Formation and causes of NO x pollution on the east side of the Taihang Mountains in China

Tropospheric column amounts of NO2 from OMI/Aura products were applied to investigate the seasonal variation of NO x on the east side of the Taihang Mountains (ESTM) in China. The NO2 concentration varied seasonally, with winter maxima and summer minima. To explore the causes of the heavy NO2 pollution in this region, wind field simulations were conducted by using the Fifth-Generation NCAR/Penn State Mesoscale Model (MM5). MM5 simulations showed that wind directions in the ESTM region were dominated by northeasterly and northwesterly winds throughout the year and to a lesser degree by southwesterlies. Elevated NO2 concentrations (higher than 50×1015 molec/cm2) occurred in three meteorological situations: convergent wind fields (5 days), northeasterly winds (2 days), and calm days (1 day) in January. Although the local anthropogenic NO x emission strength was not so high as that in other developed areas, the calms and frequent occurrence of convergent wind fields, as well as unfavorable terrains for diffusion, resulted in the accumulation of atmospheric pollutants and hence heavy NO x pollution. Furthermore, long-distance NO x transportation from the Beijing-Tianjin-Tangshan region to the ESTM area could be seen when the wind was from a northeasterly direction, especially under high wind speeds.