Missing Intermediate- and Semi-Volatile Secondary Organic Aerosol Precursors Comprise the Majority of Total Gas-Phase Organic Carbon Emissions from Oil Sands Operations

DREW GENTNER, Megan He, John Liggio, Jenna Ditto, Lexie Gardner, Tori Hass-Mitchell, Christina Chen, Peeyush Khare, Jo Machesky, Bugra Sahin, John Fortner, Katherine L. Hayden, Jeremy Wentzell, Richard Mittermeier, Amy Leithead, Patrick Lee, Andrea Darlington, Samar Moussa, Shao-Meng Li, Yale University

     Abstract Number: 461
     Working Group: Aerosol Sources and Constituents of Emerging Importance and Their Impacts across Spatial Scales

Abstract
Oil sands are an increasingly major source of unconventional oil with approximately 2 million barrels produced per day in Alberta, Canada through both open-pit surface mining and in-situ techniques. While emissions of volatile organic compounds (VOCs) are routinely reported, emissions inventories cannot explain large observed enhancements in secondary organic aerosol (SOA) production, inferring substantial “missing” emissions.

Aircraft-based measurements using a diverse suite of online and offline instruments were collected in the Athabasca Oil Sands region. Using measurements of total gas-phase organic carbon (excluding CH₄, CO, CO₂) and the Top-down Emission Rate Retrieval Algorithm (TERRA), hourly emission rates of total gas-phase organic carbon ranged 2-40 tonnes C hr^-1. This translated to approximately 200,000-500,000 tonnes C yr^-1 for the largest surface mining facilities when scaled up via established NOx inventories.

These observed total organic carbon emissions greatly exceeded (e.g., 10-20x) those reported to national and regional emissions inventories. However, traditional VOCs and oxygenated VOCs each only accounted for a minor portion of the total carbon (i.e., <20% each), measured via whole air canister samples and online mass spectrometers (i.e., PTR-ToF-MS; Iodide-CIMS). Detailed chemical speciation of concurrent offline adsorbent tubes analyzed via gas chromatography with high-resolution mass spectrometry revealed abundant complex mixtures of intermediate-volatility and semi-volatile organic compounds (IVOCs, SVOCs) with maxima ranging C₁₈-C₂₂. On average, these IVOC-SVOCs accounted for 60% of measured total gas-phase organic carbon and were consistent with the weathered composition of oil sands measured in laboratory experiments. Further analysis of both airborne and laboratory data demonstrates the influence of non-combustion-related emissions pathways across extraction, processing, and waste management.

With total gas-phase organic carbon emissions around 1% of oil production and major gaps in inventory estimates, the results highlight the importance of routine IVOC-SVOC monitoring and lifecycle-wide emissions inventories with important implications for downwind SOA production and carbon deposition.