangetic basin, northern India) during 2001–2003 for the first time. [BC] is derived from BC volume fraction and AERONET-retrieved size distribution using Maxwell–Garnett and Bruggeman mixing rules in a mixture of water, BC and (NH4)2SO4. In addition partly absorbing components like organic carbon (OC) and dust are added to the Maxwell–Garnett mixture depending on the season to investigate their role in the retrieval of [BC]. The volume fraction of each component is retrieved by matching the mixture refractive index (real, n(λ) and imaginary, k(λ)) with AERONET-retrieved refractive index. [BC] shows seasonal variations with high values () observed during the post-monsoon and winter seasons and low values () during the monsoon season. Specific absorption cross-section (α a) decreases non-linearly with the increase in [BC], however, the decrease becomes linear when other absorbing components are present. Yearly averaged [BC] and α a are 9.99±1.95, 5.52±1.07, 7.9±1.53mgm−2 and 7.9±1.83 and 9.67±3.45, 12.74±2.92m2 g−1 for 2001, 2002 and 2003, respectively, using Maxwell–Garnett mixing, which differ by ∼15% from those using Bruggeman mixing. [BC] shows diurnal variation with morning and afternoon peaks and mid-day minimum. The amplitude is subdued as it represents the total column, which is more influenced by anthropogenic activities than by boundary layer dynamics. In order to estimate [BC] accurately, OC has to be considered when the absolute difference between k(0.44μm) and k(1.02μm) becomes higher than 0.0015. The sensitivity of [BC] due to dust n(λ) becomes important during the intense dust loading period. It was found that [BC] is more sensitive to dust k(λ) than dust n(λ), as [BC] increases ∼10–13% for 10% rise in k(λ). Retrieved [BC] shows good agreement with the in situ measurements. Therefore our retrieval can be used as an alternate method to infer BC and OC specific absorption globally. [Copyright &y& Elsevier]

Abstract: Indo-Gangetic (IG) alluvial plains, one of the largest river basins in the world, suffers from the long range transport of mineral dust from the western arid and desert regions of Africa, Arabia and Rajasthan during the summer (pre-monsoon season, April–June). These dust storms influence the aerosol optical depth (AOD) across the IG plains. The Kanpur AERONET (Aerosol Robotic Network) station and Moderate Resolution Imaging Spectro-radiometer (MODIS) data show pronounced effect on the aerosol optical properties and aerosol size distribution during major dust storm events over the IG plains that have significant effect on the aerosol radiative forcing (ARF). The multi-band AOD, from AERONET and MODIS, show contrasting changes in wavelength dependency over dust affected regions. A time collocated (±30min) validation of AERONET AOD with MODIS Terra (level 2 swath product) over Kanpur, at a common wavelength of 550nm for the period 2001–2005 show moderate correlation (R 2∼0.6) during the summer season. The average surface forcing is found to change by −23Wm−2 during dust events and the top of the atmosphere (TOA) forcing change by −11Wm−2 as compared to the non-dusty clear-sky days. A strong correlation is found between AOD at 500nm and the ARF. At surface, the correlation coefficient between AOD and ARF is found to be high (R 2=0.925) and is found to be moderate (R 2=0.628) at the TOA. The slope of the regression line gives the aerosol forcing efficiency at 500nm of about −46±2.6Wm−2 and −17±2.5Wm−2 at the surface and the TOA, respectively. The ARF is found to increase with the advance of the dry season in conjunction with the gradual rise in AOD (at 500nm) from April (0.4–0.5) to June (0.6–0.7) over the IG plains. [Copyright &y& Elsevier]