Impacts of Precipitation on Aerosol Chemical Composition in a Highly Urbanized Area during the TRACER-MAP Field Campaign

CHUN-YING CHAO, Fangzhou Guo, Shan Zhou, James Flynn, Rebecca J. Sheesley, Sascha Usenko, Don Collins, Robert Griffin, Rice University

     Abstract Number: 132
     Working Group: Urban Aerosols

Abstract
Wet deposition of pollutants is one process by which precipitation impacts air quality. For particulate matter, previous studies focused on changes in size distribution rather than chemical composition because of precipitation. To better understand how precipitation impacts aerosol composition, this study (1) investigates the in-situ chemical characteristics of aerosol before, during, and after precipitation and (2) compares the removal efficiencies of aerosol species for different rain events across a field campaign.

We measured aerosol composition using an Aerodyne high-resolution time-of-flight aerosol mass spectrometer during the Tracking Aerosol Convection Interactions - Mapping of Aerosol Processes (TRACER-MAP) field campaign in Houston, Texas, from July to September 2022. We made collocated precipitation intensity measurements using an AcuRite Multi-Weather Sensor. During July and August, we measured the aerosol composition and precipitation for periods of five to nine days at five different locations across Houston that represent a range of suburban/urban/industrialized locations. In September, we conducted measurements only at a single industrialized site.

Across the campaign, we observed 17 precipitation events that had a duration of 30 minutes or longer and had rain intensity ranging from 2.6 to 33.9 mm/hr. Changes in concentrations for individual species were both positive and negative in these events. However, relative and absolute changes in sulfate and organic aerosol were larger compared to those in ammonium, nitrate, and chloride. For sulfate, the largest mass concentration difference between before and during precipitation was from 2.90 to 0.81 µg/m³ (a 58% decrease). For organic matter, the largest difference was from 6.67 to 1.99 µg/m³ (a 70% decrease). These values indicate that sulfate and organic aerosol may have different wet removal mechanisms. The results will be used to estimate scavenging coefficients of individual aerosol species, which can improve modeling aerosol concentrations under different intensities of precipitation.