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 southwest
Germany, 10 km north of the city of Karlsruhe. VOCs and semi-volatile OA particles were measured with
a proton-transfer-reaction mass spectrometer coupled to the CHemical Analysis of aeRosols ONline particle
inlet (CHARON–PTR-MS). The CHARON-measured OA mass accounted for 62+-18% on average of the total
OA mass (4.2+-2.8 μgm-3) measured concurrently with an aerosol mass spectrometer (AMS). The total concentrations
of measured VOCs ranged from 7.6 to 88.9 ppb with an average of 31.2+-13.4 ppb. Positive matrix
factorization (PMF) was used to identify major source factors of VOCs and OA particles. Three types of oxygenated
VOC (OVOC), namely aromatic OVOCs, biogenic OVOCs and aged OVOCs, contributed on average
11%+-9 %, 37%+-29% and 29%+-21% of total VOC concentrations, respectively. The results of AMS–PMF
indicated 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 biogenic
SOA (17%+-17 %) and nighttime biogenic SOA (28%+-21 %), consistently showed high contributions to the
total CHARON-measured OA mass. Nighttime particle growth was observed regularly in this area, which was
mainly attributed to the semi-volatile organic compounds and organic nitrates formed from the oxidation of
monoterpenes and sesquiterpenes. This study presents the major sources, real-time transformations of VOCs and
OA particles, and nighttime particle formation characteristics for central European forested areas.