Study of size distribution of atmospheric aerosol at Agra

Measurements on size distribution of atmospheric aerosol were made at Dayalbagh, Agra during July to September 1998. A 4-stage cascade particle sampler (CPS - 105) which fractionates particles in sizes ranging between 0.7 and >10.9 μm, was used. Samples were collected on Whatman 41 filters. The filters were analyzed for the major water-soluble ions. The anions (F, Cl, NO3 and SO4) were analyzed by Dionex DX-500 ion chromatograph while atomic absorption and colorimetric techniques were used for the analysis of cations (Na, K, Ca and Mg) and NH4, respectively. The average mass of aerosol was found to be 131.6 μg m−3 and aerosol composition was found to be influenced by terrigeneous sources. The mass size distribution of total aerosol and the ions NH4, Cl, NO3, K, Ca, Mg, SO4 and Na was bimodal while that of F was unimodal. SO4, F, K and NH4 dominated in the fine mode while Ca, Mg, Cl and NO3 were in abundance in coarse fraction. Na was found in both coarse as well as fine mode. Coarse mode SO4 and NO3 have been ascribed to contribution from re-suspension of soil and formation by heterogeneous oxidation on soil derived particles. Preponderance of K in fine mode is attributed to emissions from vegetation and from burning of plant materials. Ca, Mg, Cl and NO3 are largely soil derived and hence dominate in coarse fraction. Equivalent ratios of NH4/(SO4+NO3) were calculated for both fine and coarse aerosols. The coarse mode ratio varied between 0.7 and 1.4 while in fine mode it ranged between 1.4 and 1.9. It shows that aerosol is basic, the basicity of coarse mode is due to higher concentration of soil-derived alkaline components while the basicity in fine mode is due to neutralization of acidity by NH3.