AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
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Particle Size Distribution in New Delhi: Role of Coagulation and Nucleation
SHAHZAD GANI, Sahil Bhandari, Sarah Seraj, Zainab Arub, Gazala Habib, Lea Hildebrandt Ruiz, Joshua Apte, University of Texas at Austin
Abstract Number: 595 Working Group: Urban Aerosols
Abstract As part of the 2017 Delhi Aerosol Supersite campaign, we explore the sources and dynamic mechanisms that influence the size distributions of urban background aerosols in a polluted megacity. This investigation is motivated by the observation that fine particulate matter (PM2.5) mass concentration in Delhi, India are ten times or more higher than in many western cities, but the particle number (PN) concentrations are not similarly elevated.
We deployed a scanning mobility particle sizer in a high-income area of Delhi to collect continuous long-term measurements of particle size distributions (PSDs). In this study, we investigate diurnal and seasonal variation of PSDs. For the observed size range of 12 to 560 nm, estimated mass concentrations dropped from 120 µg m-3 during winter months to 55 µg m-3 in spring (-55%). The corresponding drop for PN concentrations was from 37,000 to 29,000 cm-3 (-20%) and for ultrafine particles (UFP, dp < 100 nm) from 21,000 to 19,000 cm-3 (-10%).
Calculations based on measured PSDs and coagulation theory suggest wintertime UFP concentrations suppression by a rapid coagulation sink. Our modeling suggests coagulation timescales of ~1 minute for 30 nm particles in winter and ~2 minutes in spring. High wintertime particle surface area, mostly from particles in the accumulation mode, provides a plausible sink for UFP. A smaller accumulation mode for warmer months result in increased UFP proportion, likely owing to a comparatively smaller coagulation sink. We also see evidence suggestive of nucleation which may also contribute to the increased UFP proportion in spring. The 24h average median-diameter of the PSD drops from ~90 to ~60 nm from winter to spring, while the daytime PSD shifts from being unimodal (peaks at ~100 nm) to bimodal (peaks at ~30, ~100 nm).