Conclusions and Prospects
In a region, average concentrations of PM exceeded the World Health Organization guideline values, which indicated a serious impact of concentrations on people and the environment of Lanzhou City. Seasonal and diurnal course of pollutants and pollution probabilities were characterized, respectively. They were characterized as the maximum in the winter and summer months in the afternoon. The annual course was largely determined by the seasonal mode of operation of the heat and power unit and vehicles. In summer months, the dependence of the increase in the concentrations of secondary pollutants on the rate of photochemical processes in the atmosphere was revealed. Daily variations in concentrations were associated with atmospheric turbulence.
Seasonal characteristic of air quality in Lanzhou had obvious differences. According to the seasonal distribution, the average concentration of PM2.5, of SO2, of NO2, and of CO quarter value had the same dynamics, i.e., winter > autumn > spring > summer. The mass concentration value of PM10 varied from heavy to light in seasons, winter > spring > autumn > summer, while difference between winter and spring concentration values was not high; spring dust weather increased concentration of PM10; however, strong wind was of importance in the removal of gaseous pollutants and PM2.5; thus the concentrations of PM2.5, of SO2, of NO2, and of CO in spring were a little lower than those in autumn.
The concentrations of all pollutants (in addition to the properties of their sources) significantly depended on the complex of meteorological quantities. It was necessary to attribute the following conditions to unfavorable meteorological conditions of dispersion of impurities in Lanzhou; on the surface of the Earth, unfavorable meteorological conditions of dispersion of impurities were the following: quiet or low wind, prevailing temperature gradients and wind direction, relative air humidity exceeding 60%, fogs and hazes at the height of the leading stream of 3 km, and speeds of west and northwest winds of 9–19 m/s. The range of increased concentrations of nitrogen oxides, maximum of sulfur, and carbon containing in urban atmosphere was mainly related to meteorological parameters. This correlation allowed us to construct a multidimensional regression model of atmospheric pollution.
Atmospheric pollution did not depend only on a complex of meteorological quantities, but it also affected the thermodynamic state of the air, creating “heat islands,” as a feedback. The data obtained from MODIS and Landsat 8 satellites revealed that microclimatic measurements allowed to register increasing air temperature in the city center and in the industrial zone by 1–3 °C comparing to its margins. “Heat island” led to an upward movement of air in the city center during the day, which helped to reduce the concentration of impurities near the surface of the Earth, to some extent. The nature of this phenomenon is related to both the direct release of heat into the atmosphere by municipal services and the radiation heating of the polluted air. The NDVI index indicated a lack of vegetation in urban area.
Aerosol optical depth data indicated that the environmental situation of the city had tended to improve, in comparison with other regions of the country, but it had still been at a level indicating the need for continuous monitoring of aerosols in the atmosphere of the city. Maximum aerosol contamination was observed in spring, due to sandstorms at the current time, indicating a high level of concentration of natural aerosols. The minimum one was registered in the summer-autumn period.
Except for local pollution sources, there are external sources of pollution located outside the study area and associated with passage of contaminants from elevated and desert regions of Western and Northern China, which are based on CALIPSO, OMI, NAAPS, and HYSPLIT and have three main trajectories. They are “north,” associated with air masses passing through the Gobi Desert, Republic of Mongolia, and Inner Mongolia; “northwest,” appearing mainly in the northwestern part of Xinjiang (Gurbantunggut Desert), where air mass go through Inner Mongolia and Ningxia to Lanzhou City; and “western,” originating in the western deserts (Taklamakan Desert in Xinjiang) and in the desert of Qaidam Вasin, where air masses pass through the Tibetan Plateau and Qinghai.
Studies of the dynamics of air pollution index (API in 2001–2012 and AQI in 2013−2016) in Lanzhou City show a tendency to reduce the level of pollution. The API value decreased continuously since 2001–2005; it increased suddenly in 2006. In 2007, the API value was the lowest in 11 years, accounting for 90, air quality achieved the best value, and then the API value increased year by year; by 2011 it fell to 95. From the API average, 2007, 2008, and 2011 were the best years of air quality in the last 11 years; the API average was 90−100. In 2006, there was the most serious pollution than in 6 years before and 6 years after, and the API standard deviation also reached the highest value. In the period of 2013–2016, the AQI gradually decreased and reached the lowest value in 16 years.