Temporal trends in atmospheric PM_2.5, PM₁₀, elemental carbon, organic carbon, water-soluble organic carbon, and optical properties: impact of biomass burning emissions in the Indo-Gangetic plain

Abstract

The first simultaneous measurements and analytical data on atmospheric concentrations of PM_2.5, PM₁₀, inorganic constituents, carbonaceous species, and their optical properties (aerosol optical depth, AOD; absorption coefficient, b_abs; mass absorption efficiency, σ _abs; and single scattering albedo, SSA) from an urban site (Kanpur) in the Indo-Gangetic Plain are reported here. Significantly high aerosol mass concentration (>100 μ g m^-3) and AOD (> 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_2.5 mass. The WSOC/OC ratio varies from 0.21 to 0.65, suggesting significant contribution from secondary organic aerosols (SOAs). The mass fraction of SO₄^2- in PM_2.5 (Av: 12.5%) exceeds that of NO₃^- and NH₄⁺. Aerosol absorption coefficient (@ 678 nm) decreases from 90 Mm^-1 (in December) to 20 Mm^-1 (in April), and a linear regression analysis of the data for b_abs and EC (n = 54) provides a measure of the mass absorption efficiency of EC (9.6 m² g^-1). In contrast, scattering coefficient (@ 678 nm) increases from 98 Mm^-1 (in January) to 1056 Mm^-1 (in April) and an average mass scattering efficiency of 3.0 ± 0.9 m² g^-1 is obtained for PM₁₀ 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.