Abstract
Abstract. We present measurements of a long-range smoke transport event recorded on 20–21 July 2011 over Halifax, Nova Scotia, Canada, during the Quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites (BORTAS-B) campaign. Ground-based Fourier transform spectrometers and photometers detected air masses associated with large wildland fires burning in eastern Manitoba and western Ontario. We investigate a plume with high trace gas amounts but low amounts of particles that preceded and overlapped at the Halifax site with a second plume with high trace gas loadings and significant amounts of particulate material. We show that the first plume experienced a meteorological scavenging event, but the second plume had not been similarly scavenged. This points to the necessity to account carefully for the plume history when considering long-range transport since simultaneous or near-simultaneous times of arrival are not necessarily indicative of either similar trajectories or meteorological history. We investigate the origin of the scavenged plume, and the possibility of an aerosol wet deposition event occurring in the plume ~ 24 h prior to the measurements over Halifax. The region of lofting and scavenging is only monitored on an intermittent basis by the present observing network, and thus we must consider many different pieces of evidence in an effort to understand the early dynamics of the plume. Through this discussion we also demonstrate the value of having many simultaneous remote-sensing measurements in order to understand the physical and chemical behaviour of biomass burning plumes.
Highlights
Wildland fires have burned an average of 2.3 million hectares of the Canadian boreal forest annually over the past 25 yr, with the majority of large fires being started by lightning strikes
This paper will focus on the measurements made by the four remote-sensing instruments that were operating at Dalhousie Ground Station (DGS) during the BORTAS campaign: two Fourier transform spectrometers (FTSs) providing solar absorption measurements of trace gases, a sun photometer providing aerosol optical depth during daylight hours, and a star photometer providing the same during clear nights
Runs were initialized at the DGS and along the aircraft flight track and used National Centers for Environmental Prediction (NCEP) Global Data Assimilation System (GDAS) reanalysis fields to model the transport to roughly locate the origins of the plumes
Summary
Wildland fires have burned an average of 2.3 million hectares of the Canadian boreal forest annually over the past 25 yr, with the majority of large fires being started by lightning strikes (data from the Canadian Wildland Fire Information System; http://cwfis.cfs.nrcan.gc.ca/) These biomass burning events are a significant source of carbonaceous aerosols and trace gases to the atmosphere. As lofting of air via meso-scale convection often occurs during LRT, clouds may form and aerosol particles may be scavenged by cloud/rain drops and potentially removed from the atmosphere via wet deposition. These processes are critical in determining aerosol lifetimes In this paper we present observational evidence of a biomass burning plume that experienced a period of precipitation scavenging during transport to the DGS.
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