Abstract
Abstract. As part of the BAQS-Met 2007 field campaign, Aerodyne time-of-flight aerosol mass spectrometers (ToF-AMS) were deployed at two sites in southwestern Ontario from 17 June to 11 July 2007. One instrument was located at Harrow, ON, a rural, agriculture-dominated area approximately 40 km southeast of the Detroit/Windsor/Windsor urban area and 5 km north of Lake Erie. The second instrument was located at Bear Creek, ON, a rural site approximately 70 km northeast of the Harrow site and 50 km east of Detroit/Windsor. Positive matrix factorization analysis of the combined organic mass spectral dataset yields factors related to secondary organic aerosol (SOA), direct emissions, and a factor tentatively attributed to the reactive uptake of isoprene and/or condensation of its early generation reaction products. This is the first application of PMF to simultaneous AMS measurements at different sites, an approach which allows for self-consistent, direct comparison of the datasets. Case studies are utilized to investigate processing of SOA from (1) fresh emissions from Detroit/Windsor and (2) regional aerosol during periods of inter-site flow. A strong correlation is observed between SOA/excess CO and photochemical age as represented by the NOx/NOy ratio for Detroit/Windsor outflow. Although this correlation is not evident for more aged air, measurements at the two sites during inter-site transport nevertheless show evidence of continued atmospheric processing by SOA production. However, the rate of SOA production decreases with airmass age from an initial value of ~10.1 μg m−3 ppmvCO−1 h−1 for the first ~10 h of plume processing to near-zero in an aged airmass (i.e. after several days). The initial SOA production rate is comparable to the observed rate in Mexico City over similar timescales.
Highlights
Quantification of aerosol effects on climate (Albrecht et al, 1989; Twomey et al, 1984), visibility (Eidels-Dubovoi, 2002; Pryor et al, 1997), and health (Pope et al, 2006; Gamble, 1998; Peters et al, 1997) is complicated by uncertainties in particle sources and atmospheric transformations
The present study investigates aerosol processing from comparison of OA composition measurements by time-offlight aerosol mass spectrometers (ToF-AMS) at two locations in southwestern Ontario, Canada, during the 2007 BAQSMET (Border Air Quality Study-Meteorology) field campaign
oxygenated organic aerosol (OOA)-1 and OOA-2 have recently been identified with volatility, with OOA-1 denoted as “low-volatility OOA” (LV-OOA) and OOA-2 denoted as “semivolatile OOA” (SVOOA) (Jimenez et al, 2009)
Summary
Quantification of aerosol effects on climate (Albrecht et al, 1989; Twomey et al, 1984), visibility (Eidels-Dubovoi, 2002; Pryor et al, 1997), and health (Pope et al, 2006; Gamble, 1998; Peters et al, 1997) is complicated by uncertainties in particle sources and atmospheric transformations. Major uncertainties have been demonstrated in the formation and evolution of secondary organic aerosol (SOA) in polluted regions (Volkamer et al, 2006). Significant advances have been made in identifying the role of gas/particle partitioning in SOA formation (Robinson et al, 2007). Inclusion of these processes in SOA models has improved the agreement between modelled and measured SOA mass in polluted regions; discrepancies remain between modelled and measured particle properties (e.g. composition, volatility) (Dzepina et al, 2009). Takegawa et al, 2006), and fixed or mobile sites at which photochemical age is estimated utilizing the ratios of reactive hydrocarbons Studies of the atmospheric processing of aerosol have previously been conducted using methods such as mobile sampling platforms (e.g. Jimenez et al, 2009), multiple stationary sites (e.g. Takegawa et al, 2006), and fixed or mobile sites at which photochemical age is estimated utilizing the ratios of reactive hydrocarbons
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