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

Particle number size distribution has critical importance in characterising the number, size, surface area, volume, and evolution of aerosols in the atmosphere. Atmospheric new particle formation (NPF) is one the largest source of aerosol numbers to the terrestrial atmosphere and it greatly impacts the evolution of particle number size distributions. In this study, we analysed at least one year of asynchronous measurements of particle number size distributions from six different locations in India. We found that NPF frequently occurred at all locations in the pre-monsoon season (March through May) and is the least common in the post-monsoon season (October-November). Considering all the sites (mountain background, mountain semi-rural, coastal semi-urban and urban), the particle formation rate of lowest detectable size (JLDS) varied by more than an order of magnitude (0.01 - 0.6 cm-3 s-1) and the growth rate between the lowest detectable size and 25 nm (GRLDS-25nm) by about three orders of magnitude (0.2 - 17.2 nm h-1). The site-specific JLDS and GRLDS-25nm are positively correlated, indicating their co-dependence on gas-phase production rates of low-volatility vapours, driven by the source and atmospheric conditions. Our results also show that NPF events significantly modulate the shape of particle number size distributions, particularly in the pre-monsoon season. We also provide the pest possible estimate of the increase in CCN concentration during an NPF event. The NPF-associated CCN concentrations were higher in urban locations than the mountain background sites. Although using asynchronous measurements, our results implicate the process-level characterisation of particle number size distribution and CCN concentrations in different locations in India.