Characteristics of size-segregated particle number concentrations in an urban location in India

Atmospheric aerosols exhibit considerable variability in size, spanning a broad range with a difference of four orders of magnitude between the smallest and largest particles. The spatiotemporal heterogeneity in natural and anthropogenic emission sources results in significant variability in aerosol properties, making it extremely hard to precisely quantify aerosol’s climatic effect. New particle formation (NPF, via gas-to-particle conversion) is the largest source of aerosol numbers to the terrestrial atmosphere, and therefore ultrafine particles (particles less than 100 nm diameter). The growth of newly formed particles or primary particles can significantly alter the total aerosol mass and composition which has implications for Earth’s radiation budget and hydrological cycle via aerosol’s direct and indirect effects, respectively. In the Indian context, NPF has been poorly studied in the absence of state-of-the-art instrumentation to characterize atmospheric NPF events. The main objective of this study is to quantitatively assess the role of NPF in the size-segregated particle number concentration in an urban location, Hyderabad. Here, we have used long-term (2019-2022) measurements of particle number size distributions from the nano Condensation Nucleus Counter (nCNC) in the size range of 1 to 3 nm and the Scanning Mobility Particle Sizer (SMPS)  in the size range from 10 nm to 514 nm. Measurements were conducted at the University of Hyderabad campus site. The observation days were broadly categorized into three event types, namely NPF, non-event and undefined based on the visual inspection of the contour plot of particle number size distributions. We additionally utilized in-situ measurements of particulate matter with a diameter smaller than 2.5 µm (PM_2.5) to identify polluted days following the NAAQS criteria (days with PM_2.5 > 60 µg m^-3). The size-segregated particle number concentrations in the cluster mode (sub-3nm), nucleation mode (10-25 nm), Aitken mode (25-100 nm), and accumulation mode (100-514 nm) were calculated for each identified event type. The size-segregated particle number concentrations showed a distinct seasonal pattern, with the highest particle number concentrations during the spring (March - May) and the lowest during the winter months (December - February). The highest particle number concentrations in spring coincide with the highest frequency of NPF event occurrence.  Amongst them, the cluster-mode particles constituted the largest fraction of particle number concentrations and the accumulation-mode particles constituted the lowest. The cluster mode particle number concentrations were found to be the highest during NPF event days than other event types. The positive association between cluster mode particles and PM_2.5 indicates that NPF is not inhibited at high pre-existing particle concentrations unlike in the USA and EUROPE. This suggests that the balance between the precursor vapour concentrations and the pre-existing particle concentrations decides when NPF will trigger in the polluted boundary layer under a given atmospheric condition. However, the negative association between nucleation mode particles and PM_2.5 suggests that not all cluster mode particles grow to nucleation mode size.