| Gaseous nitrous acid(HONO)is an important atmospheric trace gas,which can be photodegradated to produce hydroxyl radicals(·OH),thus promoting the generation of secondary pollutants such as ozone(O3)and fine particulate matter(PM2.5),and intensifying photochemical smog and haze pollution.Although HONO has an important impact on air quality,its pollution characteristics,impacts on atmospheric oxidation capacity and sources between environments are still not known comprehensively and systematically.This dissertation stems from the comprehensive field observation data of three typical environments in urban,rural and coastal China,with a variety of statistical methods and analysis means applied(such as empirical formula calculation,chemical budget calculation,air flow trajectory analysis,atmospheric chemical box model simulation,etc.).In this dissertation,the tempospatial variations of HONO concentration and its impact on atmospheric oxidation capacity are systematically claimed.The HONO/NOx emission ratios of two emission sources are obtained.The source intensity of HONO and the contribution of each source in multiple groups of space-time are quantified.And the influences of different parameterization schemes on the source resolution results of HONO are evaluated.The research results promote the systematic cognition of the pollution characteristics,impacts and sources of HONO in different environments,provide important parameters and data for the calculation and simulation of atmospheric chemical processes,and have guiding role in the building of scientific prevention and control policies for photochemical smog and haze pollution.The main achievements of this dissertation are as follows:1.The tempospatial variations of HONO in typical environments in China are clarified.Six periods of atmospheric observation experiments were carried out at three stations in urban Beijing(early summer of 2017 and winter of 2018),rural Yellow River Delta(winter spring and summer of 2017)and coastal Hong Kong(summer and autumn of 2018).Air masses from the ocean,coastline and continent were further screened from the coastal atmosphere of Hong Kong according to the backward air flow trajectory.The study found that the concentration level of HONO showed one spatial gradient feature(urban Beijing>rural Yellow River Delta>coastal Hong Kong,continental air mass>coastal air mass>marine air mass),two diurnal variation trends(single peak fluctuation type of night rise and day fall,sunrise peak and afternoon valley(urban Beijing,rural Yellow River Delta and continental air mass)and a stable type with little change throughout the day(coastal and marine air masses)),and three seasonal variations(early summer is slightly higher than winter(urban Beijing),winter spring is slightly higher than summer(rural Yellow River Delta),and equivalent between summer and autumn(coastal Hong Kong)).The HONO/NOx ratio,which characterizes the efficiency of HONO formation,shows the spatial trend that it is much lower in the marine air mass than in the urban,rural,continental and coastal air masses.The ratio is significantly higher in summer than winter.The diurnal trend of the HONO/NOx ratio shows regional consistency and seasonal difference.In winter,the ratio rose at night and fell during the day,reached the peak in the early morning and arrived the valley in the afternoon,showing a typical single peak shape.In summer,the ratio had a second peak in the afternoon,showing a bimodal shape,indicating an important daytime source of HONO.2.The impacts of HONO on atmospheric free radicals and atmospheric oxidation capacity are analyzed and compared between different tempospatial situations.Through empirical formula calculation,chemical calculation and atmospheric chemical box model simulation,it is found that HONO photolysis has an important contribution to atmospheric free radicals,and the contribution intensity and proportion show tempospatial differences-higher in winter than in summer,higher in cities than in rural and coastal areas,and higher in air masses from the continent than the coastline and the sea.In urban Beijing,the net contribution intensity of HONO photolysis to ·OH varied with the seasons,which were 3.86±1.46 and 0.76±0.46 ppb h-1 in early summer and winter,respectively.They were more than 10 times higher than those from O3 photolysis in both seasons.In rural Yellow River Delta,the net contribution intensity of HONO photolysis to OH was much lower than that in urban Beijing.They were 0.30±0.23 and 0.21±0.17 ppb h-1 in summer and winter-spring,respectively,which were about 20%and 2 times of those from O3 photolysis.Besides,the contribution in the morning was the largest.In coastal Hong Kong,the daytime generation rates of ROx radicals(ROx=·OH+HO2·+RO2·)increased successively in the three types of air masses from sea,coastline and continent,which were 0.34±0.09,0.71±0.26 and 1.17±0.39 ppb h-1,respectively.And