Seasonal variations in δ13C and δ18O of atmospheric CO2 measured in the urban boundary layer over Vancouver, Canada in relation to fuel emissions.

Monday, 15 December 2014
Andreas Christen1, Rick Ketler1, Joseph Lee1, Zoran Nesic2, Luitgard Schwendenmann3 and Caitlin Semmens1, (1)University of British Columbia, Geography, Vancouver, BC, Canada, (2)University of British Columbia, Biometeorology Group, Faculty Land and Food Systems, Vancouver, BC, Canada, (3)University of Auckland, Auckland, New Zealand
Recent advances in techniques to measure carbon dioxide (CO2) in urban plumes show potential for validating and monitoring emission inventories at regional to urban scale. A major challenge remains the attribution of elevated CO2 in urban plumes to different fuel and biogenic sources. Stable isotopes are a promising source of additional information.

Here, we report a full year of measurements of CO2 mixing ratios, δ13C and δ18O in CO2 in the urban boundary layer over Vancouver, Canada. The goal of the work is to link seasonally changing isotopic composition to dominant fuel sources and put the urban enhancement into the context of regional background concentrations.

Atmospheric composition in the urban atmosphere was measured continuously using a tunable diode laser absorption system (TGA 200, Campbell Scientific, Logan, UT, USA). In addition, end member signatures were determined by means of bag samples from representative fuel emission sources (gasoline, diesel, natural gas). While δ13C depends on the fuel type and origin (for Vancouver in 2013/14: δ13C gasoline 27.2‰; diesel -28.8‰; natural gas -41.6‰), δ18O is fractionated in catalytic converters (d18O gasoline vehicles -12.5‰; diesel -18.6‰; natural gas -22.7‰) and exhibits higher variability between samples. Additional signatures were determined for human, soil and plant respiration.

During the study year, monthly mean mixing ratios in the urban atmosphere ranged between 410.5 (Jul) and 425.7 ppm (Dec), which was on average 18 ppm elevated above the regional background. As expected, mean monthly δ13C was lower in winter than summer with seasonally changing intercepts between -33.6‰ (JJF) and -27.7‰ (MJJ). Making the simple assumption that natural gas and gasoline are the only major fuel sources, natural gas would contribute ~45% to emissions in winter and ~3% in early summer, which is lower than the downscaled Local Emissions Inventory (57% in winter and 20% in summer). Mean δ18O showed a clear annual cycle with higher monthly intercepts in summer (-15.6 ‰) than in winter (-20.2 ‰). Various mixing models were explored to characterize contributions to the urban enhancement and analyzed in combination with meteorological data.