Composition and origin of PM 2.5 aerosol particles in the upper Rhine valley in summer

Abstract

Abstract. We conducted a 6-week measurement campaign in summer 2016 at a\nrural site about 11\u2009km north of the city of Karlsruhe in southwest Germany\nin order to study the chemical composition and origin of aerosols in the\nupper Rhine valley. In particular, we deployed a single-particle mass\nspectrometer (LAAPTOF) and an aerosol mass spectrometer (AMS) to provide\ncomplementary chemical information on aerosol particles smaller than 2.5\u2009 µ m. For the entire measurement period, the total aerosol particle\nmass was dominated by sodium salts, contributing on average ( 36±27 )\u2009% to the total single particles measured by the LAAPTOF. The total\nparticulate organic compounds, sulfate, nitrate, and ammonium contributed on\naverage ( 58±12 )\u2009%, ( 22±7 )\u2009%, ( 10±1 )\u2009%, and\n( 9±3 )\u2009% to the total non-refractory particle mass measured by the\nAMS. Positive matrix factorization (PMF) analysis for the AMS data suggests\nthat the total organic aerosol (OA) consisted of five components, including\n( 9±7 )\u2009% hydrocarbon-like OA (HOA), ( 16±11 )\u2009%\nsemi-volatile oxygenated OA (SV-OOA), and ( 75±15 )\u2009%\nlow-volatility oxygenated OA (LV-OOA). The regional transport model\nCOSMO-ART was applied for source apportionment and to achieve a better\nunderstanding of the impact of complex transport patterns on the field\nobservations. Combining field observations and model simulations, we\nattributed high particle numbers and SO2 concentrations observed at\nthis rural site to industrial emissions from power plants and a refinery in\nKarlsruhe. In addition, two characteristic episodes with aerosol particle\nmass dominated by sodium salts particles comprising ( 70±24 )\u2009%\nof the total single particles and organic compounds accounting\nfor ( 77±6 )\u2009% of total non-refractory species, respectively, were\ninvestigated in detail. For the first episode, we identified relatively\nfresh and aged sea salt particles originating from the Atlantic Ocean more\nthan 800\u2009km away. These particles showed markers like m∕z \xa0129\n C 5 H 7 NO 3 + , indicating the influence of anthropogenic\nemissions modifying their composition, e.g. from chloride to nitrate salts\nduring the long-range transport. For a 3\u2009d episode including high organic\nmass concentrations, model simulations show that on average ( 74±7 )\u2009% of the particulate organics at this site were of biogenic origin.\nDetailed model analysis allowed us to find out that three subsequent peaks\nof high organic mass concentrations originated from different sources,\nincluding local emissions from the city and industrial area of Karlsruhe,\nregional transport from the city of Stuttgart ( ∼64 \u2009km away),\nand potential local night-time formation and growth. Biogenic (forest) and\nanthropogenic (urban) emissions were mixed during transport and contributed\nto the formation of organic particles. In addition, topography, temperature\ninversion, and stagnant meteorological conditions also played a role in the\nbuild-up of higher organic particle mass concentrations. Furthermore, the\nmodel was evaluated using field observations and corresponding\nsensitivity tests. The model results show good agreement with trends and\nconcentrations observed for several trace gases (e.g. O3 , NO2 ,\nand SO2 ) and aerosol particle compounds (e.g. ammonium and nitrate).\nHowever, the model underestimates the number of particles by an order of\nmagnitude and underestimates the mass of organic particles by a factor of\n2.3. The discrepancy was expected for particle number since the model does\nnot include all nucleation processes. The missing organic mass indicates\neither an underestimated regional background or missing sources and/or\nmechanisms in the model, like night-time chemistry. This study demonstrates\nthe potential of combining comprehensive field observations with dedicated\ntransport modelling to understand the chemical composition and complex\norigin of aerosols.