α-Pinene Secondary Organic Aerosol (SOA) Formation through Interconnected Day- and Nighttime Chemistry

The chemical reactions driving atmospheric oxidation during the day strongly influence the subsequent nighttime chemistry and vice versa. Here, the interconnection of the α-pinene daytime and nighttime chemistry was studied to answer the following question: How does the generated secondary organic aerosol evolve during the time of the day transition? The study conducted two types of simulation chamber experiments reflecting the daily changes in atmospheric oxidants: night-to-day and day-to-night processing, where day processing was reflected as OH radical oxidation, and night processing as NO3 radical oxidation. In the day-to-night transition, α-pinene oxidation produced more SOA, with yields increasing under higher relative humidity (RH), indicating that humidity enhances the SOA formation efficiency in this process, in contrast to the night-to-day process, which was less sensitive to humidity changes. The main products of the reaction of α-pinene and NO3 radicals were identified as organic nitrates (ON) in all conducted experiments, contributing to the total aerosol mass of around 40%. Under daytime conditions, the fraction of ON decreased, while that of SOA-bound peroxides became more dominant, suggesting a shift in chemical composition with the changing oxidation regime. The calculated fractional contribution of carboxylic acids was two times higher for experiments where SOA formation originated from the OH radical reaction (up to an 89% mass fraction).