Importance of secondary organic aerosol formation of α-pinene, limonene, and m-cresol comparing day- and nighttime radical chemistry

The oxidation of biogenic and anthropogenic compounds leads to the formation of secondary organic aerosol mass (SOA). The present study aims to investigate alpha-pinene, limonene, and m-cresol with regards to their SOA formation potential dependent on relative humidity (RH) under night-(NO3 radicals) and daytime conditions (OH radicals) and the resulting chemical composition. It was found that SOA formation potential of limonene with NO3 under dry conditions significantly exceeds that of the OH-radical reaction, with SOA yields of 15-30% and 10-21 %, respectively. Additionally, the nocturnal SOA yield was found to be very sensitive towards RH, yielding more SOA under dry conditions. In contrast, the SOA formation potential of alpha-pinene with NO3 slightly exceeds that of the OH-radical reaction, independent from RH. On average, alpha-pinene yielded SOA with about 6-7% from NO3 radicals and 3-4% from OH-radical reaction. Surprisingly, unexpectedly high SOA yields were found for m-cresol oxidation with OH radicals (3-9 %), with the highest yield under elevated RH (9 %), which is most likely attributable to a higher fraction of 3-methyl-6-nitro-catechol (MNC). While alpha-pinene and m-cresol SOA was found to be mainly composed of water-soluble compounds, 50-68% of nocturnal SOA and 22-39% of daytime limonene SOA are water-insoluble. The fraction of SOA-bound peroxides which originated from alpha-pinene varied between 2 and 80% as a function of RH. Furthermore, SOA from alpha-pinene revealed pinonic acid as the most important particle-phase constituent under day- and nighttime conditions with a fraction of 1-4 %. Other compounds detected are norpinonic acid (0.05-1.1% mass fraction), terpenylic acid (0.1-1.1% mass fraction), pinic acid (0.1-1.8% mass fraction), and 3-methyl-1,2,3-tricarboxylic acid (0.05-0.5% mass fraction). All marker compounds showed higher fractions under dry conditions when formed during daytime and showed almost no RH effect when formed during night.

Automated summary

The study investigated the oxidation reactions of alpha-pinene, limonene, and m-cresol under different relative humidity conditions and found that the SOA formation potential varied significantly depending on the reaction and humidity levels. Limonene showed a higher SOA yield with NO3 radicals under dry conditions, while m-cresol exhibited unexpectedly high SOA yields with OH radicals, particularly under elevated RH. Alpha-pinene, on the other hand, yielded consistent levels of SOA regardless of RH. Additionally, the chemical composition and markers of SOA were found to differ between the different compounds and reaction conditions.