Observations of particle number size distributions and new particle formation in six Indian locations

Abstract

Atmospheric new particle formation (NPF) is a crucial process driving aerosol number concentrations in the atmosphere; it can significantly impact the evolution of atmospheric aerosol and cloud processes. This study analyses at least 1 year of asynchronous particle number size distributions from six different locations in India. We also analyze the frequency of NPF and its contribution to cloud condensation nuclei (CCN) concentrations. We found that the NPF frequency has a considerable seasonal variability. At the measurement sites analyzed in this study, NPF frequently occurs in March-May (pre-monsoon, about 21% of the days) and is the least common in October-November (post-monsoon, about 7% of the days). Considering the NPF events in all locations, the particle formation rate (JSDS) varied by more than 2 orders of magnitude (0.001-0.6cm-3s-1) and the growth rate between the smallest detectable size and 25nm (GRSDS-25nm) by about 3 orders of magnitude (0.2-17.2nmh-1). We found that JSDS was higher by nearly 1 order of magnitude during NPF events in urban areas than mountain sites. GRSDS did not show a systematic difference. Our results showed that NPF events could significantly modulate the shape of particle number size distributions and CCN concentrations in India. The contribution of a given NPF event to CCN concentrations was the highest in urban locations (4.3×103cm-3 per event and 1.2×103 cm-3 per event for 50 and 100nm, respectively) as compared to mountain background sites (2.7×103cm-3 per event and 1.0×103cm-3 per event, respectively). We emphasize that the physical and chemical pathways responsible for NPF and factors that control its contribution to CCN production require in situ field observations using recent advances in aerosol and its precursor gaseous measurement techniques.