Aerosol radiative forcing over the Indo-Gangetic plains during major dust storms

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 (±30 min) validation of AERONET AOD with MODIS Terra (level 2 swath product) over Kanpur, at a common wavelength of 550 nm for the period 2001–2005 show moderate correlation (R²∼0.6) during the summer season. The average surface forcing is found to change by −23 W m⁻² during dust events and the top of the atmosphere (TOA) forcing change by −11 W m⁻² as compared to the non-dusty clear-sky days. A strong correlation is found between AOD at 500 nm and the ARF. At surface, the correlation coefficient between AOD and ARF is found to be high (R²=0.925) and is found to be moderate (R²=0.628) at the TOA. The slope of the regression line gives the aerosol forcing efficiency at 500 nm of about −46±2.6 W m⁻² and −17±2.5 W m⁻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 500 nm) from April (0.4–0.5) to June (0.6–0.7) over the IG plains.     

Aerosol indirect effect during successive contrasting monsoon seasons over Indian subcontinent using MODIS data

Aerosol indirect effect (AIE) was estimated over six Indian regions, which have been identified as main source regions of absorbing aerosol emissions, for four successive contrasting monsoon years, 2001 (normal monsoon rainfall year), 2002 (drought year), 2003 (excess monsoon rainfall year) and 2004 (below normal rainfall year). The AIE has been estimated both for fixed cloud liquid water path (CLWP) and for fixed cloud ice path (CIP) bins, ranging from 1 to 350 gm^?2 at 25 gm^?2 intervals obtained from Moderate resolution imaging spectroradiometer (MODIS). In 2002 and 2004, AIE found to be of positive (Twomey effect) in majority of the fixed CLWP and CIP bins, while in 2001 and 2003 majority of the bins were found to be showing negative indirect effect (Anti-Twomey effect). Changes in circulation patterns during contrasting monsoon seasons, bringing up air mass containing aerosols of different source origins may be the main reason for this positive and negative AIE. The study suggests that AIE could be one of the factors in modulating Indian summer monsoon. However, further research on this topic is to be carried out to establish the relationship between AIE and Indian monsoon rainfall and also AIE values may be parameterized in climate models for better prediction of monsoon.     

Radiative effects of aerosols over Indo-Gangetic plain: environmental (urban vs. rural) and seasonal variations

Aerosol radiative effects over two environmentally distinct locations, Kanpur (urban site) and Gandhi College (rural location) in the Indo-Gangetic plain (IGP), a regional aerosol hot spot, utilizing the measured optical and physical characteristics of aerosols, an aerosol optical properties model and a radiative transfer model, are examined. Shortwave aerosol radiative forcing (ARF) at the top of the atmosphere (TOA) is 30 W m(?-?2)). Shortwave atmospheric heating due to aerosols is >0.4 K/day over IGP and peaks during premonsoon at >0.6 K/day due to lower single scattering albedo (SSA) and higher surface albedo. TOA forcing is always less negative over Kanpur when compared to Gandhi College due to lower surface albedo except in postmonsoon owing to higher SSA. This happens as TOA forcing depends on SSA and surface albedo in addition to aerosol optical depth. The magnitude of longwave forcing and atmospheric cooling in an absolute sense is significantly small and contributes only about 20% or less to the net (shortwave + longwave) forcing. Aerosol radiative effects over these two locations, despite differences in aerosol characteristics, are similar, thus confirming that aerosols and their radiative influence get transported due to circulation. ARF over Kanpur and Gandhi College is an order of magnitude higher when compared to greenhouse gas forcing. A large reduction in surface reaching solar irradiance accompanied by large atmospheric warming can have implications on precipitation and hydrological cycle, and these aerosol radiative effects should be included while performing regional-scale aerosol climate assessments.     

Investigating hydrochemistry of groundwater in Indo-Gangetic alluvial plain using multivariate chemometric approaches

Groundwater hydrochemistry of an urban industrial region in Indo-Gangetic plains of north India was investigated. Groundwater samples were collected both from the industrial and non-industrial areas of Kanpur. The hydrochemical data were analyzed using various water quality indices and nonparametric statistical methods. Principal components analysis (PCA) was performed to identify the factors responsible for groundwater contamination. Ensemble learning-based decision treeboost (DTB) models were constructed to develop discriminating and regression functions to differentiate the groundwater hydrochemistry of the three different areas, to identify the responsible factors, and to predict the groundwater quality using selected measured variables. The results indicated non-normal distribution and wide variability of water quality variables in all the study areas, suggesting for nonhomogenous distribution of sources in the region. PCA results showed contaminants of industrial origin dominating in the region. DBT classification model identified pH, redox potential, total-Cr, and λ ₂₅₄ as the discriminating variables in water quality of the three areas with the average accuracy of 99.51 % in complete data. The regression model predicted the groundwater chemical oxygen demand values exhibiting high correlation with measured values (0.962 in training; 0.918 in test) and the respective low root mean-squared error of 2.24 and 2.01 in training and test arrays. The statistical and chemometric approaches used here suggest that groundwater hydrochemistry differs in the three areas and is dominated by different variables. The proposed methods can be used as effective tools in groundwater management.     

