Aerosol Chemistry over a High Altitude Station at Northeastern Himalayas, India

BACKGROUND: There is an urgent need for an improved understanding of the sources, distributions and properties of atmospheric aerosol in order to control the atmospheric pollution over northeastern Himalayas where rising anthropogenic interferences from rapid urbanization and development is becoming an increasing concern. METHODOLOGY/PRINCIPAL FINDINGS: An extensive aerosol sampling program was conducted in Darjeeling (altitude approximately 2200 meter above sea level (masl), latitude 27 degrees 01'N and longitude 88 degrees 15'E), a high altitude station in northeastern Himalayas, during January-December 2005. Samples were collected using a respirable dust sampler and a fine dust sampler simultaneously. Ion chromatograph was used to analyze the water soluble ionic species of aerosol. The average concentrations of fine and coarse mode aerosol were found to be 29.5+/-20.8 microg m(-3) and 19.6+/-11.1 microg m(-3) respectively. Fine mode aerosol dominated during dry seasons and coarse mode aerosol dominated during monsoon. Nitrate existed as NH(4)NO(3) in fine mode aerosol during winter and as NaNO(3) in coarse mode aerosol during monsoon. Gas phase photochemical oxidation of SO(2) during premonsoon and aqueous phase oxidation during winter and postmonsoon were the major pathways for the formation of SO(4)(2-) in the atmosphere. Long range transport of dust aerosol from arid regions of western India was observed during premonsoon. The acidity of fine mode aerosol was higher in dry seasons compared to monsoon whereas the coarse mode acidity was higher in monsoon compared to dry seasons. Biomass burning, vehicular emissions and dust particles were the major types of aerosol from local and continental regions whereas sea salt particles were the major types of aerosol from marine source regions. CONCLUSIONS/SIGNIFICANCE: The year-long data presented in this paper provide substantial improvements to the heretofore poor knowledge regarding aerosol chemistry over northeastern Himalayas, and should be useful to policy makers in making control strategies

Effect of dust and anthropogenic aerosols on columnar aerosol optical properties over Darjeeling (2200 m asl), eastern Himalayas, India.

The loading of atmospheric particulate matter (aerosol) in the eastern Himalaya is mainly regulated by the locally generated anthropogenic aerosols from the biomass burning and by the aerosols transported from the distance sources. These different types of aerosol loading not only affect the aerosol chemistry but also produce consequent signature on the radiative properties of aerosol.An extensive study has been made to study the seasonal variations in aerosol components of fine and coarse mode aerosols and black carbon along with the simultaneous measurements of aerosol optical depth on clear sky days over Darjeeling, a high altitude station (2200 masl) at eastern Himalayas during the year 2008. We observed a heavy loading of fine mode dust component (Ca(2+)) during pre-monsoon (Apr-May) which was higher by 162% than its annual mean whereas during winter (Dec-Feb), the loading of anthropogenic aerosol components mainly from biomass burning (fine mode SO(4)(2-) and black carbon) were higher (76% for black carbon and 96% for fine mode SO(4)(2-)) from their annual means. These high increases in dust aerosols during pre-monsoon and anthropogenic aerosols during winter enhanced the aerosol optical depth by 25 and 40%, respectively. We observed that for every 1% increase in anthropogenic aerosols, AOD increased by 0.55% during winter whereas for every 1% increase in dust aerosols, AOD increased by 0.46% during pre-monsoon.The natural dust transport process (during pre-monsoon) plays as important a role in the radiation effects as the anthropogenic biomass burning (during winter) and their differential effects (rate of increase of the AOD with that of the aerosol concentration) are also very similar. This should be taken into account in proper modeling of the atmospheric environment over eastern Himalayas

Contour plots of Aerosol Index (AI) showing dust transport over Himalaya during a) April 2008 and b) May 2008.

<p>Contour plots of Aerosol Index (AI) showing dust transport over Himalaya during a) April 2008 and b) May 2008.</p

Monthly mean columnar AODs at five different wavelengths (nm).

<p>Monthly mean columnar AODs at five different wavelengths (nm).</p

Seasonal variations in concentrations of several aerosol components.

<p>Seasonal variations in concentrations of several aerosol components.</p

Seasonal variations in fine and coarse mode aerosol concentrations.

<p>Seasonal variations in fine and coarse mode aerosol concentrations.</p

Correlations between increase in AOD (500 nm) and a) increase in dust loading during pre-monsoon and b) increase in anthropogenic aerosol loading during winter.

<p>Correlations between increase in AOD (500 nm) and a) increase in dust loading during pre-monsoon and b) increase in anthropogenic aerosol loading during winter.</p

Seasonal variations in a) AOD at 500 nm and Angstrom exponent and b) derivative of Angstrom exponent.

<p>Seasonal variations in a) AOD at 500 nm and Angstrom exponent and b) derivative of Angstrom exponent.</p

Comparisons of aerosol components between Darjeeling (2200 m) and other Himalayan sites.

<p>(Jiri, Nepal at 1900 m; Nagarkot, Nepal at 2150 m; Langtang, Nepal at 3920 m; Phortse, Nepal at 4450 m; NCO-P, Nepal at 5079 m). Data have been taken from Decesari et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0040286#pone.0040286-Decesari1" target="_blank">[20]</a>. It is seen that dust component (Ca²⁺) over Darjeeling does not significantly differ from other locations during premonsoon. This vertical homogeneity of dust aerosol could be attributed to a common transport mechanism i.e. long-range transport from distant sources. Black Carbon and Sulphate aerosols over Darjeeling show significant differences from other locations. This vertical inhomogeneity could be attributed to the local anthropogenic sources.</p

Percentage changes in aerosol component concentrations and AOD (at 500 nm) from respective annual means.

<p>Percentage changes in aerosol component concentrations and AOD (at 500 nm) from respective annual means.</p