Impact of the Loess Plateau on the atmospheric boundary layer structure and air quality in the North China Plain: a case study

The North China Plain (NCP), to the east of the Loess Plateau, experiences severe regional air pollution. During the daytime in the summer, the Loess Plateau acts as an elevated heat source. The impacts of such a thermal effect on meteorological phenomena (e.g., waves, precipitation) in this region have been discussed. However, its impacts on the atmospheric boundary layer structure and air quality have not been reported. It is hypothesized that the thermal effect of the Plateau likely modulates the boundary layer structure and ambient concentrations of pollutants over the NCP under certain meteorological conditions. Thus, this study investigates such effect and its impacts using measurements and three-dimensional model simulations. It is found that in the presence of daytime westerly wind in the lower troposphere (~1 km above the NCP), warmer air above the Loess Plateau was transported over the NCP and imposed a thermal inversion above the mixed boundary layer, which acted as a lid and suppressed the mixed layer growth. As a result, pollutants accumulated in the shallow mixed layer and ozone was efficiently produced. The downward branch of the thermally-induced Mountain-Plains Solenoid circulation over the NCP contributed to enhancing the capping inversion and exacerbating air pollution. Previous studies have reported that low mixed layer, a factor for elevated pollution in the NCP, may be caused by aerosol scattering and absorption of solar radiation, frontal inversion, and large scale subsidence. The present study revealed a different mechanism (i.e., westerly warm advection) for the suppression of the mixed layer in summer NCP, which caused severe O3 pollution. This study has important implications for understanding the essential meteorological factors for pollution episodes in this region and forecasting these severe events.

Tropospheric Ozone in Louisiana and Synoptic Circulation

Tropospheric ozone (O3) is a pollutant of increasing concern in many urban areas in the United States. There is an increasing need to understand the geographical and meteorological properties associated with O3, particularly because of the changing criteria that are being implemented by the U.S. Environmental Protection Agency to monitor O3. This research examines the relationship between O3 mixing ratios in Louisiana and surface and low-tropospheric synoptic circulation patterns. Results suggest that local conditions and synoptic influences are both important in determining the behavior of observed O3 in Louisiana during this period in which “exceedance” frequencies decreased until 2000–01, at which time they increased again. Furthermore, the expected pattern of surface anticyclonic activity, low-tropospheric ridging, weak pressure gradients, and subsidence from the lower troposphere is found to be associated with anomalously high O3, both at most local sites in the state and on days with anomalously high O3 statewide. More surprising, however, is the tendency for high O3 to be associated with low surface pressure in the Great Plains, perhaps in advance of a midlatitude wave cyclone to the north. This and other surface patterns may be linked to advection from the southeastern Texas urban–industrial corridor. A temporally increasing tendency for surface and lower-tropospheric ridging over the 1994–2001 study period provides at least a partial explanation for the absence of a frequency decline in statewide anomalously high ozone days during a time of increasing public awareness and concern for meeting the O3 standard.

Role of Deep Cloud Convection in the Ozone Budget of the Troposphere

Convective updrafts in thunderstorms prolong the lifetime of ozone (O(3)) and its anthropogenic precursor NOx [nitric oxide (NO) + nitrogen dioxide (NO(2))] by carrying these gases rapidly upward from the boundary layer into a regime where the O(3) production efficiency is higher, chemical destruction is slower, and surface deposition is absent. On the other hand, the upper troposphere is relatively rich in O(3) and NOx from natural sources such as downward transport from the stratosphere and lightning; convective overturning conveys the O(3) and NOx toward the Earth's surface where these components are more efficiently removed from the atmosphere. Simulations with a three-dimensional global model suggest that the net result of these counteractive processes is a 20 percent overall reduction in total tropospheric O(3). However, the net atmospheric oxidation efficiency is enhanced by 10 to 20 percent.