Atmospheric aerosol formation and its growth during the cold season in India

The effects of molecular diffusivity of H₂SO₄ and NH₃ vapours on nucleated particles of SO₄²⁻ and NO₃⁻ species are reported. Condensation sink and source rate of H₂SO₄ and NH₃ vapours, growth rates and ratios of real to apparent nucleation rates are calculated for SO₄^– and NO₃⁻ aerosols using fractional contributions of them in total aerosol size-distribution during the measurement period at Pune, reported in Chate and Pranesha (2004). The percentage of nucleated SO₄²⁻ and NO₃⁻ aerosols of mid-point diameter 13 nm are 2% and 3% respectively of the total particles (13 nm ≤ D _p ≤ 750 nm) for both H₂SO₄ and NH₃ diffusion. In the diameter range 75 nm ≤ D _p ≤ 133 nm, it is 48% and 45% of SO₄²⁻ and NO₃⁻ aerosols, respectively for NH₃ diffusion and 43% and 36% of SO₄²⁻ and NO₃⁻ for H₂SO₄ diffusion. Increase in percentage of nucleated particles of these species corresponding to mid-point diameter 133 nm around 0900 h IST is significantly higher than that of mid-point diameter 13 nm and it is due to photo-chemical nucleation, coagulation and coalescence among nucleated clusters. The ratios of real to apparent formation rates for SO₄²⁻ and NO₃⁻ aerosols are 12% and 11% respectively, corresponding to mid-point diameter 13 nm, 17% and 13%, for midpoint diameter 133 nm and 12% and 9.5%, for mid-point diameter 750 nm. The results indicate that nucleation involving H₂SO₄ and acidic NH₃ diffusion on SO₄²⁻ and NO₃⁻ particles is the most relevant mechanism in this region.