Aerosol organic nitrogen over the remote Atlantic Ocean

Water soluble organic nitrogen (WSON) has been measured in aerosols collected on three research cruises on the Atlantic Ocean from approximately 55°N to 45°S. Results are interpreted using air mass back trajectories and results for other aerosol components. WSON concentrations range from <1 to ～40 (median 5.6) nmol m^?3 with significant WSON concentrations in both fine (<1 μm) and coarse mode (>1 μm) aerosol. Concentrations of WSON were highest in samples containing Saharan dust, suggesting a locally significant source associated with soil dust. More generally WSON concentrations were highest in air which had recently crossed continental areas. In the whole data set, WSON is well correlated to total soluble nitrogen and represents approximately 25% of total nitrogen. This correlation implies a significant anthropogenic contribution to the organic nitrogen.     

Mitigating nitrous oxide and methane emissions from soil in rice-wheat system of the Indo-Gangetic plain with nitrification and urease inhibitors

Mitigation of methane (CH4) and nitrous oxide (N2O) emissions from soil is important to reduce the global warming. Efficacy of five nitrification inhibitors, i.e. neem (Azadirachta melia) cake, thiosulphate, coated calcium carbide, neem oil coated urea and dicyandiamide (DCD) and one urease inhibitor, hydroquinone, in mitigating N2O and CH4 emissions from fertilized soil was tested in rice-wheat system in the Indo-Gangetic plains. The closed chamber technique was used for the collection of gas samples, which were analyzed using gas chromatography. Reduction in N2O emission on the application of nitrification/urease inhibitors along with urea ranged from 5% with hydroquinone to 31% with thiosulphate in rice and 7% with hydroquinone to 29% with DCD in wheat crop. The inhibitors also influenced the emission of CH4. While application of neem coated urea, coated calcium carbide, neem oil and DCD reduced the emission of CH4; hydroquinone and thiosulphate increased the emission when compared to urea alone. However, the global warming potential was lower with the inhibitors (except hydroquinone) as compared to urea alone, suggesting that these substances could be used for mitigating greenhouse gas emission from the rice-wheat systems.     

Contribution of anthropogenic aerosols in direct radiative forcing and atmospheric heating rate over Delhi in the Indo-Gangetic Basin

Introduction  

The present work is aimed to understand direct radiation effects due to aerosols over Delhi in the Indo-Gangetic Basin (IGB) region, using detailed chemical analysis of surface measured aerosols during the year 2007.     

Measurements of Atmospheric Aerosol Optical Thickness over Water Using ERTS-1 Data

The body of information presented in this paper Is directed to those Interested in the application of satellite techniques for environmental monitoring. The apparent gradual increase of particles in the atmosphere has received considerable attention in recent years due to the possible effect of atmospheric aerosols on the earth’s climate. The ERTS-1 satellite offered the opportunity of determining the feasibility of monitoring the atmospheric aerosol optical thickness on a global basis, as suggested by theoretical studies, which showed a linear relationship between the upwelling earth-atmosphere radiance and the aerosol optical thickness. This relationship was investigated at two test sites, San Diego and the Salton Sea, using the multispectral scanner (MSS) radiance data, with ground-truth observations of the aerosol optical thickness being made with a Volz photometer at the time of the satellite overpasses. Significant results, relating the radiance over water surfaces to the atmospheric aerosol optical thickness, have been obtained. The results indicate that the MSS channels, 4, 5 and 6 centered at 0.55, 0.65 and 0.75 μm have comparable sensitivity, and that the optical thickness can be determined within ±10% with the assumed measurement errors of the MSS.     

Aerosol optical properties based on ground measurements over the Chinese Yangtze Delta Region

