Diurnal variations of boundary-layer carbon dioxide in a coastal city—Observations and comparison with model results

Climate change expected from increasing atmospheric CO2 concentrations has been studied widely. Further, it is recognized that cities are a major source of anthropogenic CO2 However, few studies of CO2 concentrations in, or near, cities have been conducted. A LI-COR infrared gas analyzer was operated in a suburban region of Vancouver during June 1993. The observed summer-time concentrations show a late afternoon minimum, and overnight maximum around the upwind background concentration. The afternoon CO2 minimum is attributed to the strength of biospheric photosynthesis and strong mixing of local anthropogenic sources within a deep mixed layer. Poor nighttime mixing, lower mixed layer depths, and biospheric respiration account for the observed nighttime maximum, often more than 80 ppmv greater than the background concentration. A numerical multiple-box transport and mixing model was developed to simulate the observed diurnal pattern of CO2 concentration at the suburban site. CO2 emissions inventories for important mobile sources, stationary sources, and biospheric sources and sinks provide input to the model for upwind fetch areas. Other CO2 inputs include advection, entrainment from above the mixed layer and modelled mixed layer depth for the CO2 mixing volume. Close agreement between observations and model results show the importance of boundary layer structure on CO2 concentrations at a specific location. In terms of CO2 the role of the city is placed in its global context.