A major regional air pollution event in the northeastern U.S. caused by extensive forest fires in Quebec, Canada

During early July 2002, wildfires in Quebec burned ~1.1x10⁶ ha of forest. The resultant smoke plume was seen in satellite images blanketing the U.S. east coast. At the same time extremely high CO mixing ratios were observed at the Atmospheric Investigation, Regional Modeling, Analysis and Prediction (AIRMAP) network sites in NH, USA and the Harvard Forest Ecological Monitoring Station (EMS) in MA, USA, representing enhancements of 525-1025 ppbv above background conditions. A biomass burning source for the event was confirmed by extremely elevated aerosol K⁺, NH₄⁺, NO₃^-, and oxalate mixing ratios at the AIRMAP sites. Additional data for aerosol K, OC and EC from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network and CO data from EPA sites indicated that the smoke plume impacted much of the U.S. east coast, from Maine to Virginia. CO mixing ratios and K concentrations at stations with 10+ year records suggested that this was the largest biomass burning plume to impact the U.S. east coast in over a decade. Furthermore, CO mixing ratios and aerosol PM_2.5 mass and scattering coefficients from the AIRMAP network and EMS indicate that this event was comparable to the large anthropogenic combustion and haze events which intermittently impact rural New England. We also present evidence that the biomass plume was superimposed on U.S. anthropogenic pollution, particularly in urban areas. The degree of enhancement of O₃, NO_y, NO₃^-, NH₄⁺ and SO₄^2- in the biomass plume showed significant variation with elevation and latitude, with all species except SO₄^2- having reduced mixing ratios at lower elevation and more southern sites. The smoke plume reaching lower elevation sites clearly experienced enhanced interaction with the surface layer compared to higher sites due to subsiding transport along isentropic surfaces. Transport in the boundary layer resulted in increased depositional loss (e.g., O₃ and NO_y) and increased incorporation of polluted air masses (e.g., SO₄^2-) compared to transport in the free troposphere. The decreases with latitude likely also reflected increased dilution of the smoke plume as it traveled further south.