Ultrafine aerosol formation and growth in a southern California desert

Ultrafine aerosol number-size distributions were collected in the northwestern Sonoran Desert from January 2018 to April 2019 to assess the frequency and intensity of particle formation and growth events in this region. Daily and seasonal patterns of particle formation and growth events were compared with meteorology, ozone mixing ratios, air mass back-trajectories, and modeled boundary layer height to determine potential environmental and anthropogenic influences. These were supplemented with measurements of gas-phase biogenic volatile organic compounds (BVOCs) during a period of active particle formation to gather insights into the potential role of condensation of highly oxidized organic molecules. Particle formation peaked in summer, trending with dominant westerly winds crossing the southern California megalopolis with correspondingly high ozone, higher peak solar radiation, and greater variability in planetary boundary layer height. The timing of the start of growth events corresponded to abrupt increases in both modeled planetary boundary layer height and measured solar radiation. Formation and/or growth events occurred on 22% of days with an average observed diameter growth rate of 5.2 ± 2.2 nm/hr. Gas-phase measurements of BVOCs suggest a low-isoprene, high-monoterpene environment that, when combined with high ozone, creates ideal conditions for aerosol formation. These observations indicate that the formation and growth of ultrafine particles is common in this California desert atmosphere and is likely influenced by the products of anthropogenic pollutants and biogenic emissions.

Methods
Date of data collection: 2018-01 to 2019-04

Geographic location of data collection:  Steele/Burnand Anza-Borrego Desert Reseach Center (33°14'25"N 116°23'19"W), Borrego Springs, CA USA.

Other details regarding these measurements can be found in the associated manuscript: Cramer K. L., Guenther, A. G., and Smith, J. N.: Ultrafine aerosol formation and growth in a southern California desert, Aerosol Science and Technology.