Airborne and ground-based measurements of aerosol optical depth of freshly emitted anthropogenic plumes in the Athabasca Oil Sands region

Abstract. In this work we report the airborne aerosol optical depth (AOD) from measurements within freshly-emitted anthropogenic plumes arising from mining and processing operations in the Athabasca Oil Sands Region (AOSR) in the context of ground-based AERONET climatological daily averaged AODs at Fort McMurray (Alberta, Canada). During two flights on June 9 and June 18, 2018, the NASA airborne 4STAR (Spectrometers for Sky-Scanning, Sun-Tracking Atmospheric Research) sunphotometer registered high fine-mode (FM, < 1 µm) in-plume AODs of up to 0.4 and 0.9, respectively, in the vicinity of the plume source (< 20 km). Particle composition shows that the plumes were associated with elevated concentrations of sulphates and ammonium. These high AODs significantly exceed climatological averages for June and were not captured by the nearby AERONET instrument (mean daily AODs of 0.10 ± 0.01 and 0.07 ± 0.02, maximum AOD of 0.12) due possibly to horizontal inhomogeneity of the plumes, plume dilution, and winds which in certain cases were carrying the plume away from the ground-based instrument. The average 4STAR out-of-plume (background) AODs deviated only marginally from AERONET daily-averaged values. While 4STAR AOD peaks were generally well correlated in time with peaks in the in-situ measured particle concentrations, we show that differences in particle size are the dominant factor in determining the 4STAR derived AOD. During the two flights of June 24 and July 5, 2018 when plumes likely travelled distances of 60 km or more, the average 4STAR FM AOD increased by 0.01–0.02 over ~ 50 km of downwind particle evolution which was supported by the increases in layer AODs calculated from the in-situ extinction measurements. Based on these observations as well as the increases in organic mass, we attribute the observed AOD increase, at least in part, to secondary organic aerosol formation. The in-plume and out-of-plume AODs for this second pair of flights, in contrast to clear differences in in-situ optical and other measurements, were practically indistinguishable and compared favorably to AERONET within 0.01–0.02 AOD. This means that AERONET was generally successful in capturing the background AODs, but missed some of the spatially constrained high-AOD plumes with sources as close as 30–50 km, which is important to note since the AERONET measurements are generally thought to be representative of the regional AOD loading. The fact that industrial plumes can be associated with significantly higher AODs in the vicinity of the emission sources than previously reported from AERONET can potentially have an effect on estimating the AOSR radiative impact.