Observation And Simulation Study On Atmospheric Ammonia And Secondary Inorganic Aerosol In The Western Yangtze River Delta,China

Fine particles(PM_2.5)have been proven to pose substantial impacts on climate by perturbating Earth-atmosphere radiation balance or acting as cloud condensation nuclei and ice nuclei,ecosystems,human health and visibility.Sulfate,nitrate and ammonium?SNA?are the predominant secondary inorganic species in PM_2.5,fast chemical production and stagnant weather condition are considered as important contributors to frequent haze pollutions in eastern China.Ammonia?NH₃?is the most important alkaline gas in the atmospheric environment due to its important role in the global nitrogen cycle and ecosystem,and its contribution to the formation of secondary inorganic aerosols as an important precursor.Based on long-term observations and field campaigns at Station for Observing Regional Processes of the Earth System?SORPES?,a suburban site of the western Yangtze River Delta,this study analyses the concentration levels and temporal variations of secondary inorganic aerosols and their gaseous precursors in this region,and discusses the key influencing factors of NH₃,the causes of high ammonia events in summer and the effect of NH₃ on the formation of secondary inorganic aerosols.By combining in-situ measurements together with a regional model?WRF-Chem?with improved diagnose scheme,the linkage between secondary production of SNA and synoptic conditions,and their joint contribution to PM_2.5 have also been investigated in this work.Continuous and long-term in-situ observation of ambient SNA and their precursors is conducted at SORPES station from 2014 to 2017.The results show that the average concentrations of PM_2.5 and its main secondary inorganic aerosols SNA were 56.6,13.8,15.0 and 9.6?g/m³,and the corresponding average concentrations of gaseous precursors SO₂,NO_x and NH₃ were 14.4,48.0 and 7.1?g/m³,respectively.Except NH₃,SNA and its precursors showed the similar seasonal variation of highest concentration in winter and lowest concentration in summer,which was closely related to the seasonal variation of meteorological conditions and emission sources,as well as the transport process caused by the East Asian monsoon.The holiday effect of Spring Festival led to the decrease of PM_2.5 concentration in February,while the small peak of SNA in June was due to the enhancement of photochemical oxidation reaction in early summer,and the increase of temperature and daytime solar radiation were favorable for the emission of NH₃,thus promoting the formation of secondary inorganic aerosols.The diurnal cycle of pollutants were mainly affected by the evolution of boundary layer,the variations of emission intensity of anthropogenic and natural sources,and the chemical production process.The NH₃ concentrations at SORPES station ranged from 0.1 to 53.3?g/m³,with the pattern of highest in summer and lowest in winter on seasonality,and higher level during daytime than nighttime on diurnal scale.NH₃ was mainly affected by factors such as agricultural activities,biomass burning,temperature,gas-to-particle conversion,and precipitation.Among them,precipitation would have a scavenging effect on NH₃,but NH₃ emissions might be enhanced after the rain.Traffic source contributed little to the NH₃ concentration of SORPES station.The high ammonia events observed at SORPES station in summer were the result of a combination of regional-scale primary emissions caused by agricultural fertilization in the Yangtze River Delta region and the transport process of NH_x?NH₃+NH₄⁺?system.The western region of the Yangtze River Delta was under ammonia-rich conditions in all seasons,which meant NH₃ was sufficient to completely neutralize the strong acids in aerosols.An extensive measurement campaign was conducted at SORPES station around December 2017.The results showed that heterogeneous reaction effectively promoted the formation rate of secondary inorganic aerosols SO₄^2-and NO₃^-,aggravating PM_2.5pollution,and the proportion of SNA increased significantly during heavy pollution events.By combining in-situ measurements on meteorology and aerosol chemical composition at three main cities together with a regional model WRF-Chem with improved diagnose scheme,we investigated the chemical formation and accumulation of main secondary composition SNA under typical synoptic conditions.It is indicated that SNA did play a vital role in haze pollution across eastern China,contributing more than 40%to PM_2.5 mass concentration.As most fast developing region,the Yangtze River Delta?YRD?was slightly polluted during stable weather and local chemical production accounting for 61%SNA pollution.While under the influence of cold front,the pollution was aggravated and advection transport became predominant process?85%?.Nevertheless,the chemical production of SNA was notably enhanced due to the uplift of air pollutant and humidity ahead of the cold front,which then facilitated the heterogeneous and aqueous-phase oxidation of the precursors.We also found the substantial difference in the phase equilibrium of nitrate over the land surface and ocean due to changes in temperature,ammonia availability and dry deposition.