Aerosol Trends Research Articles

Impact of satellite viewing-swath width on global and regional aerosol optical thickness statistics and trends

Abstract. We use the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite aerosol optical thickness (AOT) product to assess the impact of reduced swath width on global and regional AOT statistics and trends. Along-track and across-track sampling strategies are employed, in which the full MODIS data set is sub-sampled with various narrow-swath (~ 400–800 km) and single pixel width (~ 10 km) configurations. Although view-angle artifacts in the MODIS AOT retrieval confound direct comparisons between averages derived from different sub-samples, careful analysis shows that with many portions of the Earth essentially unobserved, spatial sampling introduces uncertainty in the derived seasonal–regional mean AOT. These AOT spatial sampling artifacts comprise up to 60% of the full-swath AOT value under moderate aerosol loading, and can be as large as 0.1 in some regions under high aerosol loading. Compared to full-swath observations, narrower swath and single pixel width sampling exhibits a reduced ability to detect AOT trends with statistical significance. On the other hand, estimates of the global, annual mean AOT do not vary significantly from the full-swath values as spatial sampling is reduced. Aggregation of the MODIS data at coarse grid scales (10°) shows consistency in the aerosol trends across sampling strategies, with increased statistical confidence, but quantitative errors in the derived trends are found even for the full-swath data when compared to high spatial resolution (0.5°) aggregations. Using results of a model-derived aerosol reanalysis, we find consistency in our conclusions about a seasonal–regional spatial sampling artifact in AOT. Furthermore, the model shows that reduced spatial sampling can amount to uncertainty in computed shortwave top-of-atmosphere aerosol radiative forcing of 2–3 W m−2. These artifacts are lower bounds, as possibly other unconsidered sampling strategies would perform less well. These results suggest that future aerosol satellite missions having significantly less than full-swath viewing are unlikely to sample the true AOT distribution well enough to obtain the statistics needed to reduce uncertainty in aerosol direct forcing of climate.

Revisiting AVHRR tropospheric aerosol trends using principal component analysis

The advanced very high resolution radiometer (AVHRR) satellite instruments provide a nearly 25 year continuous record of global aerosol properties over the ocean. It offers valuable insights into the long-term change in global aerosol loading. However, the AVHRR data record is heavily influenced by two volcanic eruptions, El Chichon on March 1982 and Mount Pinatubo on June 1991. The gradual decay of volcanic aerosols may last years after the eruption, which potentially masks the estimation of aerosol trends in the lower troposphere, especially those of anthropogenic origin. In this study, we show that a principal component analysis approach effectively captures the bulk of the spatial and temporal variability of volcanic aerosols into a single mode. The spatial pattern and time series of this mode provide a good match to the global distribution and decay of volcanic aerosols. We further reconstruct the data set by removing the volcanic aerosol component and reestimate the global and regional aerosol trends. Globally, the reconstructed data set reveals an increase of aerosol optical depth from 1985 to 1990 and decreasing trend from 1994 to 2006. Regionally, in the 1980s, positive trends are observed over the North Atlantic and North Arabian Sea, while negative tendencies are present off the West African coast and North Pacific. During the 1994 to 2006 period, the Gulf of Mexico, North Atlantic close to Europe, and North Africa exhibit negative trends, while the coastal regions of East and South Asia, the Sahel region, and South America show positive trends.

