Person:

Munger, J.

Urban areas are directly or indirectly responsible for the majority of anthropogenic (CO_2) emissions. In this study, we characterize observed atmospheric (CO_2) mixing ratios and estimated (CO_2) fluxes at three sites across an urban-to-rural gradient in Boston, MA, USA. (CO_2) is a well-mixed greenhouse gas, but we found significant differences across this gradient in how, where, and when it was exchanged. Total anthropogenic emissions were estimated from an emissions inventory and ranged from (1.5 to 37.3 mg·C·ha^{-1}·yr^{-1}) between rural Harvard Forest and urban Boston. Despite this large increase in anthropogenic emissions, the mean annual difference in atmospheric (CO_2) between sites was approximately 5% ((20.6 \pm 0.4 ppm)). The influence of vegetation was also visible across the gradient. Green-up occurred near day of year 126, 136, and 141 in Boston, Worcester and Harvard Forest, respectively, highlighting differences in growing season length. In Boston, gross primary production—estimated by scaling productivity by canopy cover—was ~75% lower than at Harvard Forest, yet still constituted a significant local flux of (3.8 mg·C·ha^{-1}·yr^{-1}). In order to reduce greenhouse gas emissions, we must improve our understanding of the space-time variations and underlying drivers of urban carbon fluxes.

Large intra-season differences in mixing ratios of CO and (O_3) were detected at Miyun, a rural site north of Beijing, in summer 2006. Despite an increase in mean daytime mixing ratio of CO from 500 ppbv in June to 700 ppbv in July, mean daytime (O_3) dropped from 67 ppbv in June to 50 ppbv in July and August. The observed changes in CO and (O_3) are attributed to the influence of the summer monsoonal circulation that develops over the North China Plain in July. Photochemical production of (O_3) is reduced as a consequence of increased cloudiness during July and August, as indicated by the strong negative correlation observed between (O_3) and satellite observations of cloud optical depth, with cloudiness having little effect on CO. The analysis suggests a strategy for emission controls that could be implemented in an economically efficient manner to minimize the frequency of high levels of (O_3) during summer in Beijing.