Abstract
Abstract. Submicron aerosol particles were collected during July and August 2010 in Hyytiälä, Finland, to determine the composition and sources of aerosol at that boreal forest site. Submicron particles were collected on Teflon filters and analyzed by Fourier transform infrared (FTIR) spectroscopy for organic functional groups (OFGs). Positive matrix factorization (PMF) was applied to aerosol mass spectrometry (AMS) measurements and FTIR spectra to identify summertime sources of submicron aerosol mass at the sampling site. The two largest sources of organic mass (OM) in particles identified at Hyytiälä were (1) biogenic aerosol from surrounding local forest and (2) biomass burning aerosol, transported 4–5 days from large wildfires burning near Moscow, Russia, and northern Ukraine. The robustness of this apportionment is supported by the agreement of two independent analytical methods for organic measurements with three statistical techniques. FTIR factor analysis was more sensitive to the chemical differences between biogenic and biomass burning organic components, while AMS factor analysis had a higher time resolution that more clearly linked the temporal behavior of separate OM factors to that of different source tracers even though their fragment mass spectrum were similar. The greater chemical sensitivity of the FTIR is attributed to the nondestructive preparation and the functional group specificity of spectroscopy. The FTIR spectra show strong similarities among biogenic and biomass burning factors from different regions as well as with reference OM (namely olive tree burning organic aerosol and α-pinene chamber secondary organic aerosol (SOA)). The biogenic factor correlated strongly with temperature and oxidation products of biogenic volatile organic compounds (BVOCs), included more than half of the oxygenated OFGs (carbonyl groups at 29% and carboxylic acid groups at 22%), and represented 35% of the submicron OM. Compared to previous studies at Hyytiälä, the summertime biogenic OM is 1.5 to 3 times larger than springtime biogenic OM (0.64 μg m−3 and 0.4 μg m−3, measured in 2005 and 2007, respectively), even though it contributed only 35% of OM. The biomass burning factor contributed 25% of OM on average and up to 62% of OM during three periods of transported biomass burning emissions: 26–28 July, 29–30 July, and 8–9 August, with OFG consisting mostly of carbonyl (41%) and alcohol (25%) groups. The high summertime terrestrial biogenic OM (1.7 μg m−3) and the high biomass burning contributions (1.2 μg m−3) were likely due to the abnormally high temperatures that resulted in both stressed boreal forest conditions with high regional BVOC emissions and numerous wildfires in upwind regions.
Highlights
Boreal forests produce a large amount of global SOA from the emissions of high molecular weight, reactive biogenic volatile organic compounds (BVOCs) such as monoterpenes and sesquiterpenes (Seinfeld and Pandis, 2006)
Particle-phase mass at Hyytiälä was dominated by organic components and varied significantly throughout the campaign (± 3.1 μg m−3) due to local sources and long-range transport of Russian biomass burning emissions (Fig. 1; Williams et al, 2011)
During HUMPPA-COPEC, the majority of the remaining organic mass (OM) was from transported emissions, making rural Hyytiälä not as pristine or remote in character as one might think. This meant that even though the biogenic OA was higher than previously reported at 1.7 μg m−3, its fraction of OM was only 35 % based on Positive matrix factorization (PMF) from both aerosol mass spectrometry (AMS) and Fourier transform infrared (FTIR). While this apportionment is subject to the uncertainties inherent to statistical inversions, the PMF-based separation of biomass burning and biogenic OM at Hyytiälä, Finland, has the advantage of two time series of detailed organic composition measurements that included multiple periods when either biogenic or biomass burning OM substantially outweighed the other (Fig. 3)
Summary
Boreal forests produce a large amount of global SOA from the emissions of high molecular weight, reactive biogenic volatile organic compounds (BVOCs) such as monoterpenes and sesquiterpenes (Seinfeld and Pandis, 2006). The OOA factors correlated strongly (r > 0.7) with BVOC oxidation products and the spectra were enhanced with a few mass fragments (m / z 29, 43, and 58) that have been identified in SOA from α-pinene photooxidation (Shilling et al, 2009) and from plant emission photooxidation (Kiendler-Scharr et al, 2009) These mass fragments are not specific to biogenically influenced OOA factors and have been observed in hydrocarbon-like organic aerosol (HOA), biomass burning organic aerosol (BBOA), and semivolatile oxygenated organic aerosol (SV-OOA or OOA-2) (Slowik et al, 2010; Sun et al, 2011; Zhang et al, 2005). The primary goals of the campaign include investigating summertime aerosol chemistry and photochemistry of BVOCs (Williams et al, 2011) In this analysis, we focus on the characterization of the organic aerosol during HUMPPA-COPEC 2010 using two independent analytical methods of organic measurement (FTIR and AMS techniques) and three separate statistical methods (clustering, factorization, potential source contributions) to apportion OM to biogenic and biomass burning sources. This approach allows us to provide the most robust apportionment of OM for summertime Hyytiälä completed to date and to identify the relative advantages of different OM measurement techniques
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