Measurements of carbon dioxide in an Oregon metropolitan region

Ambient concentrations of atmospheric carbon dioxide (CO2) are reported for the Portland, Oregon (USA) metropolitan region for the late July through December, 2009 period. Three stationary locations were established: a downtown location on the campus of Portland State University; a residential site in southeast Portland; and a rural station on Sauvie Island, located ∼30 km northwest of Portland in the Columbia River Gorge. Continuous measurements of CO2 at each site average 403–408 ppm and show considerable variability at each site (360–610 ppm) due to CO2 sources, sinks and meteorological variability. Within this variability, a marked 20–30 ppm diurnal cycle is observed due to photosynthetic activity and variations in the planetary boundary layer. In-city CO2 concentrations are on average enhanced by 5–6 ppm over the Sauvie Island site during upgorge wind conditions, a difference which is greatest in the afternoon. Measurements of the 13C/12C ratio of CO2 in downtown Portland are significantly depleted in 13C relative to 12C compared with background air and suggest that regional CO2 is dominated by petroleum sources (75–80%). High degrees of relationship between CO2 variability and primary air pollutants CO and NO (r2 = 0.80 and 0.77), measured by the Oregon Department of Environmental Quality at the Southeast Portland location, corroborate this finding and illustrate the importance of traffic emissions on elevated ambient CO2 concentrations.

Research highlights
► We study carbon dioxide in the Portland, Oregon metropolitan region.
► In-city concentrations are enhanced by 5–6 ppm on average above an upwind site.
► Carbon isotopic composition is depleted due to regional fossil sources.
► Correlations with primary pollutants indicate traffic emissions as large source.
► Isotopic composition indicates that petroleum is 75–80% of the total.