Comparison of aerosol optical properties from Beijing and Kanpur

Aerosol Robotic Network (AERONET) aerosol optical depth (AOD) and almucantar retrievals (single scattering albedo (SSA) and aerosol size distribution) from 2005–2009 in Beijing and Kanpur are used to analyze differences and similarities in aerosol optical properties over these two regions. The examination of monthly mean AOD (440 nm) shows that maximum and minimum values occurred in summer and winter, respectively, for Beijing, while the range in AOD in Kanpur was lower. Precipitation in both Beijing and Kanpur peaked in summer; however, the columnar water vapor (CWV) exhibited a high correlation with AOD in Beijing (R2 = 0.79) but had a weak relationship with AOD in Kanpur (R2 = 0.13). The Angstrom exponent (α, 440–870 nm) generally increased linearly as the fine mode fraction (FMF) of AOD (500 nm) increased for FMF < 90% in both regions, with a high correlation of R2 > 0.96. However a clear decrease in α for FMF > 90% found in Beijing is not shown distinctly in Kanpur, and is mainly due to the higher aerosol loading in this FMF bin at Beijing (AOD at 440 nm > 2.2) which results in a stronger coagulation of fine mode particles. Bimodal seasonally-averaged size distributions reveals similar aerosol mixtures composed of fine pollution particles and coarse dust particles in both regions. The analysis of spectral SSA as a function of α is emphasized in this paper. The average SSA at 440 nm in both regions shows a similar low dynamic range of ∼0.03 for α < 1.4. The obvious increase in SSA at 440 nm for α > 1.4 in both regions can be attributed to a higher FMF leading to fine mode coagulation. However, the distinctly smaller increase in Kanpur suggests that fine mode aerosols at Beijing are less absorbing than those at Kanpur. The visibly lower SSA at 675 nm at Kanpur compared to that at Beijing for α > 0.4 is due to a larger find-coarse mode separation radius of ∼0.76 μm in Beijing versus a value lower than ∼0.58 μm in Kanpur. Another reason lies in the weaker absorption by fine mode aerosols in Beijing. The distinctly lower near-infrared SSA in Kanpur when α < 1.4, compared to Beijing, may be attributed to lower aerosol concentrations in all α bins, resulting in less aggregation of absorbing black carbon particles on coarse particles in Kanpur. The classification of aerosol properties shows that the AOD accumulation mode in all seasons, even including spring, in Beijing and in post-monsoon and winter seasons in Kanpur can be attributed to fine particle coagulation or hygroscopic growth; during pre-monsoon and monsoon seasons in Kanpur, it is due to coarse mode particle accumulation or cloud contamination.

► Higher aerosol loading at Beijing results in stronger coagulation of fine particles.
► Fine mode aerosols at Beijing are less absorbing than those at Kanpur.
► Less BC aggregated on dust leads to weaker absorption at NIR bands in Kanpur.
► AOD growth in Kanpur arises from coarse mode particle accumulation in two seasons.