Gaseous pollutants in Beijing urban area during the heating period 2007-2008: Variability, sources, meteorological, and chemical impacts

摘要

Gaseous pollutants, NO_y/NO_x, SO_2, CO, and O_3, were measured at an urban site in Beijing from 17 November 2007 to 15 March 2008. The average concentrations (with ± 1;sigma;) of NO, NO_2, NO_x, NO_y, CO, SO_2, and O3 were 29.0 ± 2.7 ppb, 33.7 ± 1.4 ppb, 62.7 ± 4.0 ppb, 72.8 ± 4.5 ppb, 1.99 ± 0.13 ppm, 31.9 ± 2.0 ppb, and 11.9 ± 0.8 ppb, respectively, with hourly maxima of 200.7 ppb, 113.5 ppb, 303.9 ppb, 323.2 ppb, 15.06 ppm, 147.3 ppb, and 69.7 ppb, respectively. The concentrations of the pollutants show "saw-toothed" patterns, which are attributable mainly to changes in wind direction and speed. The frequency distributions of the hourly mean concentrations of NO_y, SO _2, CO, and O_3 can all be decomposed in the two Lorentz curves, with their peak concentrations representing background levels under different conditions. During the observation period, the average ratio NO _x/NO_y was 0.86 ± 0.10, suggesting that the gaseous pollutants in Beijing in winter are mainly from local emissions. Data of O _3, NO_z, and NO_x/NO_y indicate that photochemistry can take place in Beijing even in the cold winter period. Based on the measurements of O_3, NO_x, and NO_y, ozone production efficiency (OPE) is estimated to be in the range of 0-8.9 (ppb ppb~(-1)) with the mean(± 1σ) and median values being 1.1(± 1.6) and 0.5 (ppb ppb~(-1)), respectively, for the winter 2007-2008 in Beijing. This low OPE would cause a photochemical O3 source of 5 ppb day~(-1), which is small but significant for surface O_3 in winter in Beijing. Downward transport of O_3-rich air from the free troposphere is the more important factor for the enhancement of the O3 level in the surface layer, while high NO level for the destruction of O_3. The concentrations of SO_2, CO, and NO_x are strongly correlated among each other, indicating that they are emitted by some common sources. Multiple linear regression analysis is applied to the concentrations of NO_y, SO_2, and CO and empirical equations are obtained for the NO_y concentration. Based the equations, the relative contributions from mobile and point sources to NO_y is estimated to be 66 ± 30 % and 40 ± 16 %, respectively, suggesting that even in the heating period, mobile sources in Beijing contribute more to NOy than point sources.