Abstract
Abstract. Top-down approaches to measure total integrated emissions provide verification of bottom-up, temporally resolved, inventory-based estimations. Aircraft-based measurements of air pollutants from sources in the Canadian oil sands were made in support of the Joint Canada–Alberta Implementation Plan for Oil Sands Monitoring during a summer intensive field campaign between 13 August and 7 September 2013. The measurements contribute to knowledge needed in support of the Joint Canada–Alberta Implementation Plan for Oil Sands Monitoring. This paper describes the top-down emission rate retrieval algorithm (TERRA) to determine facility emissions of pollutants, using SO2 and CH4 as examples, based on the aircraft measurements. In this algorithm, the flight path around a facility at multiple heights is mapped to a two-dimensional vertical screen surrounding the facility. The total transport of SO2 and CH4 through this screen is calculated using aircraft wind measurements, and facility emissions are then calculated based on the divergence theorem with estimations of box-top losses, horizontal and vertical turbulent fluxes, surface deposition, and apparent losses due to air densification and chemical reaction. Example calculations for two separate flights are presented. During an upset condition of SO2 emissions on one day, these calculations are within 5 % of the industry-reported, bottom-up measurements. During a return to normal operating conditions, the SO2 emissions are within 11 % of industry-reported, bottom-up measurements. CH4 emissions calculated with the algorithm are relatively constant within the range of uncertainties. Uncertainty of the emission rates is estimated as less than 30 %, which is primarily due to the unknown SO2 and CH4 mixing ratios near the surface below the lowest flight level.
Highlights
Aircraft-based measurements have been previously used to derive emission rates from point and area sources of compounds including CO2, CH4, CO, NOx, and SO2
From flux–gradient relations, it can be shown that wind speeds follow a stability-dependent log profile (Garratt, 1996), which can be compared to a least-squares fit of U to ln(z) as
U∗ is the friction velocity; κ = 0.4, z is the flight altitude; zg is the ground height beneath the flight path; d is a displacement height and z0 is the roughness length, which are both characteristic of the terrain and surface characteristics; and is a stability correction, which depends on atmospheric conditions
Summary
Aircraft-based measurements have been previously used to derive emission rates from point and area sources of compounds including CO2, CH4, CO, NOx, and SO2 (see Table 1 for references). This analysis is accomplished by flying downwind and/or around the source, in some cases at multiple heights, and inferring the emissions rate based on a mass-balance analysis. This top-down approach offers an advantage over a bottom-up, inventory-based estimation as it attempts to capture the total integrated emissions, some of which may be missed by inventories or difficult to assess, for large and complex industrial facilities spanning tens to hundreds of square kilometres that are comprised of a large number of activities.
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