Temporal and spatial patterns of carbon dioxide mixing ratios in a subtropical urban environment during spring
The aim of this study was to quantify the temporal and spatial variability of urban CO2
mixing ratios in the low- to medium-density subtropical city of Auckland, New Zealand.
The relations between CO2 mixing ratios and urban land use are examined using a
combination of fixed and mobile measurements. Spring CO2 mixing ratios were
measured in the morning, afternoon and at night along a transect route covering
common urban land uses. Fixed measurements were made at a central urban location, a
residential and a rural location. While results from the fixed measurements show
increased mixing ratios in the central urban location during daytime, the results from the
mobile measurements have shown that CO2 mixing ratios were strongly dependent on
the temporal variability of local CO2 emissions and uptake associated with different
urban land use. Thus, the CO2 dome reported in previous studies was not observed in
this setting. Traffic was likely the dominant influence on morning CO2 mixing ratios
along the mobile measurement route, with highest values observed in industrial areas.
Afternoon CO2 mixing ratios showed a slight decrease with distance from the Central
Business District (CBD). During the night, CO2 mixing ratios were influenced by
biogenic CO2 emissions, particularly when wind speed was low, reaching a maximum
in the rural area. Overall, this study shows that a few mobile measurements are
sufficient to reveal pronounced spatial patterns of urban CO2 mixing ratios, which
cannot be detected by one or two fixed measurement stations.
Urban atmospheric CO2; fixed and mobile measurements; urban land use types; urban
vegetation; CO2 dome; near-surface CO2