The characteristics of Aerosol Optical Depth (AOD) and Angstrom exponent were analyzed and compared using Cimel sunphotometer data from 2007 to 2008 at five sites located in the Yangtze River Delta region of China. The simultaneous measurements between Lin’an and ZFU showed a very high consistency of AOD at all wavelengths. The differences are less than 0.02 for Angstrom exponent and AOD at all wavelengths. The mean values of AOD at 440 nm at the Pudong, Taihu and Lin’an were about 0.74 ± 0.43, 0.85 ± 0.46, and 0.89 ± 0.46, respectively. The mean values of Angstrom exponents were about 1.27 ± 0.30, 1.20 ± 0.28 and 1.32 ± 0.35, respectively. The variation of monthly averaged AOD over Pudong showed a single peak distribution, with the maximum value occurring in July (AOD_440nm 1.26 ± 0.61) and minimum in January (AOD_440nm 0.50 ± 0.27). However, the variations of monthly averaged AOD at Taihu and Lin’an showed a bi-modal distribution. There were peak values of AOD occurring in July (AOD_440nm 1.41 ± 0.49) and September (AOD_440nm 1.22 ± 0.52) for Taihu. For Lin’an, the two peak values of AOD occurred in June (AOD_440nm 1.17 ± 0.69) and September (AOD_440nm 1.28 ± 0.46). The AOD accumulated mainly between 0.30–0.90(68%), 0.30–1.20(75%) and 0.30–1.20 (～75%) at Pudong, Taihu, and Lin’an, respectively. The Angstrom exponent accumulated mainly between 1.10–1.60 (75%), 1.10–1.50 (63%) and 1.20–1.60, 50% (50%) at Pudong, Taihu, and Lin’an, respectively.The synchronized observation showed that the AOD at Pudong was larger than those at Dongtan by 0.03, 0.03, 0.04, 0.07, and 0.08 at wavelengths of 1020 nm, 870 nm, 670 nm, 500 nm and 440 nm, respectively. The synchronized observations at Pudong, Taihu and Lin’an showed that the three stations had high level AOD with means at 440 nm about 0.68, 0.73, and 0.78, respectively. The relationship between MODIS retrieved and ground-based measured AOD shows good agreement with R² ranging from 0.68 to 0.79 at Pudong, Taihu, Lin’an and Dongtan. The MODIS results were overestimated comparing the ground measurements at Pudong, Taihu, and Dongtan but exceptional at Lin’an.The analysis results between aerosol optical properties and wind measurement at Pudong showed that the wind speed from the east correlates with the lower observed AOD. The back trajectory analysis indicates that more than 50% airmasses were from the marine area at Pudong, while back trajectories distribution is relatively homogeneous at Lin’an.     

Aerosol optical thickness of yellow sand over the yellow sea derived from NOAA satellite data

To derive the optical thickness of desert yellow sand from China crossing over the sea, first of all a comparison was made of the NOAA satellite data in a clear area with that in the area polluted by yellow sand. Secondly, the upward radiance emerging from a model atmosphere-ocean system was computed with and without yellow sand, where it is assumed that the yellow sand is homogeneously concentrated in the sublayer at altitudes between 2 and 4 km. Finally, the optical thickness was derived by comparing the satellite data with the computed upward radiation.Out of the sunglint, the optical thickness of the dust was estimated to range from 0.17 to 0.60 in the visible and near infrared channels, but the radiance computed is rather overestimated in the sunglint. To estimate the effect of water vapor absorption on the radiation in the near infrared channel, use was made of 11 μm data. However, it was difficult to estimate this effect precisely using the one channel, partly due to insufficient information on surface temperatures and verticle distributions of temperature and water vapour. The combined use of 3.7 and 11-μm data should help to remedy this deficiency.     

Aerosol ion characteristics during the Big Bend Regional Aerosol and Visibility Observational study

The ionic compositions of particulate matter with aerodynamic diameter < or = 2.5 microm (PM2.5) and size-resolved aerosol particles were measured in Big Bend National Park, Texas, during the 1999 Big Bend Regional Aerosol and Visibility Observational study. The ionic composition of PM2.5 aerosol was dominated by sulfate (SO4(2-)) and ammonium (NH4+). Daily average SO4(2-) and NH4+ concentrations were strongly correlated (R2 = 0.94). The molar ratio of NH4+ to SO4(2-) averaged 1.54, consistent with concurrent measurements of aerosol acidity. The aerosol was observed to be comprised of a submicron fine mode consisting primarily of ammoniated SO4(2-) and a coarse particle mode containing nitrate (NO3-). The NO3- appears to be primarily associated with sea salt particles where chloride has been replaced by NO3-, although formation of calcium nitrate (Ca(NO3)2) is important, too, on several days. Size-resolved aerosol composition results reveal that a size cut in particulate matter with aerodynamic diameter < or = 1 microm would have provided a much better separation of fine and coarse aerosol modes than the standard PM2.5 size cut utilized for the study. Although considerable nitric acid exists in the gas phase at Big Bend, the aerosol is sufficiently acidic and temperatures sufficiently high that even significant future reductions in PM2.5 SO4(2-) are unlikely to be offset by formation of particulate ammonium nitrate in summer or fall.     

A Simplified Aerosol Coagulation Model

A simplified aerosol coagulation model has been developed, to determine the mutual coagulation of many discrete size classes. The model has been applied to computation of the growth of light scattering particles from a small monodisperse aerosol, and prediction of aerosol size and concentration downwind of sources. These computations are in good agreement with field observations. This model is sufficiently simplified to allow programming In contemporary desk calculators and is an extremely practical tool for application of aerosol technology to air pollution problems.     

Aerosol maps from GOME data

In this paper, we present a methodology to calibrate the surface reflectance seen by satellite and validate the aerosol optical properties retrieved by the GOME instrument. Data are also visualized in maps by a tool properly developed, named GOMEView. The validation procedure is based on ground measurements obtained by sunphotometers. Results show that calibration of the surface reflectance is crucial to obtain the best results, i.e. in agreement with the ground measurements. Aerosol data have also been classified on the basis of their optical properties evidencing for instance, the presence of desert aerosol over the sea along the west coast of Sahara. Cloud retrievals were also analyzed in terms of their occurrence and amount.