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.