Observation And Sources Of Nitrous Acid In Ji'nan

Nitrous acid(HONO)is one of the important trace gases and plays a central role in the atmospheric chemistry as a key reservoir of the hydroxyl radical(OH).Its concentration level directly affects atmospheric oxidation capacity,then further changes the atmospheric composition and regional air quality.The cognition of HONO source is still not very clear.Many field observations and modeling studies both suggest that there is a strong unknown daytime HONO source,but there are still many significant uncertainties concerning the identification and formation mechanisms of these missing sources.The North China Plain(NCP)is one of hotspots of photochemical pollution and haze,which is characterized as high concentration of O3,NOx and particles.It is favor of the study of potential HONO sources.Therefore,continuous measurements of HONO and related parameters were conducted from September 2015 to August 2016 at an urban site in Ji'nan,the capital city of Shandong province.During this period,Ji'nan experienced hazes in winter,dust storms in spring and photochemical smog in autumn,which is helpful to analyze the characteristics and budget of ambient HONO in detail in different situations.In the study,ambient HONO was measured on-line using long optical path absorption spectroscopy(LOPAP),and other related gases,aerosols and meteorological parameters were measured simultaneously.The results showed that there was a high concentration of HONO in the atmosphere in Ji'nan.The average concentration of HONO(± SD)was 1.15± 1.07 ppbv during the observation period,0.99±0.95 ppbv during daytime(06:00-18:00)and 1.29±1.16 ppbv during nighttime(18:00-06:00).HONO showed well-defined seasonal and diurnal variation patterns with clear wintertime and nighttime concentration peaks.Elevated HONO concentrations(e.g.,over 5 ppbv)were frequently observed with a maximum value of 8.36 ppbv.Based on the selected fresh emission flumes,this study analyzed the vehicles emission ratio,which indicates that vehicles emission contributed to high HONO mixing ratio significantly.The effect of vehicles emission on daytime HONO budget can not be ignored.A total cases of 11 were summarised according the four criteria.The calculated HONO/NOx ratios of direct vehicle emissions varied in the range of 0.29%-0.87%,with a mean value of 0.53%±0.02%.Traffic emission had the largest contribution on the nighttime HONO concentration in winter,up to 21%.The heterogeneous formation strength of HONO at night was calculated systematically by the statistics of cases with increasing HONO concentration in steady meteorological condition.An average nighttime NO2-to-HONO conversion rate(khet)was derived to be 0.0068 ±0.0045 h-1 and the uptake coefficient of NO2 on the bulk surfaces was 1.40±2.4×10-6 from 107 HONO formation cases.According to the correlation analysis of khet and aerosol surface,Nighttime HONO was more likely to originate from the heterogeneous reaction of NO2 on the ground surface.And average every four NO2 molecules deposited to the ground surface released one HONO molecule.A detailed HONO budget analysis was investigated for the daytime HONO unknown source strength based on the measurements of total species and the above calculations.Considering the integrality and availability of observed data,this study evaluated an unexplained daytime(11:00-14:00)missing source of 2.95 ppb h-1 in summer,which is about seven times larger than the homogeneous reaction of NO with OH and accounts for over 80%of ambient HONO concentration.The possible formation pathways of unknown source in term of the correlation between and NO2,JNO2 and S/Va was studied.The result suggests the photo-enhanced heterogeneous formation of HONO on the ground surface may play an important role in ambient HONO mixing ratio in summer in Ji'nan.The contributions to OH radical of HONO photolysis and ozone photolysis were also studied in autumn and summer.We found that the net OH generation rate of HONO photolysis was 1.88 ppb/h in summer and 0.78 ppb/h in autumn,which both were approximately 2.4 times greater than that of O3 photolysis.HONO photolysis was the uppermost source of local OH radical throughout the daytime,which suggests HONO played an important role in the atmospheric oxidation capacity in Ji'nan.With a continuous observation for one year,not only does this study provide a large amount of cases to analyse HONO sources,but gives a system evaluation of the characteristics of HONO,the direct vehicle emission,the nighttime heterogeneous conversion,the daytime missing souces and the contribution of HONO to OH radical.The results will further enrich the knowledge of unknown HONO sources and reveal the significant role of HONO in atmospheric photochemistry of NCP.These data could provide to some degree reference for the study in the future and offer some supports to the improvement of air quality model and control of regional environmental problems like photochemical smog and haze.