Understanding the Primary Emissions and Secondary Formation of Gaseous Organic Acids in the Oil Sands Region of Alberta, Canada
John Liggio1, Samar G. Moussa1, Jeremy Wentzell1, Andrea Darlington1, Peter Liu1, Amy Leithead1, Katherine Hayden1, Jason O'Brien1, Richard L. Mittermeier1, Ralf Staebler1, Mengistu Wolde2, and Shao-Meng Li11Air Quality Research Division, Environment and Climate Change Canada, Toronto, Ontario, Canada, M3H 5T4 2National Research Council Canada, Flight Research Laboratory, Ottawa, Canada, K1A 0R6
Received: 09 Mar 2017 – Accepted for review: 10 Mar 2017 – Discussion started: 13 Mar 2017
Abstract. Organic acids are known to be emitted from combustion processes and are key photochemical products of biogenic and anthropogenic precursors. Despite their multiple environmental impacts, such as on acid deposition and human/ecosystem health, little is known regarding their emission magnitudes or detailed chemical formation mechanisms. In the current work, airborne measurements of 18 gas-phase low molecular weight organic acids were made in the summer of 2013 over the oil sands region of Alberta, Canada, an area of intense unconventional oil extraction. The data from these measurements were used in conjunction with emission retrieval algorithms to derive the total and speciated primary organic acid emission rates, as well as secondary formation rates downwind of oil sands operations. The results of the analysis indicate that approximately 12 tonnes day−1 of low molecular weight organic acids, dominated by C1–C5 acids, were emitted directly from off-road diesel vehicles within open pit mines. Although there are no specific reporting requirements for primary organic acids, the measured emissions were similar in magnitude to primary oxygenated hydrocarbon emissions measured previously (≈ 20 tonnes day−1), for which there are reporting thresholds. Conversely, photochemical production of gaseous organic acids significantly exceeded the primary sources, with formation rates up to ≈ 184 tonnes day−1 downwind of the oil sands facilities. The formation and evolution of organic acids from a Lagrangian flight were modeled with a box model, incorporating a detailed hydrocarbon reaction mechanism extracted from the Master Chemical Mechanism (v 3.3). Despite evidence of significant secondary organic acid formation, the explicit chemical box model largely underestimated their formation in the oil sands plumes, accounting for 39 %, 46 %, 26 % and 23 % of the measured formic, acetic, acrylic and propionic acids respectively, and with little contributions from biogenic VOC precursors. The model results, together with an examination of the carbon mass balance between the organic acids formed and the primary VOCs emitted from oils sands operations, suggest the existence of significant missing oil sands related secondary sources/precursor emissions and/or an incomplete mechanistic and quantitative understanding of how they are processed in the atmosphere.
Liggio, J., Moussa, S. G., Wentzell, J., Darlington, A., Liu, P., Leithead, A., Hayden, K., O'Brien, J., Mittermeier, R. L., Staebler, R., Wolde, M., and Li, S.-M.: Understanding the Primary Emissions and Secondary Formation of Gaseous Organic Acids in the Oil Sands Region of Alberta, Canada, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-220, in review, 2017.