Abstract
Abstract. Vertical column densities (VCDs) of SO2 retrieved by a Pandora spectral sun photometer at Fort McKay, Alberta, Canada, from 2013 to 2015 were analysed. The Fort McKay site is located in the Canadian oil sands region, approximately 20 km north of two major SO2 sources (upgraders), with total emission of about 45 kt yr−1. Elevated SO2 VCD values were frequently recorded by the instrument, with the highest values of about 9 Dobson Units (DU; DU = 2.69 × 1016 molecules cm−2). Comparisons with co-located in situ measurements demonstrated that there was a very good correlation between VCDs and surface concentrations in some cases, while in other cases, elevated VCDs did not correspond to high surface concentrations, suggesting the plume was above the ground. Elevated VCDs and surface concentrations were observed when the wind direction was from south to southeast, i.e. from the direction of the two local SO2 sources. The precision of the SO2 measurements, estimated from parallel measurements by two Pandora instruments at Toronto, is 0.17 DU. The total uncertainty of Pandora SO2 VCD, estimated using measurements when the wind direction was away from the sources, is less than 0.26 DU (1σ). Comparisons with integrated SO2 profiles from concurrent aircraft measurements support these estimates.
Highlights
Sulfur dioxide (SO2) plays an important role in Earth’s atmospheric chemistry and climate
The examples presented above indicate that the opposite may not be always true: elevated vertical column densities may be related to plumes that are above the ground and may not produce high surface concentrations
In order to study variability and changes of Vertical column densities (VCDs) of major pollutants such as SO2 in the Canadian oil sands region, a Pandora sun photometer was installed at Fort McKay in August 2013
Summary
Sulfur dioxide (SO2) plays an important role in Earth’s atmospheric chemistry and climate. The Brewer spectrophotometer operating in the direct-sun (DS) mode measures UV at five wavelengths between 306 and 320 nm to retrieve column ozone and SO2 Such measurements have been used to monitor volcanic SO2 (Krueger et al, 1995) and anthropogenic SO2 in extreme pollution events (De Backer and Muer, 1991; Zerefos et al, 2000). In 2013, a Pandora instrument, along with other instrumentation, was deployed to the oil sands region where relatively high SO2 column amounts are common (McLinden et al, 2012; Simpson et al, 2010) This made it possible to establish optimal retrieval procedures for the Pandora SO2 measurements, estimate uncertainties, and study the relationship between total columns and surface concentrations
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