Investigating the Applications of the CO:CO2ff Emission Ratio to Develop a Greater Understanding of Fossil CO2 Sources in Auckland, New Zealand
Authors:   , Young Hayden 
Published: July 2022

<p><b>As the need for climate action becomes increasingly more important, cities have become focal points for observing carbon emissions derived from human activity. Despite making up 3% of Earth's surface area, urban areas contribute ~70% of the world's fossil fuel-derived carbon dioxide (CO2ff). Since cities display a wide range of emission sources, it is useful to measure their emission ratios of CO to CO2ff (CO:CO2ff) to identify local sources, as each has its own unique signature.</b></p>
<p>As part of the CarbonWatch-NZ research programme, air samples were collected in flasks at 28 sites around Auckland and measured to determine CO:CO2ff. The sample locations were categorised into seven site types to observe the emission ratio around Auckland, namely motorway, urban, suburban, and downwind sites. Flasks collected at the motorway sites were expected to reflect the emission ratio of traffic and observed a CO:CO2ff of 14.3 +- 0.7 ppb/ppm. By comparing the traffic CO:CO2ff to CO:CO2ff measured at other sites, the relative influence of traffic at each site type was investigated. The similarity between the suburban CO:CO2ff (13.5 +- 1.3 ppb/ppm) and the traffic CO:CO2ff suggests that suburban emissions are strongly dominated by traffic during the daytime hours, the period of flask collection. The lower CO:CO2ff at the urban sites (10.9 +- 0.5 ppb/ppm) suggests that urban emissions are made up of more than just traffic, with strong contributions from residential, commercial, and industrial sources reducing the observed emission ratio. Finally, the downwind sites were believed to have the best overall representation of CO:CO2ff for Auckland city (10.8 +- 0.7 ppb/ppm) and showed a good agreement to the expected CO:CO2ff calculated from the 2016 Auckland air emissions inventory (11 ppb/ppm). The observed distribution of sources provides an independent validation of the expected CO2ff emission sources determined from the Mahuika model, a high-resolution CO2ff inventory for Auckland. </p>
<p>Another application of CO:CO2ff is that when combined with high-resolution in situ CO data, it can be used to estimate high-resolution CO2ff (CO2ffCO) at a given location. CO2ff can normally only be calculated by collecting flask samples of air which is unrealistic for high-resolution measurements. One caveat is that the greater the diurnal fluctuation in CO:CO2ff, the greater the uncertainty in CO2ffCO. To determine if CO:CO2ff was stable enough in Auckland to estimate CO2ffCO accurately, CO2ff was calculated from flask samples over a 48-hour period at the Auckland University of Technology and compared to CO2ffCO measured using in situ CO data and the urban CO:CO2ff calculated previously. Results showed a fairly good agreement between the values suggesting that CO can plausibly be used as a high-resolution tracer for CO2ff in Auckland.</p>
<p>To measure CO2ff, 14C measurements were made for each flask sample. Since measurement precision is a critical component of atmospheric research, the effect of sample size on 14C measurement precision was also investigated. For smaller samples, measurement precision decreases and contamination has a greater overall effect. To identify the mass threshold above which samples can be confidently processed in the laboratory, samples ranging from 0.08 to 0.71 mgC were analysed in XCAMS, the accelerator mass spectrometer at the National Isotope Centre. The mass threshold was determined to be 0.3 mgC.</p>