Measurements of VOCs by proton transfer reaction mass spectrometry at a rural Ontario site: Sources and correlation to aerosol composition

Proton transfer reaction mass spectrometry (PTR-MS) volatile organic compound (VOC) measurements were made at the Centre for Atmospheric Research Experiments at Egbert, Ontario, over a five week period in May to June 2007. VOCs were also analyzed by gas chromatography and with a fluorescence detector sensitive to formaldehyde. This is a well positioned site for contrasting biogenic versus anthropogenic sources. In particular, with southerly flow from Southern Ontario, the VOC signal was observed to have strong anthropogenic influence, as seen through high levels of aromatics, acetylene and NOx. With flow from the northwest, the VOCs had substantial biogenic character, with elevated mixing ratios of monoterpenes, isoprene, and possibly 2-methyl-3-buten-2-ol (MBO) with increasing temperatures. Positive matrix factorization (PMF) was applied to the PTR-MS data set yielding two noninstrumental factors, one related to primary anthropogenic emissions and one arising from secondary atmospheric oxidation. There is a tight correspondence between the composition of the submicron organic aerosol and the VOCs. In particular, (1) organic aerosol loadings highly correlate with long-lived VOC tracers such as acetone, (2) the degree of oxygenation of the organic aerosol scaled with the photochemical age of the air, as determined through the toluene-to-benzene ratio, and (3) there is a tight correlation between PMF factors from the AMS and PTR-MS data sets, with the HOA AMS factor scaling with the anthropogenic emissions PTR-MS factor and the OOA AMS factor matched to the photochemical oxidation PTR-MS factor. Finally, the PTR-MS PMF factors were used for VOC source apportionment.