Investigating aerosol properties in Peninsular Malaysia via the synergy of satellite remote sensing and ground-based measurements

Spatio-temporal variation and trends in atmospheric aerosols as well as their impact on solar radiation and clouds are crucial for regional and global climate change assessment. These topics are not so well-documented over Malaysia, the fact that it receives considerable amounts of pollutants from both local and trans-boundary sources. The present study aims to analyse the spatio-temporal evolution and decadal trend of Aerosol Optical Depth (AOD) from Terra and Aqua MODIS sensors, to identify different types and origin of aerosols and explore the link between aerosols and solar radiation. AOD and fine-mode fraction (FMF) products from MODIS, AOD and Ångström Exponent (AE) values from AERONET stations along with ground-based PM10 measurements and solar radiation recordings at selected sites in Peninsular Malaysia are used for this scope. The MODIS AODs exhibit a wide spatio-temporal variation over Peninsular Malaysia, while Aqua AOD is consistently lower than that from Terra. The AOD shows a neutral-to-declining trend during the 2000s (Terra satellite), while that from Aqua exhibits an increasing trend (~0.01 per year). AERONET AODs exhibit either insignificant diurnal variation or higher values during the afternoon, while their short-term availability does not allow for a trend analysis. Moreover, the PM10 concentrations exhibit a general increasing trend over the examined locations. The sources and destination of aerosols are identified via the HYSPLIT trajectory model, revealing that aerosols during the dry season (June to September) are mainly originated from the west and southwest (Sumatra, Indonesia), while in the wet season (November to March) they are mostly associated with the northeast monsoon winds from the southern China Sea. Different aerosol types are identified via the relationship of AOD with FMF, revealing that the urban and biomass-burning aerosols are the most abundant over the region contributing to a significant reduction (~−0.21MJm−2) of the solar radiation.

Anthropogenic aerosols increasing over India

AbstractAerosol particles in the Earth's atmosphere scatter and absorb light differently at different wavelengths, thereby affecting the amount of incoming sunlight that reaches the planet's surface and the amount of heat that escapes, potentially altering the planet's climate. Most recent regional studies of aerosol trends have used satellite data to examine aerosol levels over ocean regions; fewer regional studies have measured aerosol over land.

Spatio-temporal variations of optical properties of aerosols in East Asia measured by MODIS and relation to the ground-based mass concentrations observed in central Korea during 2001∼2010

Long-term variations and trends of atmospheric aerosols in the East Asian region were analyzed by using aerosol optical depth (AOD or τ), and angstrom exponent (AE or α) obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) from 2001 to 2010. The increased emission of anthropogenic fine aerosols in east China resulted in the high AOD in this region during summer. The steady increasing emission of anthropogenic fine aerosols caused an increasing trend of AOD in east China, and the large-scale transport of sandstorms and smoke plume caused by forest fires affected intense inter-annual variations of AOD in the East Asian region. While in the central part of South Korea, located in the lee side of the East Asian continent, AE tended to rise to a level higher than in east China, the ground-based mass concentrations continued to decline. A noticeable decrease of PM10 mass concentration in spring and winter in central Korea is most likely attributable to decreases in sandstorms in the source region of East Asia. However, the ratio of PM2.5 mass concentration to PM10 increases overall with a high level in summer. Aerosol types were classified into dust, smoke plume, and sulphate by using satellite data over Cheongwon in central Korea. The columnar AOD, with different aerosol types, was compared with the ground-based mass concentrations at Cheongwon, and the relatively high level of the correlations presented between PM2.5 and AOD produced in sulphate. Growth and increases of fine hygroscopic aerosols generated as gas-to-particle conversion particularly in summer contribute to increases of columnar AOD in the East Asian region.

Seasonal Air Quality Profile of Size-Segregated Aerosols in the Ambient Air of a Central Indian Region

Seasonal distribution trends of size-segregated aerosols i.e. submicron (PM1), fine (PM2.5) and coarse (PM2.5-10) and their relationship with meteorological variables employing correlation analysis were studied in the ambient air of central India from July 2009 to June 2010. The annual mean concentrations of PM1, PM2.5 and PM2.5-10 were found to be 65.7, 135.0 and 118.5μgm(-3), respectively. The annual mean PM2.5 concentration is three times higher than the National Ambient Air Quality Standards of India (NAAQS). Higher concentrations of PM1, PM2.5 and PM2.5-10 were found during winter due to enormous biomass burning especially during night time due to the use of combustible goods like fire wood and dung cake in the open space by the peoples to keep themselves warm and lower concentrations were observed during monsoon when there were high precipitations. PM2.5 showed high positive correlation with PM1 (r=0.69) and moderate correlation with PM2.5-10 indicating that variation in PM2.5 mass is governed by the variation in PM1 mass or vice versa.

