10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Long-Term Field Observations of Aerosol Chemical Composition in the Boreal Forest
LIINE HEIKKINEN, Mikko Äijälä, Matthieu Riva, Krista Luoma, Tuukka Petäjä, Douglas Worsnop, Mikael Ehn, University of Helsinki
Abstract Number: 668 Working Group: Source Apportionment
Abstract In the past decades the atmospheric fine particulate matter (PM) concentrations have caught attention due to the impacts on climate, health and visibility. Atmospheric aerosols have many sources and they can be directly emitted into the atmosphere as particles or they can be formed from the oxidation of gaseous precursors during new particle formation events. The atmospheric PM loading is greatly enhanced by the partitioning or reactive uptake of gaseous species, where the organics play a key role through secondary organic aerosol (SOA) formation. Biogenic volatile organic compounds (BVOCs) are a major source of SOA especially in regions with dense vegetation. Also anthropogenic emissions have a significant impact on both organic and inorganic PM composition. The aerosol chemical composition greatly impacts both toxicity and the physicochemical properties of aerosols, such as light scattering abilities or hygroscopicity. Therefore, we deployed an Aerodyne Aerosol Chemical Speciation Monitor (ACSM) for a long-term measurement to the BVOC-rich SMEAR (Station Measuring Ecosystem Atmosphere Relations) II station located in the boreal forest in Southern Finland to capture the chemical composition of aerosol particles and its seasonal variation. To understand the true nature of the seasonal variation of the chemical species present in non-refractory submicron PM and detected by the ACSM, a long data sets is crucial to minimize the influence of short-term pollution episodes. In this way, a better understanding of the chemical processes governing the aerosol within the atmosphere is achievable. Therefore, we analyzed a large data set collected from 2012 to 2017 with approximately a 65% data coverage having all the four seasons well represented. The analysis reveals that the chemical composition of aerosol sampled throughout this period, is highly dominated by the organics. Overall this fraction has the highest contribution to the non-refractory PM1 especially during summer when the BVOC emissions are the highest and the oxidation processes are the strongest within the boreal forest. A more detailed analysis of the organic aerosol composition was carried out by performing positive matrix factorization (PMF) in order to isolate the different sources (e.g. biogenic vs anthropogenic). This mass spectral analysis highlighted a more pronounced anthropogenic influence on the organic aerosol composition during the cold seasons while during the summer the aerosol was mainly biogenic in origin, as expected. In this study, a detailed analysis of the PMF results as well as an analysis of the seasonal variation of both inorganic and organic non-refractory species are presented. The PMF factors were verified and better understood by comparing them to the vast variety of other parameters measured at the SMEAR-II station as well as mass spectra generated from atmospheric simulation chamber experiments. Finally, a robust characterization of the aerosol chemical composition is presented giving a reliable estimation of its true seasonal variation over the boreal forest.