Impacts of NO on Urban Air Quality and Causes of Its High Ambient Levels: Insights from a Relatively Long-Term Data Analysis in a Typical Petrochemical City in the Bohai Bay Region, China.

The ambient levels of NO in urban areas in China in recent years have generally shown a downward trend, but high NO concentrations still exist under certain conditions, and the causes for such phenomenon and its impact on air quality remain unclear. Taking Dongying, a typical petrochemical city in the Bohai Bay of China, as an example, this paper analyzed the influence of NO on urban air quality and investigated the causes for the formation of NO with high concentrations. The results indicated that higher daily NO concentrations (>40 μg/m) mainly occurred during January-April and September-December each year, and higher hourly NO concentrations mainly occurred during the nighttime and morning rush hour in Dongying from 2017 to 2023. With the increase in daily NO concentrations, the daily air pollution levels showed a general increasing trend from 2017 to 2023. The occurrence of high NO values in Dongying was affected by the combination of unfavorable meteorological conditions, local emissions and regional transports, and localized atmospheric chemical generation. High-pressure and uniform-pressure weather patterns in 2017-2022, along with land-sea breeze circulation in 2022, contribute to high NO concentrations in Dongying. Boundary layer heights (BLH) in spring (-0.43) and winter (-0.36), wind direction in summer (0.21), and temperature in autumn (-0.46) are the primary meteorological factors driving NO-HH (High hourly NO values), while BLH (-0.47) is the main cause for NO-HD (High daily NO values). The titration reaction between NO with O is the main cause for NO-HH in spring, summer and autumn, and photochemical reactions of aromatics have a significant influence on NO-HD. NOx emissions from the thermal power and petrochemical industry in Dongying and air pollution transports from western and southwestern Shandong Province (throughout the year) and from the Bohai Sea (during spring and summer) had serious adverse impact on high NO values in 2022. The results of the study could help to provide a scientific basis for the control of NO and the continuous improvement of air quality in Dongying and similar petrochemical cities.