Observation of sub-3nm particles and new particle formation at an urban location in India

Abstract

The formation of aerosol particles with diameters smaller than 3 nm (sub-3nm) is the dominant source of particle number in the atmosphere. But, observations of sub-3nm particles are scarce globally, particularly in India. Here, we present the direct observational evidence of sub-3nm particles in India. Simultaneous observations of particle size distributions in the size range from 1.2 to 3 nm and from 10 to 514 nm were made at an urban location, Hyderabad, in India during April–June 2019. New particle formation (NPF, the formation of sub-3nm particles) occurred frequently (63.9% of the total observation days). However, only 26.7% of the days displayed sub-3nm particle formation with subsequent growth of these particles to larger sizes (>10 nm), which is indicative of the regional NPF and growth (NPF&G) events, and these events had a formation rate of particles of diameter 1.4 nm (J1.4) of 30.7 ± 73.6 cm−3s−1. The rest of the NPF event days had a higher J1.4 of 58.3 ± 99.2 cm−3s−1, but the particles did not grow larger than 10 nm. Both NPF&G and NPF days had similar sulfuric acid concentrations, sub-3nm growth rates, condensation sink, and sub-3nm coagulation sink values. The wind speed and solar radiation were also similar on NPF&G and NPF days, but temperature, relative humidity, and wind direction had some differences between the types of events as explained by airmass back trajectory calculations. While organic vapors were not measured in this study, we hypothesize that the differences in airmass direction and history might have dissimilar organic vapors for condensational growth between the types of events, which determine when NPF events include subsequent growth to larger sizes.