Optical remote sensing of industrial gas emission fluxes

Doctoral thesis, 2016

Mobile optical remote sensing techniques offer promising possibilities to quantify and geographically attribute local industrial gaseous emissions to the atmosphere. Studies have shown that such emissions are often poorly understood, underestimated, and thereby not properly accounted for in emission inventories and regional atmospheric chemistry models, especially for emissions of VOCs. A better understanding and quantification of industrial VOC emissions is crucial for combating ground-level ozone, a serious problem facing most of the world's larger urban areas. This thesis presents results from a number of measurement campaigns primarily conducted in the area around Houston, Texas, USA, which has one of the world's largest concentrations of oil, gas and petrochemical industries. In the campaigns, the two flux measurement methods Solar Occultation Flux (SOF) and Mobile DOAS were used to quantify emissions of VOCs (alkanes and alkenes), SO2, NO2, and formaldehyde (HCHO) from the largest industrial conglomerates in the area. Measured emissions are compared to industry estimates reported to emission inventories, showing discrepancies of up to an order of magnitude for VOCs but not for SO2 and NO2. Emission factor models are used to estimate effects on VOC emissions of unrepresentative meteorological conditions during the measurement campaigns. These effects are determined to be too small to explain the discrepancies between measurements and reported emissions seen for VOCs. A plume chemistry model is applied to a number of cases where formaldehyde were detected together with significant amounts of alkenes in order to assess whether the formaldehyde could be explained as a secondary pollutant from the oxidation of alkenes. The results of the modeling shows that secondary emissions can only explain a small fraction of the measured formaldehyde flux in most cases, suggesting that most of the formaldehyde emissions measured from local sources were primary emissions. Secondary emissions are, however, still believed to be the largest source of formaldehyde further downwind from sources. Mobile multi-axis DOAS measurements of absolute vertical columns of NO2 and HCHO were performed in the Houston Ship Channel as a part of the 2013 NASA DISCOVER-AQ campaign in Houston. An improved methodology for deriving the vertical columns from the measured slant columns is suggested and used. The measurements are compared to several other measurement methods employed in the campaign and a significant level of agreement is found.