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 ultrafine particle formation and growth events in this region. Daily and seasonal patterns of events were compared to 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 to growth. Ultrafine particle formation peaked in summer, trending with temperatures exceeding 30°C and dominant westerly winds crossing the southern California megalopolis with correspondingly high ozone. The initiation of particle 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−1. Gas-phase measurements of BVOCs suggest a low-isoprene, high-monoterpene environment that, when combined with high ozone, creates ideal conditions for ultrafine particle formation and growth. These observations suggest 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. This study provides a much-needed dataset for an understudied region, while also showing that ultrafine particle formation and growth can occur at high ambient temperatures.