Evolution of aerosol properties impacting visibility and direct climate forcing in an ammonia-rich urban environment

Airborne measurements of sub‐micron aerosol and trace gases downwind of Los Angeles are used to investigate the influence of aging on aerosol properties relevant to climate forcing and visibility. The analysis focuses on the Los Angeles plume, which in addition to strong urban emissions is influenced by local agricultural emissions. Secondary organic aerosol formation and repartitioning of semi‐volatile ammonium nitrate were identified as key factors controlling the optical behavior observed. For one case study, ammonium nitrate contributed up to 50% of total dry extinction. At 85% relative humidity, extinction in the fresh plume was enhanced by a factor of ∼1.7, and 60–80% of this was from water associated with ammonium nitrate. On this day, loss of ammonium nitrate resulted in decreasing aerosol hygroscopicity with aging. Failing to account for loss of ammonium nitrate led to overestimation of the radiative cooling exerted by the most aged aerosol by ∼35% under dry conditions. These results show that changes to aerosol behavior with aging can impact visibility and climate forcing significantly. The importance of ammonium nitrate and water also highlight the need to improve the current representation of semi‐volatile aerosol species in large‐scale climate models. Key Points The visibility and climate impacts of aerosol change with aging Ammonium nitrate formation/partitioning is important in the Los Angeles region