Latest decade’s spatial–temporal properties of aerosols over China based on Multiangle Imaging SpectroRadiometer observations

The analysis of the spatial–temporal variability and trends of aerosols over China based on ∼11 years (February 2000 to December 2010) of Terra-Multiangle Imaging SpectroRadiometer (MISR) Level 3 aerosol products is the focus. The study shows that the MISR aerosol optical depth (AOD) is in good agreement with corresponding AOD from AERONET stations and suggests that MISR aerosol products have higher applicability compared with MODIS AOD products over China. The spatial–temporal distribution and trends for annual and seasonal AOD values over the middle and eastern regions of China are the main focus. The results of our analysis show the significant characteristics and seasonal variation of the aerosol distribution over the study areas. The seasonal distribution in AOD over the study areas is not consistent with the MODIS data. An insignificant trend (5.3%) is observed over the whole study area during the study period. The larger positive tendencies are found in the Yangtze River Delta and the Pearl River Delta (PRD); the values increased by 24.4% and 18% and are considered to be significant with a confidence level of >95% . However, in contrast to the recently reported negligible AOD trends of 0.002 per decade using MODIS data over Sichuan, significant increasing trends of 0.046 per 11 years for MISR over the same areas have been measured. The analysis of seasonal variation revealed the seasonal trend in the AOD and the main types of aerosols. During spring, the largest increasing trend (30.2%) is observed in the PRD with a stable composition of fine and coarse aerosols. A pronounced decreasing trend is found over Shanxi and decreased by −18.7% in spring during the period, which has not been previously reported. MODIS-Terra currently shows artificial negative AOD trends over land. Therefore, trends derived from MODIS-Terra may not reflect the realistic decadal aerosol changes and variability. The study shows that seasonal distribution and trend analyses are inconsistent when using MISR AOD products and MODIS AOD products over study areas, which indicates that the analysis of aerosol variation using MISR sensors is necessary.

Variability of carbonaceous aerosols in remote, rural, urban and industrial environments in Spain: implications for air quality policy

Abstract. We interpret here the variability of levels of carbonaceous aerosols based on a 12 yr database from 78 monitoring stations across Spain specially compiled for this article. Data did not evidence any spatial trends of carbonaceous aerosols across the country. Conversely, results show marked differences in average concentrations from the cleanest, most remote sites (around 1 μg m−3 of non-mineral carbon (nmC), mostly made of organic carbon (OC) with very little elemental carbon (EC), around 0.1 μg m−3; OC / EC = 12–15), to the highly polluted major cities (8–10 μg m−3 of nmC; 3–4 μg m−3 of EC; 4–5 μg m−3 of OC; OC / EC = 1–2). Thus, urban (and very specific industrial) pollution was found to markedly increase levels of carbonaceous aerosols in Spain, with much lower impact of biomass burning and of biogenic emissions. Correlations between yearly averaged OC / EC and EC concentrations adjust very well to a potential equation (OC = 3.37 EC0.326, R2 = 0.8). A similar equation is obtained when including average concentrations obtained at other European sites (OC = 3.60EC0.491, R2 = 0.7). A clear seasonal variability in OC and EC concentrations was detected. Both OC and EC concentrations were higher during winter at the traffic and urban sites, but OC increased during the warmer months at the rural sites. Hourly equivalent black carbon (EBC) concentrations at urban sites accurately depict road traffic contributions, varying with distance from road, traffic volume and density, mixing-layer height and wind speed. Weekday urban rush-hour EBC peaks are mimicked by concentrations of primary gaseous emissions from road traffic, whereas a single midday peak is characteristic of remote and rural sites. Decreasing annual trends for carbonaceous aerosols were observed between 1999 and 2011 at a large number of stations, probably reflecting the impact of the EURO4 and EURO5 standards in reducing the diesel PM emissions. This has resulted in some cases in an increasing trend for NO2 / (OC + EC) ratios as these standards have been much less effective for the abatement of NOx exhaust emissions in passenger diesel cars. This study concludes that EC, EBC, and especially nmC and OC + EC are very good candidates for new air quality standards since they cover both emission impact and health-related issues.

