Aircraft Measurements of the Deposition of Gaseous Organic Carbon: Towards an Understanding of the Impacts on Ecosystem Health and Air Quality Processes

Abstract:

Traditionally, measurements of the deposition of gases have been obtained through ground based approaches, often using gradient or eddy flux methods. Such studies have focused on inorganic species such as SO₂ and ozone, with very limited investigation of the deposition of organic gases. However, the deposition of organic carbon represents an important atmospheric loss process and a potential input of carbon to ecosystems, both of which have not been fully explored or understood. A recently developed approach utilizing aircraft data provided an opportunity to quantify the magnitude of deposition for total organic gas-phase carbon. High time resolution measurements of total gas-phase organic carbon (excluding methane) were made aboard an aircraft using a pair of gas analyzers (Picarro G2401-M) which measured CO₂, CO, and CH₄, one of which sampled behind a Platinum catalyst which quantitatively combusted all gaseous organic species to CO₂. The difference between combusted and non-combusted sample streams allowed for the determination of total gas-phase carbon, which was used as input into a Top-down Emission Rate Retrieval Algorithm (TERRA). The difference between TERRA derived transfer rates for successive flight screens was used to derive the total organic carbon deposition rates and deposition velocities during a given 3-4 hour flight. This approach was used during a recent field campaign in the Alberta Oil Sands region (spring and summer of 2018), which contains the world’s largest deposits of commercially extractable bitumen and is a significant source of gas-phase organic emissions, at times upwind of sensitive ecosystems. The results of the current work demonstrated that upwards of ≈30-50 Tonnes/hr of gas-phase organic carbon is emitted from oil sands activities, of which approximately 30-50% is deposited to the surface within 3-4 hours, corresponding to 12-25 Tonnes C/hr deposited to downwind ecosystems. This may represent an important atmospheric input, which has previously been assumed to be a negligible carbon source. The results also suggest that the lifetime of atmospheric organic carbon may be shorter than previously considered. The implications of this gaseous carbon deposition to nearby ecosystems and subsequent air quality processes through regional air quality modelling will be discussed.