Sources Of Nitrous Acid In Typical Mountain And Marine Atmosphere And Its Impacts On Atmospheric Oxidation Capacity

Nitrous acid(HONO)is an important trace gas in the atmosphere,which is a key reservoir of atmospheric hydroxyl radicals(OH),and has an important impact on the formation of regional secondary photochemical pollutants such as ozone(O3)and secondary aerosols as well as the atmospheric oxidation capacity.Currently,the field observational studies of HONO were mainly conducted over polluted regions(e.g.,urban,suburban and rural regions),with few efforts of HONO(i.e.,characteristics,sources and its impacts on atmospheric oxidation capacity)made over remote or clean regions.Based on multi-period field observations conducted at the Mount Tai Station and the Cape Verde Atmospheric Observatory over North Atlantic Ocean,it is conducted to analyze the characteristics and sources of HONO and its impacts on the atmospheric oxidation capacity in remote regions,investigate the dominant processes affecting HONO in different geographical environments such as urban,suburban,coastal,alpine and oceanic based on the principal component analysis(PCA)multivariate statistical analysis method.Based on the observations from Mt.Tai,this study systematically focus on the characteristics,sources and its impacts on the atmospheric oxidation capacity of HONO in the upper boundary layer and lower free troposphere of the North China Plain.As the atmospheric background station in the North China Plain,the ambient HONO concentration at Mt.Tai was generally higher than that of other continental alpine background stations at abroad,indicating that the serious atmospheric pollution in the upper boundary layer and the lower free troposphere of the North China Plain.The diurnal variations of HONO,NOx,O3 and PM2.5 at Mt.Tai all showed a broad noontime or afternoon maxima and lower nighttime concentrations,indicating that the vertical transport under the coupling effect of boundary layer evolution and valley winds has an important influence on the condition of Mt.Tai.A photo-stationary state(PSS)analysis suggested existing a strong potential daytime HONO source(Pother)in winter and spring with a production rate of 0.45±0.25 ppb h-1 and 0.64±0.49 ppb h-1,respectively.Further studies indicated that the photo-enhanced heterogeneous conversion of NO2 to HONO on the aerosol surface was the primary source of daytime HONO at Mt.Tai.The impact of HONO photolysis on atmospheric oxidation capacity at Mt.Tai was quantitatively assessed using the MCM atmospheric photochemical model.The results showed that HONO photolysis is the predominant primary source of OH radical and played a major role in the radical chemistry at Mt.Tai,with a proportion of 44.4%(winter)and 25.8%(spring)for total primary radical(ROx)production rates,which were much higher than O3 photolysis with an OH contribution of 1.9%(winter)and 9.0%(spring),respectively.Based on the field observation at Cape Verde Atmospheric Observation(CVAO)in autumn 2017,we investigated the characteristics,sources and impacts on the atmospheric oxidation capacity of HONO in the marine boundary layer.The concentrations of pollutants at CVAO during the observation period were much low,reflecting the feature of CVAO as an ocean background station.HONO showed a noontime peak and low nighttime concentrations,indicating that existing a strong potential HONO source in the remote marine boundary layer.The multiphase chemical model(SPACCIM)was further evaluated the sources of HONO in the oceanic atmosphere.The results showed that the gas-phase process of NO and OH,the conversion process of NO2 on the oceans,the oxidation process of NH3 in the atmosphere,and the reaction of NO2 with organics of aerosol surface were not the important sources for ambient HONO in the marine boundary layer.While the photocatalytic adsorption process of reactive nitrogen species on the dust aerosols can better reproduce the daily trends and concentration levels of HONO and NOx,indicating the photocatalytic mechanisms on the dust aerosols have an important influence on the ambient HONO and NOx in the remote marine boundary layer,especially the HNO3/pNO3-photolysis process.Due to the low level of HONO concentration at Cape Verde,the contribution of HONO photolysis to OH concentration was low,accounting for only 1.9%of the total OH primary source,which was much lower than that of O3 photolysis to OH radical contribution(74.5%).Based on the HONO diurnal variations in different environments on a global scale,three main processes affecting HONO formation were successfully extracted:NOx-dominated process,photodecomposition process,and soil-related process.Generally,the concentration level of HONO was related to the pollution condition:that is,the HONO concentration in the heavily polluted urban areas was much higher than that in rural,coastal,and remote mountain and marine areas;in addition,the diurnal variations of HONO in urban,suburban and rural inland sites showed a "groove shape" pattern,with higher concentration at night but much lower concentration in the daytime;while in remote sites such as the oceans and high mountains showed a "unimodal shape" trend,with higher daytime HONO concentration and lower nighttime concentration;however,in many coastal sites,the diurnal HONO patterns over coastal regions were found to be "bimodal shape" with high concentrations occurring both at night and in the daytime.In combination with PCA,three main processes affecting HONO were extracted:NOx-dominated processes(including HONO formation through the reactions associated with NO and NO2,photo-enhanced heterogeneous reactions of NO2,and even direct emission),photodecomposition process(including photodecomposition of pNO3and nitrogen-containing compounds),and soil emission-related processes.In addition,NOxdominated processes played a dominated role in polluted inland areas,a photodecomposition process was dominated in marine or mountain environments.HONO variations in coastal regions were governed by either NOx-dominated processes or photodecomposition process,determined by the effects of the sea-land breeze.In summary,since the unique geographical environment and pollution conditions over mountain and marine regions,the characteristics and sources of ambient HONO showed significant difference from urban,suburban and other polluted areas:the concentration levels of HONO at Mt.Tai and Cape Verde were much lower than those in the urban,suburban and other polluted areas,and the diurnal variation of HONO showed a higher concentration in the daytime and lower at nighttime,which is totally opposite with the trend in the polluted areas.On the other hand,the ground surface is considered to be the most important medias for HONO formation over inland sites,while in remote areas,the atmospheric aerosol surfaces served as an important media on the HONO formation;and the relatively important role of dominant processes of HONO in various geographical environments results in the different HONO behaviors.This study enriches the scientific understanding of remote clean environment of HONO and provide data support and scientific basis for the study of HONO and atmospheric environmental problems in clean regions.