Real-time assessment of wintertime organic aerosol characteristics and sources at a suburban site in northern France

Abstract

A high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed during wintertime (5 February to 15 March 2016) at a suburban site in Douai, northern France, in order to investigate the characteristics and sources of the organic matter (OM). The campaign average concentration of non-refractory submicron particulate matter (NR-PM1) was 11.1 ± 9.3 μg m−3, and composed of 38% OM, 36% nitrate, 16% ammonium and 9% sulfate. The average values for the OM:OC, O:C and H:C ratios were 1.60 ± 0.15, 0.32 ± 0.11 and 1.55 ± 0.14, respectively, indicating a moderate level of aerosol oxidation. The positive matrix factorization (PMF) source apportionment method was applied to the high-resolution organic aerosol (OA) mass spectra, resulting in four factors: a hydrocarbon-like (HOA) factor; one associated with oxidized biomass burning (oBBOA); and two oxygenated factors (OOA) denoted as less oxidized (LO-OOA) and more oxidized (MO-OOA), with average contributions to OA of 20%, 28%, 17% and 35%, respectively. The oBBOA factor was found to be mainly local as shown by non-parametric wind regression (NWR) analysis, and to correlate well with relative humidity (RH), suggesting fast aqueous processing of locally emitted primary biomass burning emissions. During most part of the campaign, the sampling site was affected by different air masses. However, during the last period of the campaign (5–16 March 2016) the site was heavily impacted by air masses from Eastern Europe which were rich in secondary inorganic and organic aerosols. The H:C versus O:C (Van Krevelen, VK) diagram highlighted that the aerosol followed an oxidation process throughout the whole campaign, with an average slope of −1.05. The impact of continental air masses towards the end of the campaign confined the aerosol towards a narrower space in the VK diagram, suggesting a homogenization of the different aerosol sources due to OA ageing during transport. Several nocturnal NPF (new particle formation)-like events were observed and associated to the fast processing of BBOA emissions and the formation of ammonium nitrate.