Characterization of a large biogenic secondary organic aerosol event from eastern Canadian forests
1University of Toronto, Department of Chemistry, Toronto, ON, Canada
2Environment Canada, Science and Technology Branch, Toronto, ON, Canada
3Dalhousie University, Department of Physics and Atmospheric Science, Halifax, NS, Canada
4Harvard-Smithsonian Center for Astrophysics, Atomic and Molecular Physics Division, Cambridge, MA, USA
Abstract. Biogenic secondary organic aerosol levels many times larger than past observations have been measured 70 km north of Toronto during a period of increasing temperatures and outflow from Northern Ontario and Quebec forests in early summer. A regional chemical transport model approximately predicts the event timing and accurately predicts the aerosol loading, identifying the precursors as monoterpene emissions from the coniferous forest. The agreement between the measured and modeled biogenic aerosol concentrations contrasts with model underpredictions for polluted regions. Correlations of the oxygenated organic aerosol mass with tracers such as CO support a secondary aerosol source and distinguish biogenic, pollution, and biomass burning periods during the field campaign. Using the Master Chemical Mechanism, it is shown that the levels of CO observed during the biogenic event are consistent with a photochemical source arising from monoterpene oxidation. The biogenic aerosol mass correlates with satellite measurements of regional aerosol optical depth, indicating that the event extends across the eastern Canadian forest. This regional event correlates with increased temperatures, indicating that temperature-dependent forest emissions can significantly affect climate through enhanced direct radiative forcing and higher cloud condensation nuclei numbers.