Time-resolved measurements of black carbon light absorption enhancement in urban and near-urban locations of southern Ontario, Canada

摘要

In this study a photoacoustic spectrometer (PA), a laser-inducedincandescence instrument system (LII) and an Aerosol Mass Spectrometer wereoperated in parallel for in-situ measurements of black carbon (BC) light absorptionenhancement. Results of a thermodenuder experiment using ambient particlesin Toronto are presented first to show that LII measurements of BC are notinfluenced by the presence of non-refractory material thus providing trueatmospheric BC mass concentrations. In contrast, the PA response is enhancedwhen the non-refractory material is internally mixed with the BC particles.Through concurrent measurements using the LII and PA the specific absorptioncross-section (SAC) can be quantified with high time resolution (1 min).Comparisons of ambient PA and LII measurements from four different locations(suburban Toronto; a street canyon with diesel bus traffic in Ottawa;adjacent to a commuter highway in Ottawa and; regional background air in andaround Windsor, Ontario), show that different impacts from emission sourcesand/or atmospheric processes result in different particle light absorptionenhancements and hence variations in the SAC. The diversity of measurementsobtained, including those with the thermodenuder, demonstrated that it ispossible to identify measurements where the presence of externally-mixednon-refractory particles obscures direct observation of the effect ofcoating material on the SAC, thus allowing this effect to be measured withmore confidence. Depending upon the time and location of measurement (urban,rural, close to and within a lake breeze frontal zone), 30 min averageSAC varies between 9 ± 2 and 43 ± 4 m2 g?1. Causes of thisvariation, which were determined through the use of meteorological andgaseous measurements (CO, SO₂, O₃), include the particle emissionsource, airmass source region, the degree of atmospheric processing.Observations from this study also show that the active surface area of theBC aggregate, which is measured by the LII as the PPS, is an importantparameter for inferring the degree of particle collapse of a BC particle. Inaddition, PPS could be a useful measurement for indicating the importance ofrecently emitted BC (e.g. from gasoline or diesel engines) relative to thetotal measured BC in the atmosphere.