Spatiotemporal variability of the near-surface CO 2 concentration across an industrial-urban-rural transect, Nanjing, China

Urban lands are CO emission hotspots. In this paper, we report the CO concentration observations along an industrial-urban-rural transect and in a network of sites in the urban center, in Nanjing, China. The mean CO concentration was highest at the industrial site, not at the densely populated urban center (urban: 429.2±8.7ppm, rural: 421.2±10.0ppm, industrial: 443.88±18.3ppm), based on four sampling periods in four different seasons in 2014 and 2015. At the urban sites, a reversed weekend effect was observed, whereby the weekend CO concentration was higher than the weekday concentration by a mean of 0.9ppm over the four measurement periods and by 8.1ppm in the spring, suggesting higher traffic volume on weekends than on weekdays. The vertical CO gradient was weak above the urban canopy layer, with a mean difference of only 1.1ppm between the 60-m and 110-m measurement heights, reflecting efficient mixing in both daytime and nighttime periods. The average along-wind concentration gradient was 0.25±0.87ppmkm at the height of 110m according to the observations made at five urban sites. Based on a simple box model, we estimated an anthropogenic surface flux of about 0.4mgCO m s for the urban center.