A global survey of the effect of cloud contamination on the aerosol optical thickness and its long‐term trend derived from operational AVHRR satellite observations

Subpixel cloud contamination is one of the major issues plaguing passive satellite aerosol remote sensing. Its impact on the aerosol optical thickness (AOT) retrieval has been analyzed/evaluated by many studies. However, the question of how it influences the AOT trend remains to be answered. In this paper, four long‐term advanced very high resolution radiometer (AVHRR) AOT data sets from 1981 to 2009 over global oceans for four different definitions of clear sky, respectively, are produced by applying a two‐channel aerosol retrieval algorithm to the AVHRR clear‐sky reflectances derived by combining NOAA Pathfinder Atmosphere's Extended AVHRR climate data record level‐2b all‐sky reflectances with the cloud probability parameter determined from the Bayesian probabilistic cloud detection technique. A global analysis of the effect of cloud contamination on the AVHRR AOT retrieval as well as on its long‐term trend is then performed by comparing the results from the four data sets. It was found that cloud contamination imposes not only a positive bias on AOT values but also a positive bias on its long‐term trend such that negative trends become less negative and positive trends become more positive. A cloud probability value of ≤1% has been identified as an optimal criterion for clear‐sky definition to minimize the cloud contamination in the AVHRR aerosol retrieval while still retaining strong aerosol signals. In order for a satellite aerosol product to be useful and reliable in aerosol trend detection, the cloud contamination effect on aerosol trends needs to be studied/evaluated carefully along with the effects of calibration error, surface disturbance, and aerosol model assumptions.

Buildup of aerosols over the Indian Region

AbstractClimate change has great significance globally in general and South Asia in particular. Here we have used data from a network of 35 aerosol observatories over the Indian region to generate the first time regional synthesis using primary data and estimate the aerosol trends. On an average, aerosol optical depth (AOD) was found increasing at a rate of 2.3% (of its value in 1985) per year and more rapidly (~4%) during the last decade. If the trends continue so, AOD at several locations would nearly double and approach unity in the next few decades leading to an enhancement in aerosol‐induced lower atmospheric warming by a factor of two. However, a regionally averaged scenario can be ascertained only in the coming years, when longer and denser data would become available. The regional and global climate implications of such trends in the forcing elements need to be better assessed using GCMs.

(A)ATSR Solar Channel On-Orbit Radiometric Calibration

Data from the solar reflectance channels of the Along Track Scanning Radiometer (ATSR) series of instruments are being used in applications for monitoring trends in many climate variables, for example, clouds and aerosols. In order to provide quantitative information, the radiometric calibrations of the sensors must be consistent, stable, and ideally traced to international standards with uncertainties quantified. In this paper, the authors describe the current methodology used to monitor the long-term drifts and determine the relative biases of the ATSR solar channel radiometric calibrations. Top-of-atmosphere bidirectional reflectance factors (BRFs) over quasi-stable desert and ice sites are extracted from level-1 images and compared against a reference BRF model derived from averages of measurements over the site from a reference sensor. This enables comparisons to be performed where there is limited or no temporal overlap between sensors. The results of the drift monitoring and intercomparisons are used to provide lookup tables to be applied by users for existing products and in subsequent reprocessing of ATSR data. The method is extended to perform comparisons against the Medium Resolution Imaging Spectrometer and the Moderate Resolution Imaging Spectrometer. Results of the comparisons are presented and show that the sensors are stable throughout the mission lifetime and biases relative to the Advanced ATSR are presented. Improvements to the methodology are discussed to account for spectral mismatches of the sensors under comparison and to increase the range of view angles that the BRF model presently covers.

