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Abstract. 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 ± 1σ ) of NO, NO 2 , NO x , NO y , CO, SO 2 , and O 3 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 O 3 source of 5 ppb day −1 , which is small but significant for surface O 3 in winter in Beijing.Correspondence to: X. Xu (xuxb@cams.cma.gov.cn) Downward transport of O 3 -rich air from the free troposphere is the more important factor for the enhancement of the O 3 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 NO y than point sources.

the average concentrations of NO x , CO, SO 2 , O 3 , and O x (= O 3 + NO 2 ) are 29.0 ± 19.4 ppb, 1.52 ± 0.73 ppm, 17.1 ± 12.8 ppb, 28.5 ± 13.8 ppb, and 47.1 ± 8.5 ppb, respectively. The levels of these gases varied greatly from season to season, with the levels of NO x , CO, and SO 2 being higher and the level of O 3 being lower in winter and the levels of NO x , CO, and SO 2 being lower and the level of O 3 being higher in summer. Levels of O 3 and SO 2 often exceed Grades I or II of the Chinese Ambient Air Quality Standards. According to estimation of source contributions, biomass burning seems to be one of the major sources for the air pollutants, particularly CO and NO x . The ground measurements of NO 2 and O 3 at Gucheng correlate well with the satellite measurements of tropospheric columns NO 2 and O 3 , respectively, which suggests that the site has good regional representativeness. Therefore the site is well situated for monitoring regional background concentrations of air pollutants in the North China Plain and an ideal upwind site for studying air quality in Beijing. In August, over 80% of trajectories of more polluted air masses moved northward from Gucheng and passed over Beijing.

Abstract. The real-time measurements of NH 3 and trace gases were conducted, in conjunction with semi-continuous measurements of water-soluble ions in PM 2.5 at a rural site in the North China Plain (NCP) from May to September 2013 in order to better understand chemical characteristics of ammonia and the impact of secondary ammonium aerosols on formation in the NCP. Extremely high NH 3 and NH + 4 concentrations were observed after a precipitation event within 7-10 days following urea application. Elevated NH 3 levels coincided with elevated NH + 4 , indicating that NH 3 likely influenced particulate ammonium mass. For the sampling period, the average conversion / oxidation ratios for NH + 4 (NHR), SO 2− 4 (SOR), and NO − 3 (NOR) were estimated to be 0.30, 0.64, and 0.24, respectively. The increased NH 3 concentrations, mainly from agricultural activities and regional transport, coincided with the prevailing meteorological conditions. The high NH 3 level with NHR of about 0.30 indicates that the emission of NH 3 in the NCP is much higher than needed for aerosol acid neutralisation, and NH 3 plays an important role in the formation of secondary aerosols as a key neutraliser. The hourly data obtained were used to investigate gas-aerosol partitioning characteristics using the thermody-