Observations of New Particle Formation and Growth in the Sonoran Desert

KRISTEN CRAMER, Alex Guenther, James Smith, Univerisity of California, Irvine

     Abstract Number: 366
     Working Group: Remote and Regional Atmospheric Aerosol

Abstract
Desert environments comprise one-third of the land area on Earth, but little is known about the source of aerosol particles in the desert. To address this need for a more comprehensive view of atmospheric new particle formation, we measured ultrafine particle size distributions in the Sonoran desert near the town of Borrego Springs, CA. Size distribution measurements were performed during a 16-month period (1/2018 – 4/2019), and were supplemented by an analysis of volatile organic compounds (VOC) on September 8, 2018. Seasonally averaged growth rates ranged from 2.7 nm/hr in the winter to 5.5 nm/hr in the summer, with a maximum observed growth rate of 9.9 nm/hr. Particle formation event frequency also peaked in the summer months, with the appearance of sub-20 nm diameter particles occurring on 29% of days and regional small particle growth events occurring on 49% of days. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to calculate back trajectories for the summer, and showed influxes in air from the San Diego and Los Angeles air basins, which correlates with increased ozone at the site. VOC measurements featured no isoprene, while monoterpenes and sesquiterpenes dominated at concentrations similar to those measured in some forested regions. The lack of isoprene, which suppresses the formation of C20 dimers of monoterpene oxidation products, suggests that unique particle formation mechanisms dominated by monoterpene chemistry may occur in the Sonoran desert. These findings further our understanding of anthropogenic pollutants impacting remote environments and demonstrate the need for further study of atmospheric composition in the southern California desert bioregion.