Long-Term Aerosol Climate Data Record Derived from Operational AVHRR Satellite Observations

Aerosol optical thickness (AOT) was retrieved using the Advanced Very High Resolution Radiometer (AVHRR) PATMOS-x Level-2b gridded radiances and the two-channel algorithm of the National Climatic Data Center (NCDC). The primary retrieval product is AOT at 0.63 μm channel. AOT is also retrieved at 0.83 μm or 1.61 μm channel for consistent check. The retrieval was made during day time, under clear sky and snow-free conditions, and over the global oceans. The spatial resolution is 0.1×0.1 degree grid and the temporal resolution is both daily and monthly. The resultant AVHRR AOT climate data record (CDR) spans from August 1981 to December 2009 and provides the longest aerosol CDR currently available from operational satellites. This dataset is useful in studying aerosol climate forcing, monitoring long-term aerosol trends, and evaluating global air pollution and aerosol transport models over the global ocean.

Surface response to stratospheric aerosol changes in a coupled atmosphere–ocean model

AbstractPrevious work with a simple climate model has suggested a global cooling impact of increasing stratospheric aerosol. Here we use a comprehensive Earth System Model including coupled atmosphere and ocean components to show that increasing stratospheric aerosol since the late 1990s has reduced global warming by at least 0.07 C to present and that a further global cooling impact will occur if the observed stratospheric aerosol trend continues to the end of this decade. This result confirms the previous work and suggests that climate models that do not account for stratospheric aerosol increase will overestimate global warming to a small but notable degree. An additional new finding is that increasing stratospheric aerosol since the late 1990s has reduced the rise in global mean precipitation.Finally, regional patterns of change in simulations with stratospheric aerosol increase to year 2020 show ~40% less equatorial precipitation increase and ~60% greater surface pressure decrease around Antarctica, relative to simulations without such stratospheric aerosol changes.

The effects of recent control policies on trends in emissions of anthropogenic atmospheric pollutants and CO<sub>2</sub> in China

Abstract. To examine the effects of China's national policies of energy conservation and emission control during 2005–2010, inter-annual emission trends of gaseous pollutants, primary aerosols, and CO2 are estimated with a bottom-up framework. The control measures led to improved energy efficiency and/or increased penetration of emission control devices at power plants and other important industrial sources, yielding reduced emission factors for all evaluated species except NOx. The national emissions of anthropogenic SO2, CO, and total primary PM (particulate matter) in 2010 are estimated to have been 89%, 108%, and 87% of those in 2005, respectively, suggesting successful emission control of those species despite fast growth of the economy and energy consumption during the period. The emissions of NOx and CO2, however, are estimated to have increased by 47% and 43%, respectively, indicating that they remain largely determined by the growth of energy use, industrial production, and vehicle populations. Based on application of a Monte-Carlo framework, estimated uncertainties of SO2 and PM emissions increased from 2005 to 2010, resulting mainly from poorly understood average SO2 removal efficiency in flue gas desulfurization (FGD) systems in the power sector, and unclear changes in the penetration levels of dust collectors at industrial sources, respectively. While emission trends determined by bottom-up methods can be generally verified by observations from both ground stations and satellites, clear discrepancies exist for given regions and seasons, indicating a need for more accurate spatial and time distributions of emissions. Limitations of current emission control polices are analyzed based on the estimated emission trends. Compared with control of total PM, there are fewer gains in control of fine particles and carbonaceous aerosols, the PM components most responsible for damages to public health and effects on radiative forcing. A much faster decrease of alkaline base cations in primary PM than that of SO2 may have raised the acidification risks to ecosystems, indicating further control of acid precursors is required. Moreover, with relatively strict controls in developed urban areas, air pollution challenges have been expanding to less-developed neighboring regions. There is a great need in the future for multi-pollutant control strategies that combine recognition of diverse environmental impacts both in urban and rural areas with emission abatement of multiple species in concert.

