Heterogeneous Photochemical Reaction Of NO₂ On Humic Acid

HONO is a very important species for atmospheric chemistry for its photolysis to produce OH raddicals.However,unexpected high daytime HONO concentrations were observed and its source mechanism is not fully been understood.Recently laboratorial experiment found that the reduction of NO2 on humic acid(HA)can produce HONO.HA are heterogeneous natural macromolecules produced by microorganism through decomposing residual body.Through wind erosion and dissolution process,HA are abundant not only in soil but also in atmosphere and rivers.Because HA are ubiquitous in terrestrial and aquatic,the formation of HONO on HA film under irradiation predicted to have a large contribution to atmospheric HONO concentrations.This issue is aimed at the heterogeneous chemical reaction between NO2 and HA,including the NO2 uptake,the HONOformation and HA structure changes.The conversion of NO2 to HONO on HA under simulated sunlight was investigated using a flow tube reactor at ambient pressure.NOx anayler was used to detect NO2,HONO and NO concentrations.In the absence of light,there was little HONO formation.However,upon exposure to simulated sunlight,a strongly enhanced NO2 uptake and HONO formation were observed.The uptake coefficient(?)of NO2,linearly increased with irradiation intensity and HA mass in the range of 0-2.0 ?g/cm^,while it decreased with the NO2,O2 aging and O2 aging concentration,and remained unchanged with pH.The HONO yield was found to be independent of irradiation intensity,HA mass,and NO2,O2 aging and O3 aging concentration,while linearly decreased with pH.The temperature(278-308 K)had little influence on both y and HONO yield.Additionally,y increased continuously with relative humidity(RH,7-70%).and a maximum HONO yield was observed at 40%RH.A mixture of O2 and NO2 or O3 and NO2 simultaneously entered reactor,y was decreased with O2 and O3concentration.The HONO yield was found to be independent of O2 concentration,while non-linearly decreased with O3 concentration.According to the characterizationof HA compositions,it was concluded that the pH may influence the transfer of the protons and the equilibrium of HONO with NO2-by varying the content of the phenol andcarboxyl in HA.