Temporal trends in atmospheric PM<SUB>2.5</SUB>, PM<SUB>10</SUB>, elemental carbon, organic carbon, water-soluble organic carbon, and optical properties: impact of biomass burning emissions in the Indo-Gangetic plain

The first simultaneous measurements and analytical data on atmospheric concentrations of PM<SUB>2.5</SUB>, PM<SUB>10</SUB>, inorganic constituents, carbonaceous species, and their optical properties (aerosol optical depth, AOD; absorption coefficient, b<SUB>abs</SUB>; mass absorption efficiency, σ <SUB>abs</SUB>; and single scattering albedo, SSA) from an urban site (Kanpur) in the Indo-Gangetic Plain are reported here. Significantly high aerosol mass concentration (&gt;100 μ g m<SUP>-3</SUP>) and AOD (&gt; 0.3) are seen as a characteristic feature throughout the sampling period, from October 2008 to April 2009. The temporal variability in the mass fractions of carbonaceous species (EC, OC, and WSOC) is pronounced during October-January when emissions from biomass burning are dominant and OC is a major constituent (~30%) of PM<SUB>2.5</SUB> mass. The WSOC/OC ratio varies from 0.21 to 0.65, suggesting significant contribution from secondary organic aerosols (SOAs). The mass fraction of SO<SUB>4</SUB><SUP>2-</SUP> in PM<SUB>2.5</SUB> (Av: 12.5%) exceeds that of NO<SUB>3</SUB><SUP>-</SUP> and NH<SUB>4</SUB><SUP>+</SUP>. Aerosol absorption coefficient (@ 678 nm) decreases from 90 Mm<SUP>-1</SUP> (in December) to 20 Mm<SUP>-1</SUP> (in April), and a linear regression analysis of the data for b<SUB>abs</SUB> and EC (n = 54) provides a measure of the mass absorption efficiency of EC (9.6 m<SUP>2</SUP> g<SUP>-1</SUP>). In contrast, scattering coefficient (@ 678 nm) increases from 98 Mm<SUP>-1</SUP> (in January) to 1056 Mm<SUP>-1</SUP> (in April) and an average mass scattering efficiency of 3.0 ± 0.9 m<SUP>2</SUP> g<SUP>-1</SUP> is obtained for PM<SUB>10</SUB> samples. The highest bscat was associated with the dust storm event (April 17, 2009) over northern Iraq, eastern Syria, and southern Turkey; thus, resulting in high SSA (0.93 ± 0.02) during March-April compared to 0.82 ± 0.04 in October-February. These results have implications to large temporal variability in the atmospheric radiative forcing due to aerosols over northern India