Nucleation Closure Study from 2023 Pittsburgh Field Campaign

DARREN CHENG, Dominic Casalnuovo, Christine Troller, Coty Jen, Carnegie Mellon University

     Abstract Number: 142
     Working Group: Aerosol Chemistry

Abstract
Nucleation is the process where gas molecules in the atmosphere react and nucleate to form new particles. Since nucleation produces roughly half of global cloud condensation nuclei, understanding nucleation is important to accurately model Earth’s climate. Traditionally, characterizing the nucleation rate first requires identifying a nucleation event, a period where particles at the smallest detectable size appear and grow to larger sizes. Subsequently, the nucleation rate is then calculated over the course of the nucleation event. However, nucleation rates can vary widely during the nucleation event due to differences in atmospheric conditions and the concentration and composition of precursor gases. Nucleation can also occur outside of a well-defined nucleation event. Here, we present a closure study comparing nucleation rates calculated using particle size distribution and gas phase nucleation precursor data. During a field campaign in the Fall of 2023 in Pittsburgh, PA, particle size distributions were measured by the Multi-ANalYzer Condensation Particle Counter (MANY-CPC) and a pair of stepping particle mobility sizers (SMPS). The MANY-CPC provides 1 Hz measurement of the sub-3 nm size distribution and the SMPS from 2 – 300 nm every five minutes. Ambient gas phase composition was measured using a nitrate chemical ionization mass spectrometer (CIMS). The particle-based nucleation rate was calculated using the concentrations of sub-3 nm particles measured by MANY-CPC and the concentration of the condensation sink measured by the SMPS. The precursor-based nucleation rate was calculated using the measured concentration of sulfuric acid and unsteady state nucleation potential model. Our results suggest that near-instantaneous atmospheric nucleation rates can approach the sulfuric acid collision-rate limit during NPF events but only when accounting for the non-ideal detection efficiency of the MANY-CPC. Through these results, we can also differentiate between sulfuric acid-driven nucleation events and nucleation driven by other atmospheric precursors.