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Organic acids are known to\udbe emitted from combustion processes and are key photochemical products of\udbiogenic and anthropogenic precursors. Despite their multiple environmental\udimpacts, such as on acid deposition and human–ecosystem health, little is\udknown regarding their emission magnitudes or detailed chemical formation\udmechanisms. In the current work, airborne measurements of 18 gas-phase\udlow-molecular-weight organic acids were made in the summer of 2013 over the\udoil sands region of Alberta, Canada, an area of intense unconventional oil\udextraction. The data from these measurements were used in conjunction with\udemission retrieval algorithms to derive the total and speciated primary\udorganic acid emission rates, as well as secondary formation rates downwind of\udoil sands operations. The results of the analysis indicate that approximately\ud12 t day−1 of low-molecular-weight organic acids, dominated by\udC1–C5 acids, were emitted directly from off-road diesel vehicles\udwithin open pit mines. Although there are no specific reporting requirements\udfor primary organic acids, the measured emissions were similar in magnitude\udto primary oxygenated hydrocarbon emissions, for which there are reporting\udthresholds, measured previously ( ≈  20 t day−1). Conversely,\udphotochemical production of gaseous organic acids significantly exceeded the\udprimary sources, with formation rates of up to  ≈  184 t day−1\uddownwind of the oil sands facilities. The formation and evolution of organic\udacids from a Lagrangian flight were modelled with a box model, incorporating\uda detailed hydrocarbon reaction mechanism extracted from the Master Chemical\udMechanism (v3.3). Despite evidence of significant secondary organic acid\udformation, the explicit chemical box model largely underestimated their\udformation in the oil sands plumes, accounting for 39, 46, 26, and 23 % of\udthe measured formic, acetic, acrylic, and propionic acids respectively and\udwith little contributions from biogenic VOC precursors. The model results,\udtogether with an examination of the carbon mass balance between the organic\udacids formed and the primary VOCs emitted from oil sands operations, suggest\udthe existence of significant missing secondary sources and precursor\udemissions related to oil sands and/or an incomplete mechanistic and\udquantitative understanding of how they are processed in the atmosphere

Understanding the primary emissions and secondary formation of gaseous organic acids in the oil sands region of Alberta, Canada