CO2 Fluxes and Concentrations in a Residential Area in the Southern Hemisphere

Abstract:

While cities are generally major sources of anthropogenic carbon dioxide (CO₂) emissions, recent research has shown that parts of urban areas may also act as CO₂ sinks due to CO₂ uptake by vegetation. However, currently available results are related to a large degree of uncertainty due to the limitations of the applied methods and the limited number of studies available from urban areas, particularly from the southern hemisphere. In this study, we explore the potential of eddy covariance and tracer measurements (¹³C and ¹⁴C isotopes of CO₂) to quantify and partition CO₂ fluxes and concentrations in a residential urban area in Auckland, New Zealand.

Based on preliminary results from autumn and winter (March to July 2014) the residential area is a small source of CO₂ (0.11 mol CO₂ m^-2 day^-1). CO₂ fluxes and concentrations follow a distinct diurnal cycle with a morning peak between 7:00 and 9:00 (max: 0.25 mol CO₂ m^-2 day^-1/412 ppm) and midday low with negative CO₂ fluxes (min: -0.17 mol CO₂ m^-2 day^-1/392 ppm) between 10:00 and 15:00 local time, likely due to photosynthetic CO₂ uptake by local vegetation. Soil CO₂ efflux may explain that CO₂ concentrations increase and remain high (401 ppm) throughout the night. Mean diurnal winter δ¹³C values are in anti-phase with CO₂ concentrations and vary between -9.0 - -9.7‰. The depletion of δ¹³C compared to clean atmospheric air (-8.2‰) is likely a result of local CO₂ sources dominated by gasoline combustion (appr. 60%) during daytime. A sector analysis (based on prevailing wind) of CO₂ fluxes and concentrations indicates lower CO₂ fluxes and concentrations from the vegetation-dominated sector, further demonstrating the influence of vegetation on local CO₂ concentrations.

These results provide an insight into the temporal and spatial variability CO₂ fluxes/concentrations and potential CO₂ sinks and sources from a city in the southern hemisphere and add valuable information to the global database of urban CO₂ fluxes.