Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles<br/>Aerosol particles (tiny liquid or solid particles in the atmosphere, ranging in size from a few nanometers to several micrometers in diameter) and aerosol-cloud interactions are the most uncertain aspect of the climate system, and models routinely underpredict aerosol particle formation. Clouds are well known to alter aerosol size, chemical composition and radiative properties, but the processes are poorly characterized. By adding water to particles collected on filters, we recently found a new, substantial and unrecognized source of OH in cloud droplets. During the first few minutes following cloud droplet formation and in the presence of daylight, material in aerosols produces a burst of hydroxyl radicals within the first two minutes. However, filter collection requires several hours to obtain sufficient mass, and during this period the particles are likely loose activity. Here we describe a new sampling approach in which particles are grown into nascent cloud droplets and then deposited into water containing the OH probe, giving  a more realistic picture of the process in a real cloud. We also characterize uptake of gas phase oxidants into the direct-to-liquid sampler via several pathways. The direct-to-liquid sampling produces substantially higher signals than the filter-based method, indicating that the OH burst will be the dominant source of OH radicals under most daylight conditions, and will be far larger than other known sources under many conditions, depending largely on the composition of the aerosol particle.

Direct-to-liquid measurement of the hydroxyl radical burst in Nascent cloud droplets

ACS Spring 2022

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