Publication

Urban lands are CO2 emission hotspots. In this paper, we report the CO2 concentration observations along an industrial-urban-rural transect and in a network of sites in the urban center, in Nanjing, China. The mean CO2 concentration was highest at the industrial site, not at the densely populated urban center (urban: 429.2 +/- 8.7 ppm, rural: 421.2 +/- 10.0 ppm, industrial: 443.88 +/- 18.3 ppm), 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 CO2 concentration was higher than the weekday concentration by a mean of 0.9 ppm over the four measurement periods and by 8.1 ppm in the spring, suggesting higher traffic volume on weekends than on weekdays. The vertical CO2 gradient was weak above the urban canopy layer, with a mean difference of only 1.1 ppm 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 +/- 0.87 ppm km(-1) at the height of 110 m according to the observations made at five urban sites. Based on a simple box model, we estimated an anthropogenic surface flux of about 0.4 mg CO2 m(-2) s(-1)for the urban center. (C) 2018 Published by Elsevier B.V.