Diurnal and Seasonal Variation of Spectral Properties of Aerosols over Dehradun, India (original) (raw)
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SEASONAL VARIATIONS OF AEROSOL OPTICAL DEPTH OVER INDIAN SUBCONTINENT
Atmospheric aerosols are considered to be one of most complex and important parameter affecting the climate change. According to IPCC Scientific understanding of Atmospheric Aerosol forcing on climate change is very low. Estimating the impact of Atmospheric Aerosols on climate is very complex as they differ in chemical composition and optical properties viz. Scattering and absorbing. Precipitation phenomena is influenced by aerosols hence study of atmospheric aerosols over Indian sub continent is necessary as the Indian economy is highly dependent on Monsoon rainfall. AOD (Aerosol Optical Depth) from MODIS (MODerate Resolution Imaging Spectroradiometer) onboard Terra and Aerosol Index (AI) from OMI (Ozone Monitoring Instrument) onboard Aqua to study the seasonal variation of Aerosols. Study of Aerosols during Monsoon is highlighted to investigate their behaviour during this season. Aerosol loading is found to be more in the study region during Monsoon(June, July, August, September) and less in Post monsoon (October, November). Effect of Topography on Aerosol loading is observed and Aerosol loading is less over elevated regions. The COV (Coefficient of Variation of AOD is high over elevated regions showing high variability. High Aerosol Index over Arabian Sea and western India, and wind direction during monsoon indicates high aerosol loading transported through westerlies. This also supports the statement that the high values of AOD is not due to cloud contamination but due to presence of high amount of Aerosols and most of them are found to be transported from Arabian desert and Africa through Arabian sea.
Journal of Earth System Science, 2012
Investigations of aerosol optical and physical characteristics using the application ofÅngström formula and second order polynomial fit were carried out from April 2006 to March 2009 at Mohal in the Kullu valley. The measurements of spectral aerosol optical depths (AODs) were conducted using multiwavelength radiometer (MWR). The AOD at 0.5 μm wavelength on daily basis (mean ± standard deviation) for the entire three-year study period is obtained as 0.24 ± 0.08. Seasonal variations show the highest AOD at 0.5 μm wavelength with ∼0.34 ± 0.08 during pre-monsoon (April-July), followed by ∼0.26 ± 0.08 during monsoon (August-September), ∼0.21 ± 0.05 during post-monsoon (October-November) and ∼0.20 ± 0.07 during winter (December-March). The seasonal values indicate that the AOD at 0.5 μm wavelength is decreasing from pre-monsoon to winter with a notable reduction around 41%. TheÅngström parameters using least square method is not found appropriate for size distribution particularly when coarse mode aerosols dominate. The coefficients of second order polynomial fit are more appropriate for the discrimination of aerosol size or irrespective to the dominance of either of the aerosols size. The difference in coefficient of polynomial fit is used to get confirmation on the dominant mode during different seasons. Study reveals that about 93%, 72% and 59% of AOD spectra are dominated by a wide range of fine mode fractions or mixture of modes during post-monsoon, winter and monsoon, respectively. On the other hand, during pre-monsoon, 72% of AOD spectra are found to be dominated by coarse mode aerosols.
Seasonal asymmetry in diurnal variation of aerosol optical characteristics over Pune, western India
Journal of Geophysical Research, 2007
There is a large day-today variability in aerosol optical properties due to varied meteorological conditions that exhibit different diurnal asymmetry in different synoptic seasons. Aerosol optical depth (AOD) and Å ngstrom exponent (AE) measured for a period of 3 years at Pune, an urban site in western India, were analyzed to study the diurnal variation and its seasonal dependence. Mean AOD at 500 nm in the premonsoon (March-May) period was about 0.42, while in the winter (December, January, and February) it was $0.38. AOD departures from the daily mean showed ±20% variation (which is equivalent to AOD of 0.08) in both seasons. A systematic seasonal asymmetry in diurnal variation was found. AOD is higher in the morning and lower in the afternoon during winter, mainly because of higher relative humidities, calm winds, and associated ground-based inversions that are conducive for haze, which persists till noon. However, in premonsoon, AODs are lower in the forenoon hours and higher in the afternoon hours because of higher air temperatures, strong convection, and stronger winds raising more dust locally and transporting it from Saudi Arabia and Afghanistan regions. The difference between Moderate Resolution Imaging Spectroradiometer (MODIS) Terra and Aqua derived AOD showed similar seasonal asymmetry in forenoon-afternoon difference in aerosol loading as observed at the experimental site.
