Xiaohong Yao

Mass Absorption Cross-Section of Ambient Black Carbon Aerosol in Relation to Chemical Age

Three differing techniques were used to measure ambient black carbon (BC) aerosols in downtown Toronto through 20 December 2006 to 23 January 2007. These techniques were thermal analysis, as performed by a Sunset Labs OCEC Analyzer (OCEC); light attenuation, as performed by an Aethalometer (AE); and photoacoustic analysis, as performed by a Photoacoustic Instrument (PA). These measurements of ambient PM 2.5 were used to investigate the effects of coating thickness on BC Mass Absorption Cross-section (MAC). MAC values were determined by comparing 880 nm and 370 nm AE measurements and PA measurements of b abs (absorption coefficient, Mm–1) to the OCEC measurements. Based on mass size distributions and supporting criteria, the PM 2.5 was classified as fresh, semi-aged, or aged. The average MAC values in these categories, based on the PA measurements, were 9.3 ± 1.8, 9.9 ± 2.0, and 9.3 ± 2.2 m 2 /g (mean ± standard deviation), respectively, suggesting that any difference in coating thickness as a result of aging, on the time scale observed, did not produce a difference in MAC. In a second type of experiment, a thermodenuder was installed upstream of the AE, PA, and OCEC and samples were heated to 340°C in order to evaporate volatile and semi-volatile components within the coating. Based on the PA measurements, the average MAC values of these heated samples, for the fresh, semi-aged, and aged categories were 7.7 ± 2.2, 6.9 ± 2.2, and 9.1 ± 2.0 m 2 /g, respectively. Similar differences in MAC were also observed by the AE. The decrease in MAC in the fresh and semi-aged samples was interpreted in terms of the degree of coating of the PM 2.5 . Results agreed well with predictions made by absorption amplification theory and had ramifications for calibration of filter-base attenuation and photoacoustic instruments.

Cloud and Fog Processing Enhanced Gas-to-Particle Partitioning of Trimethylamine

An aerosol time-of-flight mass spectrometer (ATOFMS) was used to detect trimethylamine (TMA) in 0.52-1.9 μm particles at urban and rural sites in Southern Ontario during the summer and winter of 2007. During the summer, TMA-containing particles were observed exclusively during high relative humidity or fog events at both the urban and rural sites. In the wintertime, greater concentrations of TMA-containing particles were linked to cloud processing of aerosol in air masses originating from over agricultural and livestock areas. A laboratory study revealed that, at high relative humidity (∼ 100%), gas phase TMA at concentrations ranging from 2 to 20,000 ppt partitions preferentially to acidic particles present in the ambient air. On the basis of the field and laboratory studies, it appears that gas phase TMA present in ambient air partitions onto pre-existing particles preferentially during periods of acidic cloud/fog processing, leading to the presence of TMA-containing particles in the 0.52-1.9 μm size range.

Roles of SO2 oxidation in new particle formation events

The oxidation of SO2 is commonly regarded as a major driver for new particle formation (NPF) in the atmosphere. In this study, we explored the connection between measured mixing ratio of SO2 and observed long-term (duration>3 hr) and short-term (duration<1.5 hr) NPF events at a semi-urban site in Toronto. Apparent NPF rates (J30) showed a moderate correlation with the concentration of sulfuric acid ([H2SO4]) calculated from the measured mixing ratio of SO2 in long-term NPF events and some short-term NPF events (Category I) (R2=0.66). The exponent in the fitting line of J30~[H2SO4]n in these events was 1.6. It was also found that SO2 mixing ratios varied a lot during long-term NPF events, leading to a significant variation of new particle counts. In the SO2-unexplained short-term NPF events (Category II), analysis showed that new particles were formed aloft and then mixed down to the ground level. Further calculation results showed that sulfuric acid oxidized from SO2 probably made a negligible contribution to the growth of >10 nm new particles.