Unexpected increasing AOT trends over northwest Bay of Bengal in the early postmonsoon season

The main point of our study is that aerosol trends can be created by changes in meteorology without changes in aerosol source strength. Over the 10 year period 2000–2009, in October, Moderate Resolution Imaging Spectroradiometer (MODIS) showed strong increasing aerosol optical thickness (AOT) trends of approximately 14% yr−1 over northwest Bay of Bengal (BoB) in the absence of AOT trends over the east of the Indian subcontinent. This was unexpected because sources of anthropogenic pollution were located over the Indian subcontinent and aerosol transport from the Indian subcontinent to northwest BoB was carried out by prevailing winds. In October, winds over the east of the Indian subcontinent were stronger than winds over northwest BoB, which resulted in wind convergence and accumulation of aerosol particles over northwest BoB. Moreover, there was an increasing trend in wind convergence over northwest BoB. This led to increasing trends in the accumulation of aerosol particles over northwest BoB and, consequently, to strong AOT trends over this area. In contrast to October, November showed no increasing AOT trends over northwest BoB or the nearby Indian subcontinent. The lack of AOT trends over northwest BoB corresponds to a lack of trends in wind convergence in that region. Finally, December domestic heating by the growing population resulted in positive AOT trends of similar magnitude over land and sea. Our findings illustrate that in order to explain and predict trends in regional aerosol loading, meteorological trends should be taken into consideration together with changes in aerosol source strength.

Decadal climatological trends of aerosol optical parameters over three different environments in South Korea

AbstractDecadal climatology of aerosol optical parameters derived from Moderate Resolution Imaging Spectroradiometer (MODIS) on board Terra satellite has been analysed during past decade (February 2000 to February 2010) over three different environments in South Korea. Seoul, a continental urban station showed a significant decadal decrease of around 22% in aerosol optical depth (AOD) at 550 nm during observational period. The corresponding decrease in AOD found to be 18.7% over Busan, a coastal station. However Jeju, a volcanically sedimented island station in Korea showed negligible changes in AOD (1.4%). Fine mode fraction (FMF) and Angstrom exponent (ANG) parameters also showed a significant decadal reduction, indicating the decrease in fine mode aerosols, which are majorly produced due to anthropogenic activities in land areas. On analysing the decadal trends in seasonal climatology, it is found that aerosol parameters in general were showing a decreasing trend in different seasons except during spring. The decadal trend in meteorological parameters did not show a perfect alignment with aerosol trends. This suggests that the decadal decrease in aerosols could be mainly attributed to reduction in anthropogenic aerosol emissions. The surface temperature over the stations showed a significant increase of 8% to 11%. This study suggests that, in spite of positive effects in pollution perspective, the reduced AOD may in part contribute the increasing trend of surface temperature. Copyright © 2012 Royal Meteorological Society

Climatic effects of 1950–2050 changes in US anthropogenic aerosols – Part 1: Aerosol trends and radiative forcing

