Effect of the "coal to gas" project on atmospheric NOX during the heating period at a suburban site between Beijing and Tianjin

To explore the impact of the "Coal to Gas" project on air pollution around Beijing, the pollution observation data during the heating periods (from Nov.15 to Mar.15) of 2017 and 2018 at Xianghe Observatory in Langfang, Hebei Province of China, were analyzed in combination with meteorological data and fuel incremental statistics. According to this project, Xianghe County reduced coal consumption and increased gas consumption by 26.71 million cubic meters in 2018. Our observations showed that some of the air pollutants in the Langfang area were effectively improved by changing fuel from coal to natural gas, including SO2, PM2.5, PM10 and CO; the average concentrations of them in the heating period of 2018 significantly dropped compared with those in 2017, with decreases of 46.5%, 13.1%, 21.0% and 10.5%, respectively. However, the concentration of nitrogen oxides (NOX = NO+NO2) increased significantly. The average NOX concentration increased from 78.7 to 116.1 mu g/m(3), while the concentrations of NO2 and NO increased by 24.8% and 76.8%, respectively. Such a significant increase in NOX will bring about changes in the atmospheric oxidation process and influence human health. Changes in meteorological elements were not the main cause of elevated NOX because the total averaged PLAM changing rate and the PLAM changing rates in most of the months indicated a more favorable condition for the diffusion of pollutants. Through the comparison of daily changes in various pollutants during holidays and working days, the impact of motor vehicle emissions on the increase in NOX was essentially ruled out. Compared with that in the heating period of 2017, the significantly higher NOX in 2018 mainly appeared in early morning, which was consistent with the main heating period, and the differences in NOX between holidays and working days were found to be related to sectional heating. At the same time, it was found that the average concentration of NO2 from Nov. 2018 to Mar. 2019 increased in all 12 cities in the Beijing-Tianjin-Hebei region compared with the same period of the previous year, and more than half of the cities had similar variation characteristics of NO2 than that of Langfang. Heating gas was the main factor causing the increase in NOX concentration in Langfang city during the heating period in 2018. Excluding the impact of meteorological conditions, the "Coal to Gas" project brought about 51.8% increase in NOX in the southeast area of Beijing during the heating period of 2018, as well as 8.8%, 16.7%, 42.2% and 10.5% decrease of PM2.5, PM10, SO2 and CO, respectively.