| Summary: | Climate change expected from increasing atmospheric CO₂ concentrations has been
studied widely (IPCC, 1990). 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 at the Sunset Tower in a
suburban region of Vancouver during different time periods in 1993 and 1994. Sampling
revealed important information on seasonal and diurnal variations. The observed summertime
concentrations show a clear diurnal signal around the expected upwind background
concentration, and are described by a late afternoon minimum, and overnight
maximum. The afternoon CO₂ minimum is attributed to the strength of biospheric photosynthesis
and strong mixing of local anthropogenic sources within a large mixed layer.
Poor nighttime mixing, lower mixed depths, and biospheric respiration account for the
observed nighttime maximum, often more than 80 ppmv greater than the background
concentration.
A simple numerical multiple-box transport 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
are calculated as input to the model for upwind fetch areas. Other CO₂ inputs include
advection, entrainment from above the mixed layer and determination of the mixed layer
depth.
Results of both observations and modelling show large diurnal variation in CO₂ concentrations,
and the importance of boundary layer structure (as defined by the mixed
layer) on concentrations at a specific location. In terms of CO₂, the role of the city is
placed in it global context.
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