Temporal And Spatial Distribution Characteristics Of Indoor And Outdoor Atmospheric Nitrogen Dioxide In Suburban Campuses Of Northern Industrial Citie

Nitrogen dioxide（NO₂）,an atmospheric pollutant harmful to human health and the atmosphere.The common source of atmospheric NO₂ in cities and surrounding areas are fossil fuel combustion.Till now,atmospheric NO₂ concentration around the road has been studied extensively,but the spatiotemporal heterogeneity effects of traffics on atmospheric NO₂have not been carefully distinguished.Considering the high nitrogen deposition of some cities in China,NO_x originated from the green spaces may be an important source of local atmospheric NO₂.However,the relationship of soil NO_xemissions on atmospheric NO₂ was rarely reported.Besides the ambient NO₂pollution,indoor NO₂ pollution mainly originated from the fuel burning and cigarette smoke.Foreign countries have carried out more detailed research on NO₂,while the research in China was not up to the mark,especially the lack of research on the characteristics of NO₂ pollution in the centralized catering industry.Here,we monitored the indoor and outdoor atmospheric NO₂ concentrations over four seasons in Shenyang University of Chemical Technology by using passive Willems-type atmospheric NO₂ sampler.We measured 12 outdoor sampling points（later adjusted to 13,centered on these outdoor sampling points,with different green areas with a radius of 10 m,and the distance from roads and parking lots was different）,the two-week average NO₂ concentration of the 5indoor sampling points was recorded synchronously,and the NO₂ concentration of the Shenyang ambient air quality monitoring station was recorded synchronously.The results showed that except for winter,soil NO_x emissions in campus green spaces contributed significantly to the NO₂concentration of the ambient atmosphere.The ratio of NO₂ increments to background values caused by soil NO_x emissions increased with the climate warming after the spring until the end of spring,and then decreasing sequentially until the autumn.The NO₂ concentration at the end of spring was 1.02～1.22 fold compared with background values,indicating that the contribution of soil sources exceeded than that of other anthropogenic sources.Intriguingly,the temperature is the highest in summer,but increasement of NO₂ concentration was lower than spring.The reasons could be the continuous loss of soil nitrogen reservoirs through microbial transformation,vegetation absorption and soil water leaching induced reduction of soil nitrogen reservoirs.Due to the soil nitrogen reservoir and temperature reduction,the background value and the increasement of NO₂was very low in autumn.The NO₂ increase caused by soil NO_x emissions reached its highest value in the early spring（up to 25.3μg/m³）and then decreased sequentially to reach the lowest value in the autumn.Besides the soil originated NO₂,the intensity of atmospheric convection effect can’t be ignored.Due to the abundance of soil nitrogen reservoirs,soil NO_xemissions increase with the climate warming,weak atmospheric convective effect,and impotent mixed dilution effect of pollutants emitted by pollution sources,the NO₂concentration was highest in the early spring（exceeded than the 40μg/m³ specified in the ambient air quality standard GB 3095-2012）.To reduce the NO₂ pollution,NO_xemissions from anthropogenic sources should be tightly controlled,besides reducing anthropogenic NO_x emissions directly.In addition,motor vehicle NO_x emissions cause0.4 to 0.48 increment compared with background value.However,the motor vehicle induced increasement is obviously affected by the speed/direction of wind,and the high concentration of NO_x emissions in the cold start stage of the motor vehicle will also increase the impact of the pollution source on the atmospheric NO₂ concentration.Further studies are urgently needed to identify the atmospheric NO₂ pollution characteristics near the parking lot.Compared with the outdoor atmospheric NO₂concentration,the NO₂ concentration in the canteen and the north and south guard room on campus was significantly improved,indicating that gas stoves and smoking are important indoor pollution sources.The indoor NO₂ concentration in the canteen and the guard room showed a seasonal variation characteristic,basically following the characteristics of winter>autumn and spring>summer,indicating that ventilation condition is a crucial to indoor NO₂ concentration.To ensure the human health,serious NO₂ pollution caused by gas combustion and smoking can be alleviated by strengthening ventilation,improving energy composition,replacing gas with electric energy,improving the industry standard of gas,and restricting personal behavior,such as prohibiting indoor smoking.In summary,it is important to identify the source and the influencers of indoor and outdoor NO₂ pollution,providing a solid foundation for future prevention of air pollution.