Characterization of volatile organic compounds and submicron organic aerosol in a traffic environment
Authors: Saarikoski Sanna , Hellén Heidi , Praplan Arnaud P. , Schallhart Simon , Clusius Petri , Niemi Jarkko V. , Kousa Anu , Tykkä Toni , Kouznetsov Rostislav , Aurela Minna , Salo Laura , Rönkkö Topi , Barreira Luis M. F. , Pirjola Liisa , Timonen Hilkka 
Published: March 2023

Abstract. Urban air consists of a complex mixture of gaseous and particulate
species from anthropogenic and biogenic sources that are further processed
in the atmosphere. This study investigated the characteristics and sources
of volatile organic compounds (VOCs) and submicron organic aerosol (OA) in a traffic environment in Helsinki, Finland, in late summer. The anthropogenic VOCs (aVOCs; aromatic hydrocarbons) and biogenic VOCs (bVOCs; terpenoids) relevant for secondary-organic-aerosol formation were analyzed with an online gas chromatograph mass spectrometer, whereas the composition and size distribution of submicron particles was measured with a soot particle aerosol mass spectrometer. This study showed that aVOC concentrations were significantly higher than
bVOC concentrations in the traffic environment. The largest aVOC
concentrations were measured for toluene (campaign average of 1630 ng m−3) and p/m xylene (campaign average of 1070 ng m−3), while the dominating bVOC was α-pinene (campaign average of 200 ng m−3). For particle-phase organics, the campaign-average OA concentration was 2.4 µg m−3. The source apportionment analysis extracted six factors for OA. Three OA factors were related to primary OA sources – traffic (24 % of OA, two OA types) and a coffee roastery (7 % of OA) – whereas the largest fraction of OA (69 %) consisted of oxygenated OA (OOA). OOA was divided into less oxidized semi-volatile OA (SV-OOA; 40 % of OA) and two types of low-volatility OA (LV-OOA; 30 %). The focus of this research was also on the oxidation potential of the
measured VOCs and the association between VOCs and OA in ambient air.
Production rates of the oxidized compounds (OxPR) from the VOC reactions
revealed that the main local sources of the oxidation products were O3 oxidation of bVOCs (66 % of total OxPR) and OH radical oxidation of aVOCs and bVOCs (25 % of total OxPR). Overall, aVOCs produced a much smaller portion of the oxidation products (18 %) than bVOCs (82 %). In terms of OA factors, SV-OOA was likely to originate from biogenic sources since it correlated with an oxidation product of monoterpene, nopinone. LV-OOA consisted of highly oxygenated long-range or regionally transported OA that had no correlation with local oxidant concentrations as it had already spent several days in the atmosphere before reaching the measurement site. In general, the main sources were different for VOCs and OA in the traffic
environment. Vehicle emissions impacted both VOC and OA concentrations. Due
to the specific VOCs attributed to biogenic emissions, the influence of
biogenic emissions was more clearly detected in the VOC concentrations than
in OA. In contrast, the emissions from the local coffee roastery had a
distinctive mass spectrum for OA, but they could not be seen in the VOC
measurements due to the measurement limitations for the large VOC compounds.
Long-range transport increased the OA concentration and oxidation state
considerably, while its effect was observed less clearly in the VOC
measurements due to the oxidation of most VOC in the atmosphere during the
transport. Overall, this study revealed that in order to properly characterize the impact of different emission sources on air quality, health, and climate, it is of importance to describe both gaseous and particulate emissions and understand how they interact as well as their phase transfers in the atmosphere during the aging process.