Study On The Impact Of Traffic Emissions On Atmospheric Hono In Beijing

OH radicalsare the most important oxidants in the atmosphere,which can promote the removal of primary pollutants and the formation of secondary pollutants.Nitrous acid(HONO)is an important precursor of OH radicals.Its photolysis is the main source of OH radicals in the morning.Usually,it can account for nearly 60%of OH production,and in some cases,this proportion can even reach 92%.Therefore,the source and influence of HONO in the atmosphere is a hot topic in the atmospheric chemistry community.In recent years,researches on atmospheric HONO mainly include concentration,formation mechanism,source apportionment and impacts of HONO on secondary pollutants formation etc.through field observation,laboratory study and model simulation.Although significant progress has been made in HONO observations and source apportionment,the current model simulations are still unable to well explain the high values of HONO which were often observed in field observations,indicating that there are still unknown sources of HONO,and the contribution of different sources of HONO there are still large differences,and the parameterization scheme of the model also needs to be further optimized.In addition,several studies have proposed that HONO can promote the formation of secondary aerosols during pollution events in winter.However,there is no evidence supported by field observations.This study conducted long-term observations of HONO,related pollutants and meteorological parameters in Beijing.However,we found the direct evidence that HONO can promote secondary aerosol formation during haze events,explained the contribution of various sources to HONO yield,and proposed that vechile was an important source of HONO in Beijing through analyzing the budget of HONO and the characteristics of pollutants in different periods.The main findings are:1.The emission factor of HONO from vehicle(HONO/NOx)is 1.09%in Beijing urban area.Traffic-related sources(direct emissions and conversion of NO in exhaust gas)are an important source of HONO on pollution days in winter Beijing.Heterogeneous reaction of NO2 is also an important source of HONO in the night,while the ground surfaces are the main cotributor to heterongeous reaction when compared with particle surfaces,but the uncertainty of the calculation method may reduce the importance of this source.2.In the pollution events in Beijing during winter,the increase in the mass concentration of organic aerosol(OA)and nitrate from early morning to noon has a good correlation with the consumption of HONO.This means that HONO will promote the formation of aerosols during the pollution events in Beijing.This promotion effect can be explained the oxidation of corresponding precursors by OH from photolysis of HONO.3.During COVID-19,the photolysis of nitrate is the most important source of HONO during the daytime.The drastic reduction in traffic emissions has led to a significant decrease in the production rate of HONO from traffic emission-related sources and the concentration of HONO.It was further confirmed that traffic emissions before the epidemic were an important source of HONO in Beijing’s atmosphere.4.Our results indicate that reducing the emissions of HONO and NO from vehicles may be an effective way to reduce the formation of secondary aerosols and the severe haze events in winter Beijing.