Spatio-temporal Analysis of the factors affecting NOx concentration during the evaluation cycle of high pollution episodes in Tehran metropolitan

Abstract

Tehran, the capital of Iran, experiences high levels of pollution, particularly during the cold seasons. Recent studies have shown an increase in the concentration of nitrogen compounds in Tehran. However, the role of advection, deposition, vertical diffusion processes and chemical conversion of NOx in this region requires further investigation. In this study, The Weather Research and Forecasting model with Chemistry (WRF-Chem) was employed to numerically simulate each of these processes during a severe pollution episode in winter from November 28, 2021, to December 8, 2021. During this high pollution episode, the weather conditions were calm, and based on the Air Quality Index (AQI), the air quality was unhealthy for sensitive groups. The presence of a ridge at the 500 hPa level, the placement of a low-level high-pressure cell over the region, and consequently, atmospheric subsidence and low wind speed create favorable conditions for severe pollution events in Tehran. Additionally, temperature inversion during the night and a decrease in the boundary layer height contribute to increased surface NOx concentration during this period. When comparing different processes influencing NOx concentrations, it has been observed that advection has the most significant impact, while dry deposition and chemical conversion have the least effect. On stable days advection increases NOx concentration in central and northeast of Tehran urban area during daytime. The maximum vertical diffusion is observed in the southern and western regions, corresponding to the emission pattern of NO and NO2. The examination of the vertical profile of advection and vertical diffusion shows that vertical diffusion dominates near the surface layer, while advection dominates at higher altitudes in terms of NOx concentration changes. Under unstable conditions (Dec-02), with arrival of the cold front and the influence of convective clouds, the impact of convective transport becomes more pronounced in reducing NOx concentrations in the southern and central regions of Tehran urban area. The highest NOx deposition is observed in the early morning in the southern and southwestern suburban areas of Tehran, correlating with wind direction variations, NOx concentration, and vegetation coverage. The results indicate that nearly 98% of the total NOy deposition (dry and wet deposition of NO, NO2, HNO3, and NO3) in Tehran's urban and suburban area is attributed to dry deposition of NO2.