Studies On Dry And Wet Deposition Of Atmospheric Reactive Nitrogen And Air Pollution Control Effects In China

In China, anthropogenic reactive nitrogen (Nr) emissions have increased dramatically with rapid economic growth since the late 1970s, which directly resulted in an increase of atmospheric N deposition, leading to adverse impacts on human health and terrestrial and aquatic ecosystems. Additionally, excess anthropogenic Nr emissions also can induce intense air pollution. In order to prevent further deterioration of air quality, the 12th Five-Year Plan (2011-2015) of China targeted reduction of national NOx as well as controls on SO2 and primary particle emissions. This study used airborne concentration measurements and inferential models and precipitation guage method to systematically investigated the spatial and temporal variations of atmospheric and rainwater Nr concentrations and N deposition fluxes. In addition, characteristics of Nr species and PM2.5 for three typical land-use types in the North China Plain was exmined, and the effectiveness of emissions reduction measures taken prior to Beijing Parade for improvement of local and regional air quality was evaluated. The intention of this study was to provide important scientific basis for curbing atmospheric Nr pollution and N deposition in different regions of China as well as air pollution in mega cities. The main results of this study are summarized as follows:1. During period of 2010-2014, large spatial variations in annual mean Nr(sum of NH3, HNO3, NO2, particulate (p) NH4+ and NO3-) concentrations (1.3-47.0 μg N m-3), mean rainwater N (sum of NH4+-N and NO3--N) concentration (0.6-6.0 mg N L-1), dry (1.1-52.2 kg N ha-1 yr-1), wet/bulk (1.5-32.5 kg N ha-1 yr-1) and total (2.9-83.3 kg N ha-1 yr-1) N deposition fluxes were observed across the 43 monitoring sites in China. On a national basis, the annual mean concentrations and deposition fluxes of Nr species ranked by the same order of urban> rural> background sites and by regions as north China (NC)> southeast China (SE)> southwest China (SW)> northeast China (NE)> northwest China (NW)> Tibetan Plateau (TP). Average dry and wet (i.e. bulk) N deposition fluxes were 20.6±11.2 (mean± standard deviation) and 19.3±9.2 kg N ha-1 yr-1 across China, with reduced N deposition dominating both dry and wet deposition. Across the six regions, significantly positive correlations were found between NH3 emissions and NHx deposition fluxes, and between NOx emissions and NOy deposition fluxes at regional scale.2. During 2011-2014, annual concentrations of total atmospheric Nr overall showed an increasing trend, but annual mean NO2 concentration showed a declining trend and almost no changes in annual mean HNO3 concentrations and NH4+-N and NO3-N in precipitation. Compared the year 2014 with 2011, the concentrations of NH3,pNH4+ and pNO3- increased largely at the urban, rural and background sites in eastern China, mainly attributable to increases of their respective concentrations at each land use type in north area of eastern China. In contrast, NO2 concentrations reduced slightly, and degree of the reduction was somewhat larger in south area than in north area of eastern China. Annual variations of dry deposition fluxes of all measured Nr species (except HNO3) were fairly consistent with those of their respective concentrations. By contrast, there were no obvious changes in annual wet deposition fluxes of NH4+-N and NO3-N, and the changes in wet deposition fluxes of NH4+-N and NO3--N were not fully consistent with those of their concentrations. The annual ratios of reduced to oxidized N in dry depositon and total depositionboth showed increasing trends, suggesting enhanced influence of agricultural and non-agricultural NH3 emissions on atmospheric N deposition.3. The characteristics of NH3, HNO3, NO2, and PM2.5 were sumultaniously investigated at two urban, one suburban and two rural sites in the North China Plain, which was strongly interfered by anthropogenic activities. The results showed no significant spatial differences for NH3 and HNO3 but significantly higher NO2 concentration at the urban sites. At each site, annual average concentrations of NH3 and NO2 showed increasing and decreasing trends, respectively; while there was no obvious trend in annual HNO3 concentrations. All three land use types faced serious atmospheric PM2.5 pollution, with more than 70% of sampling days exceeding the Chinese Grade Ⅱ standard value of 75 μg m-3 for daily PM2.5 concentration. Concentrations of water-soluble ions in PM2.5 ranked differently between the non-rural and rural sites. The three dominant ions were NH4+, NO3-, and SO42- and mainly existed as (NH4)2SO4, NH4HSO4 and NH4NO3, and their total concentrations (sum of NH4+, NO3-, and SO42-) averaged 48.6±44.9,41.2±40.8,49.6±35.9μg m-3 at the urban, suburban and rural sites, respectively. PM2.5 was neutral at the non-rural sites but acidic at the rural sites. Seasonal variations of the gases and aerosols exhibited different patterns, depending on source strength and meteorological conditions. Compared with SO2 and NO2, NH3 played more important roles in formation of secondary inorganic aerosols and enhanced concentration of PM2.5.4. Average NH3 and NO2 concentrations across all sites in Beijing both reduced significantly during the emission control period (referred to as the Parade Blue period) compared with no control periods. Average NH3 and NO2 concentrations at sites near traffic were strongly correlated and showed positive and significant responses to traffic reduction measures. Daily concentrations of PM2.5 and secondary inorganic aerosol (sulfate, ammonium, and nitrate) at the urban and rural sites both decreased during the Parade Blue period. Daily concentrations of PM2.5, PM10, NO2, SO2 and CO reported by the Ministry of Environmental Protection of China have shown the largest decrease (34～72%) in Beijing, a smaller decrease (1～32%) in cities of North China (excluding Beijing), and a slight increase (6-16%) in other cities outside North China during the emission control period, reflecting the positive effects on air quality of taking emission controls. GEOS-Chem chemical transport model analysis demonstrated relatively smaller contribution of favorable meteorological conditions to air quality improvement in Beijing during the Parade Blue period. Our results indicate that synergistic controls of aerosol precursors (NH3, SO2 and NOX) locally and regionally are key to curbing air pollution in Beijing and likely in other mega cities worldwide.