the contribution rate of HONO photolysis to ROx also increased sequentially with 14.7%,29.6%and 52.1%,respectively.This result promotes the in-depth understanding of the tempospatial variations of the photochemical impact of atmospheric HONO,and shows important guiding role for scientifically reducing the level of atmospheric oxidation.3.The emission coefficients or contributions of typical HONO emission sources in different environments are obtained.By screening the local emission plumes in urban Beijing,rural Yellow River Delta and coastal Hong Kong,the emission ratios of HONO/NOx of urban motor vehicles and coastal ships are obtained,and the important contribution of biomass combustion to HONO in rural area is also studied.In urban Beijing,11 fresh vehicle emission plumes were screened out.The vehicle emission ratio of HONO/NOx ranged from 1.34%to 2.15%,with an average of 1.59±0.26%.In rural Yellow River Delta,a two-days case that seriously affected by biomass burning during the summer harvest was captured.And it was found that biomass burning could increase the concentration of HONO by two times at noon.In coastal Hong Kong,272 ships were screened as fresh ship emission plumes.The range of ship emission ratio of HONO/NOx was 0.21%-5.30%,with an average of 1.21±0.99%.These results provide a parameterization scheme for HONO budget calculation and model simulation.4.The source intensity of HONO and the contribution of each source under various tempospatial conditions are quantified.By the budget calculation,pseudo steady state calculation and correlation analysis,the intensity and contribution of each source to daytime HONO were studied,revealing significant temporal and spatial differences.It showed the spatial variation of urban>rural and continental>coastline>ocean,and the seasonal variation of summer>winter in urban Beijing and summer≈winter in rural Yellow River Delta.The main sources of HONO were NOx-related reactions.In urban Beijing,the source intensity of HONO was much larger in early summer than in winter,with 4.44±1.93 and 0.88±0.49 ppbv h-1,respectively.In early summer,both the aerosol loading and the atmospheric aging degree were high;the light-induced heterogeneous conversion of NO2 on the ground surface was the most important source of HONO.In winter,the NOx concentration was high and the air was relatively fresh;the homogeneous NO+·OH reaction dominated HONO formation.In rural Yellow River Delta,the source intensity of HONO did not show seasonal differences,but its source mechanism varied with seasons.The source intensity of HONO was lower than that in urban Beijing,with 0.52±0.22 and 0.54±0.28 ppbv h-1 in summer and winter-spring,respectively.The source mechanism of HONO was similar with that of Beijing.In early summer,the photo-promoted heterogeneous transformation of NO2 on the ground surface was dominant;while in winter-spring,the homogeneous NO+·OH reaction was dominant.In the three types of air masses originating from the sea,coastline and continent at the Hong Kong waterfront,the source intensities of HONO were 0.06±0.01,0.25±0.08 and 0.82±0.24 ppbv h-1,respectively.The marine air mass was seriously affected by ships,and the air mass was relatively fresh.HONO mainly come from the homogeneous NO+·OH reaction,and the sea acted as the sink of HONO.The coastal air mases mainly come from transport.Light-induced heterogeneous conversion of NO2 on the underlying surface and photolysis of adsorbed nitric acid(HNO3(ads))were the major source of coastal HONO.In the continental air masses,the concentration of nitrogen oxides was the highest,and the photo-induced heterogeneous conversion of NO2 on the ground surface was the most important source of HONO,followed by the NO+·OH reaction.In conclusion,NO2 is one of the most important precursors of HONO,and light is the key factor in promoting the formation of HONO.When the atmosphere is fresh,the contribution rate of homogeneous NO+·OH reaction to HONO is higher.While the higher the degree of atmospheric aging,the greater the contribution of photolysis of HNO3(ads)and particulate nitrate(pNO3)to HONO.The results deepen the understanding of the sources of HONO in different environments and provide theoretical guidance for scientifically reducing the concentration of HONO.5.The influence of different parameterization schemes on the calculation results of HONO sources is evaluated.Sensitivity analysis was carried out on the parameterization schemes of different sources of HONO.It was found that the photolytic uptake coefficient of NO2 on various surfaces was the most important parameter affecting the source analysis results of HONO in the daytime,and the photolysis rates of HNO3(ads)and pNO3 were another two key factors.This result points out the follow-up research direction for the realization of accurately quantifying the source of HONO. |
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