Interactions between boreal wildfire and urban emissions

Abstract

A suite of particulate, gaseous and meteorological measurements during the Pittsburgh Supersite experiment were used to characterize the impact of the 2002 Quebec wildfires on pollutant concentrations and physical and chemical processes dominant in the region. Temporal trends in the number distribution of wildfire particles (isolated using Rapid Single-ultrafine-particle Mass Spectrometry data) combined with CO, NO x and O 3 mixing ratios identified two separate periods (Periods I and II) when the measurement site was directly impacted by plumes of relatively unprocessed wildfire emissions; i.e., increases in primary ultrafine wildfire particles, CO and NO x concomitant with a decrease in O 3 from intraplume NO x titration. Carbonaceous particle number distributions predominantly associated with vehicular emissions, PM 2.5 sulfate mass concentration and SO 2 mixing ratio resolved individual components of local and regional sources. Single particle signatures indicated a period of intense atmospheric processing following Period II that caused rapid growth of the ultrafine mode due to simultaneous sulfate and secondary organic mass accumulation, resulting in significant changes to article physical and chemical properties. Particle growth was concurrent with large increases in O 3 and maxima in incoming solar radiation and ambient temperature and is posited to have occurred in situ as the air mass, containing a mixture of urban and wildfire emissions, was advected past the site. In total, the current work demonstrates significant added severity for pollution episodes in an area already burdened by large anthropogenic emission rates due to the impact of the 2002 Quebec wildfires. High levels of atmospheric processing increased sulfate accumulation and SOA formation and brought PM 2.5 mass concentrations close to, and O 3 mixing ratios in excess of, the National Ambient Air Quality Standards. Projections of increasing wildfire activity under a warming climate may increase the frequency and severity of such events. Copyright 2008 by the American Geophysical Union.