Wavelength dependence of the aerosol Angstrom exponent and its implications over Delhi, India
Aerosol Science and Technology, 2011
The aerosol optical depth (AOD), Angstrom coefficients (α and β), and the second-order Angstrom exponent (α′) obtained by Microtops-II sun photometer have been analyzed in the spectral range 0.34–0.87 μm over the urban polluted city of Delhi, India for the period 2007–2008, aiming at investigating the physical and optical properties of aerosols. The average values of AOD at 500 nm, α and β (in the range 340–870 nm) are found to be 0.78 ± 0.32, 0.78 ± 0.28, and 0.45 ± 0.21, respectively, for the entire period of observations. The AOD data show significant curvature in the lnτ versus lnλ relationship suggesting different dominant aerosol types depending on season. In order to analyze further the curvature effect and the relative dominance of aerosol size, α has been calculated in three wavelength bands, i.e., shorter (0.34–0.50 μm), longer (0.675–0.87 μm), and broad (0.34–0.87 μm) during four seasons, summer (April–June), monsoon (July–September), winter (October–January), and spring (February–March) accompanied with calculations of α′, which quantifies the deviation of logarithmic behavior of AOD with lnλ. The α′ values are found to be positive and higher in the months of October–December and mostly negative in February and March, while close to zero values of α′ are found in April–August. These results indicate that winter season exhibits dominance of fine-mode aerosols while summer relatively higher concentration of coarse-mode particles. On the other hand, monsoon and spring seasons revealed the presence of mixed type, both fine- and coarse-mode aerosols over Delhi.
Study on Aerosol Optical Depth in Winter and Summer Season in Bhaktapur
Journal of the Institute of Engineering, 2011
Aerosol optical depth in winter and summer season, 2009 was studied in Bhaktapur (Kausaltar). Spectral aerosol optical depth (AOD) measurements were estimated from a Microtops II Sun Photometer. The results indicated high values in summer and low values in winter. It also showed that AOD is larger in shorter wavelength and gradually decreases towards longer wavelength. The increasing value of AOD in summer season may be due to the accumulation of high speed dust particles. The lower values in winter months are due to the reduction of bigger particles. The spectral dependence of AOD on January 27 was found to be 0.64, 0.52, 0.48 and 0.35. The AOD at 340 nm in January and July were 0.63 and 0.62, respectively. The maximum and minimum value of AOD at 340 nm in the month of April and August were recorded as 1.62 and 0.56, respectively.Keywords: Sun photometer; Aerosol; Aerosol optical depth; Angstrom parameters; IrradianceDOI: http://dx.doi.org/10.3126/jie.v8i1-2.5122Journal of the Inst...
Aerosols optical depth and Ångström exponent over different regions in Garhwal Himalaya, India
Environmental Monitoring and Assessment, 2021
Aerosol optical depth (AOD) and Ångström exponent (AE) are observed to be important parameters in understanding the status of ambient aerosol concentration over a particular location and depend not only upon the local but also on the large-scale dynamics of the atmosphere. The present article analyses the AOD and AE parameters retrieved with Moderate Resolution Imaging Spectrometer (MODIS) and Multi-angle Imaging Spectro-Radiometer (MISR) instruments onboard satellites, for the upper (Chamoli) and foothill (Dehradun) regions of Garhwal Himalaya in Uttarakhand, India, from 2006 to 2015. Aerosol properties are investigated at monthly, seasonal, and annual scales. The monthly mean values of MODIS-derived AOD and AE were observed to be 0.18 (± 0.14) and 1.05 (± 0.43) respectively over the Dehradun region. The seasonal maximums in AOD with MODIS and MISR were observed as 0.23 ± 0.06 and 0.29 ± 0.07 respectively in the pre-monsoon season, and the minimum values (0.099 ± 0.02) were observe...
