Abstract:

The hydroxyl radical (OH) is a key oxidant in atmospheric chemistry, yet its formation in cloud droplets remains poorly understood. Recent work has identified a new source, the OH burst, characterized by rapid OH production when aerosol particles interact with cloud water under near-UV light, highlighting the importance of particle-driven aqueous-phase processes during cloud activation. This work presents on the development of an automated Direct-to-Reagent (autoDtR) system that enables remote, continuous measurements of the OH burst, incorporating the newly developed Handix Scientific Temperature Level Controller (TLC). Field deployment of the autoDtR instrument during the Department of Energy (DOE) Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) reveals variability in OH burst magnitude during both in-cloud and out-of-cloud conditions. NOAA HYSPLIT air-mass back trajectories interpolated with ERA5 liquid water content reanalysis data are used to assess the role of prior cloud processing in regulating the burst. Additionally, we examine air-quality impacts from the January 2025 Eaton and Palisades wildfires which burned a combined 15,163 hectares of wildland and 18,298 structures, respectively. We use a combined dataset of filters, hourly measurements of particulate matter, trace gases, metals, meteorology, and air-mass transport to probe fire impacts on air quality. The smoke increased airborne concentrations of arsenic, chlorine, bromine, copper and in particular lead concentrations by up to 250 times its background. Potassium, known to be a tracer for wildland smoke, was strongly associated with coarse particles and anti-correlated with smoke. We found little evidence of fire-associated pollution days following fire containment.