Abstract
Abstract. A year-long near-real-time characterization of non-refractory submicron aerosol (NR-PM1) was conducted at an urban (Atlanta, Georgia, in 2012) and rural (Look Rock, Tennessee, in 2013) site in the southeastern US using the Aerodyne Aerosol Chemical Speciation Monitor (ACSM) collocated with established air-monitoring network measurements. Seasonal variations in organic aerosol (OA) and inorganic aerosol species are attributed to meteorological conditions as well as anthropogenic and biogenic emissions in this region. The highest concentrations of NR-PM1 were observed during winter and fall seasons at the urban site and during spring and summer at the rural site. Across all seasons and at both sites, NR-PM1 was composed largely of OA (up to 76 %) and sulfate (up to 31 %). Six distinct OA sources were resolved by positive matrix factorization applied to the ACSM organic mass spectral data collected from the two sites over the 1 year of near-continuous measurements at each site: hydrocarbon-like OA (HOA), biomass burning OA (BBOA), semi-volatile oxygenated OA (SV-OOA), low-volatility oxygenated OA (LV-OOA), isoprene-derived epoxydiols (IEPOX) OA (IEPOX-OA) and 91Fac (a factor dominated by a distinct ion at m∕z 91 fragment ion previously observed in biogenic influenced areas). LV-OOA was observed throughout the year at both sites and contributed up to 66 % of total OA mass. HOA was observed during the entire year only at the urban site (on average 21 % of OA mass). BBOA (15–33 % of OA mass) was observed during winter and fall, likely dominated by local residential wood burning emission. Although SV-OOA contributes quite significantly ( ∼ 27 %), it was observed only at the urban site during colder seasons. IEPOX-OA was a major component (27–41 %) of OA at both sites, particularly in spring and summer. An ion fragment at m∕z 75 is well correlated with the m∕z 82 ion associated with the aerosol mass spectrum of IEPOX-derived secondary organic aerosol (SOA). The contribution of 91Fac to the total OA mass was significant (on average 22 % of OA mass) at the rural site only during warmer months. Comparison of 91Fac OA time series with SOA tracers measured from filter samples collected at Look Rock suggests that isoprene oxidation through a pathway other than IEPOX SOA chemistry may contribute to its formation. Other biogenic sources could also contribute to 91Fac, but there remains a need to resolve the exact source of this factor based on its significant contribution to rural OA mass.
Highlights
Characterization of the chemical composition of atmospheric fine aerosol is important, because of its adverse human health effects (Pope III and Dockery, 2006) and possible impacts on the Earth’s climate system (Forster et al, 2007)
At the Look Rock (LRK) site, average organic aerosol (OA) loadings increased from spring (3.2 μg m−3) to summer (5.3 μg m−3), and decreased in fall (2.8 μg m−3), which is likely related to biogenic VOCs (BVOCs) emissions that depend on leaf surface area, solar radiation and ambient temperature (Fig. 2; Guenther et al, 2006)
A different pattern was observed at the urban site (Fig. 1), where average OA loadings were highest during the fall (8.2 μg m−3) followed by winter (7.2 μg m−3), suggesting contributions from biomass-burning-related OA and non-biogenic sources
Summary
Characterization of the chemical composition of atmospheric fine aerosol is important, because of its adverse human health effects (Pope III and Dockery, 2006) and possible impacts on the Earth’s climate system (Forster et al, 2007). Long-term regional characterizations of ambient PM1 are required to understand its sources, formation and aging mechanisms, as well as atmospheric lifetime. This information will lead to more accurately constrained air quality models for making regulatory decisions to mitigate the potential adverse impacts of PM1. Online aerosol mass spectrometry (AMS) has been used to extensively characterize ambient non-refractory (NR)-PM1 (Zhang et al, 2007; Jimenez et al, 2009; Ng et al, 2010; Crippa et al, 2014); prior studies were limited by short measurement periods (weeks to a several months) because the need for intensive instrument maintenance required the continuous on-site presence of skilled personnel in order to generate high-quality data. The Aerodyne Aerosol Chemical Speciation Monitor (ACSM) based on the AMS technology has been modified to allow for long-term operation with less maintenance (Ng et al, 2011b). The ACSM has been recently used for longterm NR-PM1 measurements (Petit et al, 2015; Ripoll et al, 2015; Parworth et al, 201; Zhang et al, 2015) and shown to be durable and data are comparable to data collected from existing fine aerosol monitoring networks (Budisulistiorini et al, 2014)
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