Trends in aerosol optical depth over Indian region: Potential causes and impact indicators
Journal of Geophysical Research: Atmospheres, 2013
The first regional synthesis of long-term (back to~25 years at some stations) primary data (from direct measurement) on aerosol optical depth from the ARFINET (network of aerosol observatories established under the Aerosol Radiative Forcing over India (ARFI) project of Indian Space Research Organization over Indian subcontinent) have revealed a statistically significant increasing trend with a significant seasonal variability. Examining the current values of turbidity coefficients with those reported~50 years ago reveals the phenomenal nature of the increase in aerosol loading. Seasonally, the rate of increase is consistently high during the dry months (December to March) over the entire region whereas the trends are rather inconsistent and weak during the premonsoon (April to May) and summer monsoon period (June to September). The trends in the spectral variation of aerosol optical depth (AOD) reveal the significance of anthropogenic activities on the increasing trend in AOD. Examining these with climate variables such as seasonal and regional rainfall, it is seen that the dry season depicts a decreasing trend in the total number of rainy days over the Indian region. The insignificant trend in AOD observed over the Indo-Gangetic Plain, a regional hot spot of aerosols, during the premonsoon and summer monsoon season is mainly attributed to the competing effects of dust transport and wet removal of aerosols by the monsoon rain. Contributions of different aerosol chemical species to the total dust, simulated using Goddard Chemistry Aerosol Radiation and Transport model over the ARFINET stations, showed an increasing trend for all the anthropogenic components and a decreasing trend for dust, consistent with the inference deduced from trend in Angstrom exponent.
Aerosol and Air Quality Research, 2016
The present study evaluates the temporal variation of aerosol optical depth (AOD 500 nm) and the Ångström parameters [viz., Ångström exponent (AE, α), Ångström turbidity coefficient (β) and second order Ångström exponent (α′)] at a tropical observing site, Pune (18°32′N; 73°49′E, 559 m AMSL) during 2008-15. Six-year means for winter and premonsoon seasons together are found to be 0.534 ± 0.13, 1.054 ± 0.27, 0.254 ± 0.08 and 0.167 ± 1.33 for AOD 500 nm , AE, β and α′ respectively. Average month-to-month variability of AOD 500 nm , AE, β and α′ during 2008-15 depicts seasonal cycle with strong departures with respect to long-term means. Frequency distributions for AOD, AE and β are positively skewed (skewness = 0.77, 0.32 and 1.14 respectively) while it is negatively skewed for α′ (skewness =-0.18). Analysis of AE difference, curvature parameter difference (α 2-α 1) and AOD 500 nm-AE 440-870 nm contour density map reveals that the aerosol ensemble at Pune consists of four aerosol types viz., UI (urban/industrial), CM (clear maritime), DD (desert dust) and MT (mixed type). Their relative magnitudes, however, differ during winter and pre-monsoon seasons. Thus, the contour density map shows dominance of UI and relatively less occurrence of MT type aerosols during winter. In pre-monsoon, however, the aerosol scenario is driven by MT type aerosol although UI and DD type aerosols show their remarkable existence.
Journal of Geophysical Research, 2008
1] Regional and temporal variations in aerosol characteristics in 35 locations spread over seven different regions in India are studied during 2001-2005 from the daily mean MODIS Terra aerosol optical depth (AOD) and fine mode fraction (FMF) data. Northeast India has the lowest annual mean AOD of 0.28 while south comes next with 0.35. In the other regions AODs are higher than 0.35. Annual mean variations in AOD and FMFs in different regions do not show any noticeable increase or decrease during the 5-year period. High altitude locations are found to have lower AODs while densely populated, urban and industrialized locations have high AODs. Many locations show a winter low and summer high in AODs. Locations/regions dominated by pollution are found to have high FMF and high AODs, while regions in which natural (biogenic) aerosols are dominant had high FMF and range of AODs. The abundance of mechanically generated aerosols over a region results in low FMF and range of AODs. These features suggest that in addition to AOD variations knowledge on sources over a region are essential in understanding the FMF variations. Frequency distribution histograms of AODs and FMFs are consistent with the fact that aerosol sources exhibit seasonal and spatial variations over India. Dust activity peaks over north and west India during March-May which results in low FMFs as the aerosol distributions are influenced by large size dust aerosols. In Northeast FMFs are found to be higher than 0.8 throughout the year indicating the dominance of fine mode aerosols.