Abstract. We calculate decadal aerosol direct and indirect (warm cloud) radiative forcings from US anthropogenic sources over the 1950–2050 period. Past and future aerosol distributions are constructed using GEOS-Chem and historical emission inventories and future projections from the IPCC A1B scenario. Aerosol simulations are evaluated with observed spatial distributions and 1980–2010 trends of aerosol concentrations and wet deposition in the contiguous US. Direct and indirect radiative forcing is calculated using the GISS general circulation model and monthly mean aerosol distributions from GEOS-Chem. The radiative forcing from US anthropogenic aerosols is strongly localized over the eastern US. We find that its magnitude peaked in 1970–1990, with values over the eastern US (east of 100° W) of −2.0 W m−2 for direct forcing including contributions from sulfate (−2.0 W m−2), nitrate (−0.2 W m−2), organic carbon (−0.2 W m−2), and black carbon (+0.4 W m−2). The uncertainties in radiative forcing due to aerosol radiative properties are estimated to be about 50%. The aerosol indirect effect is estimated to be of comparable magnitude to the direct forcing. We find that the magnitude of the forcing declined sharply from 1990 to 2010 (by 0.8 W m−2 direct and 1.0 W m−2 indirect), mainly reflecting decreases in SO2 emissions, and project that it will continue declining post-2010 but at a much slower rate since US SO2 emissions have already declined by almost 60% from their peak. This suggests that much of the warming effect of reducing US anthropogenic aerosol sources has already been realized. The small positive radiative forcing from US BC emissions (+0.3 W m−2 over the eastern US in 2010; 5% of the global forcing from anthropogenic BC emissions worldwide) suggests that a US emission control strategy focused on BC would have only limited climate benefit.

Progress regarding climate change during recent years

Global warming has been a key international issue, particularly with respect to changes in global climate and its effects, since IPCC-AR4. Here, the latest research results and trends in radiation forcing, greenhouse gases (GHGs), aerosols, ocean water and global sea level, temperature and precipitation in the Antarctic and Arctic are reviewed with respect to changes in global climate. The literature review conducted herein was restricted to papers published in Science and Nature from 2008 to 2010. This paper supplies valuable references for studies on global change and other associated studies, as well as for the fifth assessment report of the IPCC.

Trends of pH and aerosols in the precipitation at Srinagar, Mohanbari, Allahabad, Jodhpur, Nagpur and Minicoy during the period 1981-2001

Nature of Precipitation - alkaline or acidic depends upon the concentration of major water soluble inorganic gaseous and soil derived particulates dissolved in it. If the concentration of cations is higher than that of anions the precipitations becomes alkaline and vice versa. pH is the main parameter indicating the nature of precipitation.
 If pH of rainwater <5.65 it is acidic and >5.65, it is alkaline ,both in the pH scale ranging between 0 and 14. In this paper average ionic concentration (mg/lit) and their trends have been analyzed. Srinagar, Mohanbari, Jodhpur, Allahabad, Nagpur and Minicoy have been selected for the study of chemical precipitation during the period 1981-2001. Trends of different aerosols have been analyzed at the intervals 1981-87, 1988-94 and 1995-2001. It has been observed that percentage of anions has increased which results in the increase of acidic character of the precipitation. During the interval 1995-2001, Nagpur and Mohanbari had pH values 5.16 and 5.47 respectively which were in acidic range.

A decadal regional and global trend analysis of the aerosol optical depth using a data-assimilation grade over-water MODIS and Level 2 MISR aerosol products

Abstract. Using the ten-year (2000–2009) Data-Assimilation (DA) quality Terra MODIS and MISR aerosol products, as well as 7 years of Aqua MODIS, we studied both regional and global aerosol trends over oceans. This included both operational and data assimilation grade versions of the products. After correcting for what appears to be aerosol signal drift from the radiometric calibration of both MODIS instruments, we found MODIS and MISR agreed on a statistically negligible global trend of ±0.003/per decade. Our study also suggests that AODs over the Indian Bay of Bengal, east coast of Asia, and Arabian Sea show increasing trends of 0.07, 0.06, and 0.06 per decade for MODIS, respectively. These regional trends are considered as significant with a confidence level above 95%. Similar increasing trends were found from MISR, but with less relative magnitude. These trends reflect respective increases in the optical intensity of aerosol events in each region: anthropogenic aerosols over the east coast of China and Indian Bay of Bengal; and a stronger influence from dust events over the Arabian Sea. Negative AOD trends, low in confidence levels, are found off Central America, the east coast of North America, and the west coast of Africa, which indicate that longer periods of observation are necessary to be conclusive.