Characterizing ozone and SOA formation potentials from volatile organic compounds (VOCs) emissions in an oil industry center in Canada

Past source apportionment studies focused on the contribution to VOC concentration from different sources to pinpoint the major VOC sources for emission mitigation. However, different VOC sources may have different ozone and secondary organic aerosol (SOA) formation potentials. So, from control perspective, it would be more rationale to consider the role of individual VOC source in secondary pollution in source apportionment study. In this study, we propose a tiered source identification method that considers the formation potentials of ozone and SOA and applied it in Calgary, Alberta, a site under influence from multiple competing VOC sources. The pollution characteristic, secondary pollutant formation potential, and geographical origin of VOC sources were explicitly investigated over a five-year period. Seven major sources were identified using positive matrix factorization (PMF) model, among which vehicle exhausts and fuel combustion were the dominant VOC sources responsible for the O₃ (60%) and SOA (63%) formations, suggesting that combustion of both liquid fuel (vehicle exhaust) and solid fuel (fuel combustion) has exceeded the contribution from oil and gas production and become the top contributor to ozone and aerosol pollution in Calgary. This finding was consistent with the fact of great reductions (32.2% - 99.8%) in oil and gas production in Calgary from 2013 to 2017. The source apportionment results showed that the primary VOC source has shifted from conventional oil and gas extraction to a mixture of vehicle exhausts and oil and gas extraction, indicating the effectiveness of emission control measures taken in the energy sectors. Moreover, regional transported VOC from combustion sources in the southeastern British Columbia, greatly deteriorated the VOC level and secondary pollutants formation in Calgary. To effectively alleviate secondary pollution problems, it was recommended to perform joint pollution control measures by the Alberta and British Columbia governments. These findings revealed that the tiered source identification strategy combining traditional receptor model with socioeconomic factors, emission inventory, and source region analysis, was a robust and promising tool in the interpretation of source apportionment results and optimization of secondary pollution control.