Sources of organic gases and aerosol particles and their roles in nighttime particle growth at a rural forested site in southwest Germany

The composition, sources and chemical transformation of volatile organic compounds (VOCs)and organic aerosol (OA) particles were investigated during July–August 2021 at a rural forested site in southwestGermany, 10 km north of the city of Karlsruhe. VOCs and semi-volatile OA particles were measured witha proton-transfer-reaction mass spectrometer coupled to the CHemical Analysis of aeRosols ONline particleinlet (CHARON–PTR-MS). The CHARON-measured OA mass accounted for 62+-18% on average of the totalOA mass (4.2+-2.8 μgm-3) measured concurrently with an aerosol mass spectrometer (AMS). The total concentrationsof measured VOCs ranged from 7.6 to 88.9 ppb with an average of 31.2+-13.4 ppb. Positive matrixfactorization (PMF) was used to identify major source factors of VOCs and OA particles. Three types of oxygenatedVOC (OVOC), namely aromatic OVOCs, biogenic OVOCs and aged OVOCs, contributed on average11%+-9 %, 37%+-29% and 29%+-21% of total VOC concentrations, respectively. The results of AMS–PMFindicated substantial contributions of oxygenated organic compounds to OA particle mass. Three secondary OA(SOA) factors determined by CHARON–PMF analysis, namely aromatic SOA (5%7 %), daytime biogenicSOA (17%+-17 %) and nighttime biogenic SOA (28%+-21 %), consistently showed high contributions to thetotal CHARON-measured OA mass. Nighttime particle growth was observed regularly in this area, which wasmainly attributed to the semi-volatile organic compounds and organic nitrates formed from the oxidation ofmonoterpenes and sesquiterpenes. This study presents the major sources, real-time transformations of VOCs andOA particles, and nighttime particle formation characteristics for central European forested areas.