Study of the Urban NO2 Distribution and Emission Assessment Based on Mobile MAX-DOAS Observations

Nitrogen dioxide (NO2) plays a vital role in atmospheric photochemistry. It participates in the catalytic formation of tropospheric ozone (O-3) and also contributes to the formation of secondary aerosols. As an important emission product in transportation and industrial processes, NO2 is usually regarded as a proper indicator of the intensity of the anthropogenic emission. Therefore, research on urban NO2 distribution and emissions is very important for urban air pollution control. During January and February 2018, we conducted 4 times mobile measurements based on Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) in the outer route of urban Hengshui. Furthermore, the spatial distribution of the tropospheric NO2 vertical columndensity (VCD) ranges from 0. 89 x 10(15) similar to 56. 33 x 10(15) molecule . cm(-2) besides the mean value of each measurement is from 22. 42 x 10(15) to 30. 20 x 10(15) molecule . cm(-2). It can be found the NO2 pollution sources in Hengshui are mainly near the external urban factory cluster in the southeast direction and the overpass section in the east of the outer route. The western and northern areas in Hengshui are relatively clean. If the wind comes from this area, it will play a certain cleaning effect on the pollution source area, which can reduce the NO2 concentration in the source area by more than 20%. During the mobile measurements, we also conducted stationary MAX-DOAS observations. Combined with both measurement results, we evaluated the relative contribution of pollution sources in eastern Hengshui. Combined with both measurement results, we evalu-ated the relative contribution of NO2 pollution sources area (eastern part of Hengshui), which is 30. 1%similar to 61. 9% higher than the clean area (western part) and contributes more than 7. 89 x 10(15)similar to 13. 32 x 10(15) molecule . cm(-2). With the supplementary of meteorological information simulated by the WRF model, we can calculate the NO2 emission flux in urban Hengshui, which is 0. 86 x 10(24) molecules . s(-1). This result is relatively lower than other cities in previous studies, which might be caused by two factors: one is the pollution source of Hengshui is not concentrated in the urban area; the other one is due to the research area is only 50 km(2) in this study, which is much smaller than the urban area of other studies. For the measured total flux of Hengshui, we found 96. 16% is from transportation, and 3. 84% is caused by city emission, which indicates that the main pollution source of NO2 in Hengshui City is not located in the inner city. Through the mean value of the OMI tropospheric NO2 with the backward trajectories of air mass during this campaign, we found that Hengshui was also affected by pollution transmission from northern regions (such as Baoding, Langfang) and northwest regions (such as Shijiazhuang). In general, the mobile MAX-DOAS has a good application prospect for urban pollution control, such as for finding the pollution source location, estimating the contribution, and calculating the emission flux of urban areas.

Automated summary

NO2 plays a vital role in atmospheric photochemistry and studying urban NO2 distribution and emissions is crucial for air pollution control. NO2 pollution sources in Hengshui are mainly located in the southeast near factory clusters and in the east along the outer route, while the western and northern areas are relatively clean. Transportation is the main pollution source of NO2 in Hengshui, and the lower emission flux compared to other cities may be due to dispersed pollution sources and the smaller study area.