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

Abstract

Climate change expected from increasing atmospheric CO 2 concentrations has been studied widely. Further, it is recognized that cities are a major source of anthropogenic CO 2 However, few studies of CO 2 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 CO 2 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 CO 2 concentration at the suburban site. CO 2 emissions inventories for important mobile sources, stationary sources, and biospheric sources and sinks provide input to the model for upwind fetch areas. Other CO 2 inputs include advection, entrainment from above the mixed layer and modelled mixed layer depth for the CO 2 mixing volume. Close agreement between observations and model results show the importance of boundary layer structure on CO 2 concentrations at a specific location. In terms of CO 2 the role of the city is placed in its global context.