Spatial variability of carbon dioxide in the urban canopy layer and implications for flux measurements

This contribution reports CO2 mixing ratios measured in the urban canopy layer (UCL) of a residential neighborhood in Vancouver, BC, Canada and discusses the relevance of UCL CO2 temporal and spatial variability to local-scale eddy covariance (EC) fluxes measured above the UCL. Measurements were conducted from a mobile vehicle-mounted platform over a continuous, 26-h period in the longterm turbulent flux source area of an urban EC tower. Daytime mixing ratios were highest along arterial roads and largely a function of proximity to vehicle traffic CO2 sources. At night, there was a distinct negative correlation between potential air temperature and CO2 mixing ratios. The spatial distribution of CO2 was controlled by topography and micro-scale advective processes (i.e. cold-air pooling). Mobile CO2 measurements were then used to calculate CO2 storage changes (F-S) in the UCL volume and compared to single-layer F-S estimates calculated from the EC system. In total, five variations of F-S were calculated. On average, the choice of F-S calculation method affected net measured hourly emissions (F-C) by 5.2%. Analysis of F-S using a four-year dataset measured at the EC tower show F-S was 2.8% of hourly F-C for this site on average. At this urban EC location, F-S was relatively minor compared to F-C and calculation of F-S using a single-layer method was adequate, though F-S still represents a potentially large uncertainty during individual hours. (C) 2014 Elsevier Ltd. All rights reserved.