Articles | Volume 11, issue 6
Atmos. Chem. Phys., 11, 2991–3006, 2011

Special issue: The Border Air Quality and Meteorology Study (BAQS-Met)

Atmos. Chem. Phys., 11, 2991–3006, 2011

Research article 31 Mar 2011

Research article | 31 Mar 2011

Photochemical processing of organic aerosol at nearby continental sites: contrast between urban plumes and regional aerosol

J. G. Slowik1,3,*, J. Brook4, R. Y.-W. Chang1,3, G. J. Evans2,3, K. Hayden4, C.-H. Jeong4, S.-M. Li4, J. Liggio4, P. S. K. Liu4, M. McGuire2,3, C. Mihele4, S. Sjostedt1,3,4, A. Vlasenko1,3,4, and J. P. D. Abbatt1,2 J. G. Slowik et al.
  • 1Department of Chemistry, University of Toronto, Toronto, Canada
  • 2Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Canada
  • 3Southern Ontario Centre for Atmospheric Aerosol Research, University of Toronto, Toronto, Canada
  • 4Science and Technology Branch, Environment Canada, Toronto, Canada
  • *now at: Paul Scherrer Institut, 5232 Villigen PSI, Switzerland

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.

Final-revised paper