Black carbon and its correlation with trace gases at a rural site in Beijing: Top-down constraints from ambient measurements on bottom-up emissions

The mass concentrations of black carbon (BC) were measured continuously at Miyun, a rural site near Beijing, concurrently with some trace gases (CO, CO2, NOy, SO2) during the nonheating seasons of 2010 (April to October). The average concentration of BC was 2.26 +/- 2.33 mu g m(-3). About 70%-100% of the air masses arriving at the site from June to September were from the source region of Beijing and the North China Plain (NCP), while in the spring, 40% were of continental background origin. BC had moderate to strong positive correlations with CO (R-2 = 0.51), NOy (R-2 = 0.58), and CO2 (nonsummer, R-2 = 0.54), but not with SO2 (R-2 < 0.1). The observed Delta BC/Delta CO ratio was 0.0050 +/- 0.0001 mu g m(-3)/ppbv for the regional air masses (excluding the influence of biomass burning). This ratio increased by 68% to 0.0084 +/- 0.0004 mu g m(-3)/ppbv after excluding the influence of wet deposition. Accounting further for the impact of atmospheric processes on the observation, we derived an average top-down BC/CO emission ratio of 0.0095 +/- 0.002 mu g m(-3)/ppbv for the source region of Beijing and NCP that is 18%-21% lower than the average emission ratio from the bottom-up inventory of Zhang et al. (2009), whereas the difference is substantially lower than the uncertainty of emissions for either species. The difference between the mean bottom-up and top-down emission ratios is most likely to be attributed to the residential sector, which needs to have a lower share in the total emissions of BC or a much lower BC/CO emission ratio. The industry and transportation sectors are found to be dominant sources of BC from Beijing and the NCP rather than from the residential sector as suggested by the bottom-up inventory.