| Investigating the temporal and spatial distributions and the conversion mechanisms of atmospheric nitrogen pollutants are meaningful to understand the processes of atmospheric oxidation and control the regional atmospheric pollution.The fine particulate nitrate(NO3-)and gaseous nitrous acid(HONO)are common nitrogen pollutants in the atmospheric environments.Nitrate makes up a significant fraction of atmosphernc fine particles(PM2.5),plays important roles in regional haze pollution formations.Gaseous nitrous acid serves as a key source of primary hydroxyl radicals(OH radicals),and hence plays important roles in atmospheric photochemistry and the formation of secondary pollutants.In this study,in-situ experiments were conducted in urban Ji'nan,rural Yucheng,remote Mt.Tai and Tuoji Island over the North China(NC)region,in 2014-2018,to online measure the concentrations of atmospheric pollutants;and then many statistical and analysis methods as well as multi-phase chemistry box model,trajectory model and photostationary state assumption,were deplayed to further research the temporal and spatial variation characteristics and potential formation mechanisms of atmosphernc fine particulate nitrate over the NC region and the HONO in the offshore Marine Boundary Layer(MBL).Fine nitrate concentrations exhibited clear spatial distributions during summertime over the NC region.The highest average concentration of fine NO3' was recorded at the receptor rural Yucheng site,followed by the urban Jinan and Mt.Tai site.And due to the different anthropogenic emission intensities and the aging extents of air masses,the Nitrate Conversion Ratio(NCR)in the urban area was much lower than that in the rural and then the background area.The average NO3/PM2.5 ratios in the three areas were in range of 11-14%,indicating the important contributions of nitrate in PM2.5 over the NC region.The diurnal variations of fine nitrate also exhibited distinct regional characteristics.At the low altitude Ji'nan and Yucheng site,affected by the variations of boundary layer and the semi-volatility of nitrate,the concentrations of fine nitrate normally declined during daytime and then increased during nighttime,and reached peaks in the morning.In contrast,influenced by the boundary layer variations and valley winds,the NO3-concentration at the high altitude site on Mt.Tai normally declined during nighttime and increased during daytime,and reached peak in the early night.The RACM-CARPAM multi-phase chemistry box model was deployed to further investigate the formation mechanisms of atmospheric fine particulate nitrate.Simulations of the fine particulate nitrate formation cases suggest the regional homogeneity of formation pathways over the NC region.Fine nitrate production was predominated by the HNO3 gaseous phase formation and the subsequent gas-particle partitioning during daytime,and was controlled by the N2O5 gaseous phase formation and the subsequent heterogeneous hydrolysis during nighttime.Sensitivity tests indicate that the daytime nitrate production was mainly limited by the availability of NO2,and was also promoted obviously by O3;and the nighttime nitrate formation was controlled by both NO2 and O3.The long-term trend of atmospheric pollution in the NC region is an important scientific issue in this study.Benefit from the effective control measures on the anthropogenic emissions of air pollutants,the SO2 concentration in the atmospheric environment of China has been greatly reduced in the past decade,resulting in the significant decline in the concentration of fine particulate sulfate.Nevertheless,the concentrations of fine nitrates presented gradually increasing trends during this period.The increasing fraction of NO3-in PM2.5 and the enhanced ratio of[NO3-]/[SO42-]indicate the secondary inorganic aerosol formation has been changed from the sulfate-dominate to the sulfate-nitrate-dominate and even the nitrate-dominate.Due to the large decline in sulfate concentration,the atmospheric fine particle has been transformed from poor ammonium condition to rich ammonium condition,which is favor of the nitrate formation.In order to further investigate the reason responsible for the long-term trend of atmospheric fine nitrate over the NC region,a RACM-CAPRAM model experiment on changing SO42-initial concentration was deployed according to the most important variation in the atmospheric fine particulate species during the past decade.Nitrate aerosol productions are affected by the acidity of particle phases,which is influenced by the concentrations of sulfate.Along with the large decline of SO2 anthropogenic emissions in the past decade in China,the concentration of particulate sulfate decreased rapidly,resulting in the weakening of the atmospheric aerosol acidity,and finally promoting the formation of fine particulate nitrate.In addition,the variations of particulate sulfate concentration may also affect the atmospheric N depositions.Along with the reduction of sulfate concentration,the depositions of gaseous NOy decrease to a certain extent,while the deposition of gaseous NH3 increase slowly.Thus,the reduction of SO2 anthropogenic emissions also may weaken the acid of atmospheric N deposition.A series of simulation experiments on nitrate formation were deployed under the hypothetical conditions,to further understand the roles of NO2 and 03 as well as SO42-and NH4+ and then put forward some suggestions on controlling the regional air pollution.The roles of NO2 and O3 on nitrate formation are influenced by their concentration distributions.Therefore,the primary control policies need to be formulated based on the local actual situations of air pollution.In the NC region,the further reductions of anthropogenic emissions of air pollutants(i.e.NO,and VOCs)are necessary to alleviate the pollution of nitrate aerosol in the regional atmospheric environments.In addition,NH4+ is also an important air pollutant to promote the fine nitrate formation.The productions of nitrate aerosol in the rich ammonium conditions are normally higher than that in the poor ammonium conditions.Thus,the reduction of NH3 emissions from human activities is also necessary to mitigate the pollution of nitrate aerosol over the NC region.HONO displayed distinct variation features and strong sources in the offshore MBL over the NC region.The HONO concentration was on average 0.20 ppb with a maximum value of 1.90 ppb,much higher than that over remote sea areas.The concentration of HONO exhibited distinct diurnal variations featuring with elevated concentrations in the late night and frequent concentration peaks in the early afternoon,indicating some strong production pathways during both nighttime and daytime,and also suggesting the effects of terrestrnal air masses and marine environment.During nighttime,HONO was mainly produced from the NO2 heterogeneous reaction,but its concentration was also influenced by the accumulation and the potential sea-air interactions.The NO2-HONO heterogeneous conversion rate was in range from 0.006 to 0.036 h-1 with a mean of 0.18 h-1 over the east Bohai Sea,much higher than that in the inland areas over the NC region.The fast productions and the strong linear dependence with temperature imply that the nocturnal HONO heterogeneous formation might be caused by the air-sea interactions.During daytime,the HONO concentrations were frequently increased when solar radiations were strong,and then reached peaks between 13:00-15:00.The observed daytime HONO concentrations were higher than those predicted in the photostationary state with intensive solar radiation and high temperature,indicating the existing of missing sources in the MBL.Strong or good correlations between the missing HONO production rate and temperature or photolysis frequency suggest a source of HONO from the potential photochemical heterogeneous conversions of nitrogen-containing compounds on sea microlayer.The abnormally high concentrations of daytime HONO contributed a considerable fraction to the primary OH radicals in the offshore MBL.In this study,the temporal-spatial variation characteristics and the production mechanisms of the atmospheric fine particulate nitrate over the NC region and the HONO in the offshore MBL were obtained,and are meaningful to further understand the atmospheric haze pollutions and nitrogen conversions.The related results can be used to improve the setups of models,to further research the atmospheric heterogeneous and photochemical processes.What's more,the results in this study can provide scientific evidences for controlling and alleviating the regional atmospheric pollutions. |
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