10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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An Anthropogenic Source of Organo-nitrate from Alberta Oil Sands Emissions
ALEX LEE, Max Adam, John Liggio, Shao-Meng Li, Megan Willis, Jonathan Abbatt, Travis Tokarek, Charles Odame-Ankrah, Jennifer Huo, Hans Osthoff, Kevin Strawbridge, Jeff Brook, National University of Singapore
Abstract Number: 282 Working Group: Aerosol Chemistry
Abstract The emissions from oil sands operations have been the subject of recent studies due to the potential impacts on air quality and climate. In particular, substantial production of secondary organic aerosol (SOA) has been recently reported downwind of Alberta oil sands operations. Although intermediate volatile organic compounds (IVOCs) released from these operations have been shown to be important precursors of the observed SOA, the formation mechanisms and the chemical characteristics of such SOA remain poorly understood.
In this work, we provide evidence that emissions from Alberta oil sands operations result in the formation of large amounts of particle-phase organonitrate (ON) due to the daytime chemistry of IVOCs in the presence of NOx. High-resolution aerosol mass spectrometers (HR-AMS) were deployed in both ground-based and aircraft studies to measure the chemical compositions of non-refractory particulate matter (NR-PM, including, ammonium, nitrate, sulfate, chloride and organic) in August and September, 2013. While the ground observations identified potential sources of organic aerosol (OA) due to various industrial activities in the oil sands region, the flight tracks of aircraft measurements were designed to trace the chemical evolution of OA within the plumes downstream from the oil sands facilities.
Using the –log(NOx/NOy) as a proxy of photochemical age, positive matrix factorization (PMF) of OA fragments identified a fresh SOA factor that contributed approximately 32 wt% of the total OA at the ground site, and was associated with IVOCs measured by the co-located gas-chromatograph-ion trap mass spectrometer. This fresh SOA factor was strongly correlated to the total nitrate (i.e., nitrate salts plus organo-nitrate, r2 > 0.8). The high nitrate fragments ratio (i.e., NO+/NO2+) indicates a significant contribution of ON to the total nitrate, and our estimations suggest that ON could account for up to ~50 wt% of the fresh SOA material. Diurnal patterns of the fresh SOA, nitrate, and NOx showed local peaks at approximately 10:00 and 13:00, suggesting that the observed ON was predominantly produced via photochemistry in the presence of NOx.
The aircraft measurements also observed a strong correlation between ON and OA mass loadings within the plumes that were dominated by the fresh SOA component produced through photochemistry. Using the top-down emission rate retrieval algorithm (TERRA), our preliminary results indicate that the ON production rate in a particular plume was approximately 15 tonnes per day during the summer. In addition to the above observations, photooxiation laboratory experiments were carried out to explore the formation of ON using IVOCs from bitumen vapor as SOA precursors under high NOx conditions. High ON-to-SOA ratios (~0.3-0.5) were observed in the flow tube experiments with an estimated photochemical age of 1-3 days, which are largely consistent with our field observations. Although nocturnal chemistry of nitrate radical and biogenic VOCs has been recognized as significant sources of ON in the atmosphere, our findings highlight the significance of anthropogenic ON production via photochemistry in crude oil production and refining region worldwide.