Characteristics of NO₂ Tropospheric Column Density over a Rural Area in the North China Plain

NO₂ tropospheric vertical column densities (VCD) at Gucheng Eco-meteorological Observation Experiment Station (GCH) in Hebei Province from September 2008 to September 2010 are retrieved from the scattered sunlight spectra, measured by a multi-axis differential optical absorption spectroscopy (MAX-DOAS), and its characteristics are analyzed then.It shows that the mean seasonal NO₂ tropospheric VCDs are high in winter (5.14×10¹⁶ cm^-2) and low in summer (1.28×10¹⁶ cm^-2). The mean value in winter is a little lower than that in urban Beijing and much lower than that in spring and summer. Averaged diurnal variations are always low at noon and high towards evening for the whole year. The varying amplitude in winter is the biggest among all seasons and the concentration shows a rising up trend in the late afternoon. Less variation can be seen during the day in spring and autumn, and they decrease slowly in summer, reaching a minimum value after noon. The phenomena can be explained by seasonal differences of sources and sinks of tropospheric NO₂.The tropospheric NO₂ over GCH site is mainly affected by the transport from sectors of SW, SSW, NE and ENE, where the major NO₂ emission sources in North China are located. It's interesting to note that the NO₂ tropospheric VCDs over GCH site is higher than that over Xianghe site (another rural site in North China) in winter. Backward trajectories analyses show that in winter two sites have a distinct difference of pollutant sources, which are causes of their discrepancy.Patterns of seasonal variations are quite the same for NO₂ tropospheric VCDs retrieved from MAX-DOAS and satellite observation product OMNO2d, as well as the mixing ratios of in situ NO₂ measurements. Compared with results from MAX-DOAS, a systematic underestimation of OMI satellite observation for NO₂ is found in the rural North China area, as those found in urban Beijing and Shanghai megacities. The individual NO₂ VCDs retrieved from MAX-DOAS and the surface NO₂ mixing ratios are highly correlated, with an apparently higher correlation coefficient (R=0.738) than that between OMNO2d and surface mixing ratios (R=0.639).It's evident that column density levels of tropospheric NO₂ can be much better captured by MAX-DOAS observations than other measurements. As a result, much more emphasis should be given on the development of MAX-DOAS observation network. Furthermore, the application of MAX-DOAS measurements on validation and correction of satellite data should be strengthened.