Seasonal inhomogeneity in cloud precursors over Gangetic Himalayan region during GVAX campaign

Abstract

Atmospheric aerosols are key elements in cloud microphysics, the hydrological cycle and climate by serving as cloud condensation nuclei (CCN). The present work analyzes simultaneous measure- ments of number concentration of CCN (Nccn) and condensation nuclei (Ncn) obtained at Nainital, in the Gangetic-Himalayan (GH) region, during the frameworks of Ganges Valley Aerosol Experiment (GVAX), June 2011 to March 2012. The Nccn, Ncn and activation (AR = Nccn/Ncn) at 0.31–0.33% S (supersaturation ratio), exhibit significant daily, monthly and seasonal variations within a range of 684–2065 cm⁻³ for NCCN, 1606–4124 cm⁻³ for Ncn, and 0.38–0.60 for AR, suggesting large inhomogeneity in aerosol properties, types and sources, which control the degree of aerosol potential activation. Thus, transported aerosols from the Ganges valley and abroad, the boundary-layer dynamics and atmospheric modification processes play an important role in aerosol–cloud interactions over the GH region. The Ncn and Nccn show monthly- dependent diurnal variations with afternoon maxima due to transported aerosols from the Ganges valley up to the Himalayan foothills, while the AR is lower during these hours implying lower hygroscopicities or smaller sizes of the transported aerosols. The dependence of Nccn on S is highest during Dec–Mar and lowest during monsoon (Jun–Sep), suggesting different aerosol chemical composition. Comparison between Nainital and Kanpur shows that Ncn and Nccn are much lower at Nainital, while the similarity in AR suggests aerosols of similar type, source and chemical composition uplifted from the Ganges valley to the Himalayan foothills.