Study On The Photochemical Pathway Of Oxalate And Reactive Oxygen Species In Particulate Matter

With the development of industry,the impact of atmospheric organic particulate matter on human health is becoming more and more serious.As the most abundant dicarboxylic acid in organic aerosol,oxalic acid is also considered to be the end product of atmospheric photooxidation reaction with widely concerned in recent years.In this study,the combination of online observation,offline collection and laboratory simulation was used to study the distribution of oxalic acid,its oxidation properties,secondary products after degradation,and its effects on human health.The seasonal variation of online observation oxalate average mass concentration showed that spring>summer>winter,and the maximum and minimum values appeared in May and July,respectively.Because there was more rain in July leading to the strong wet sedimentation effect,although the photooxidation reaction environment was superior.Oxalate was mainly derived from secondary generation during sampling,as well as high temperature and suitable relative humidity were conducive to the formation and existence of oxalate.In winter,oxalate was affected by winter heating in northern cities and the production of long-distance secondary aerosols.Seasonal differences may come from the duration and intensity of sunshine,which affect the photolysis of O₃ to generate superoxide radicals.Suitable relative humidity was conducive to the increase of oxalate concentration,thus the condition of 30%<RH?60%and RH>60%were beneficial to the production of oxalate in summer and winter,and the condition of RH>60%was more conducive to the existence of oxalate at night in summer.During the daytime in summer and winter,O₃ may participate in the atmospheric chemical pathway of oxalate,while NO₃·was mainly involved in the formation of oxalate at night in summer and the degradation of oxalate at night in winter.The HONO pathway was involved in the degradation of oxalate in the morning in summer.The NO₂ pathway was involved in the formation of oxalate at night in winter and the degradation of oxalate in the morning in winter.The iron content showed spring>winter>summer,night>day in spring and summer.The diurnal changes of iron form indicated that the change of iron oxidation could cause the effect of oxalate-iron oxidation system,which is also one of the pathways for the generation of·OH.In order to observe the secondary products of oxalic acid degradation,the photochemical oxidation reaction experiments with three oxalic acid-iron concentration ratios were designed.The degradation rate of oxalic acid increased with the increase of its concentration ratio.During the degradation process,secondary products of formic acid(C₁)and acetic acid(C₂)were formed,and formic acid and acetic acid are substances in the atmosphere that have an important influence on atmospheric water chemistry,aerosol formation and precipitation acidity.With the increase of the initial oxalic acid concentration,the ratio of C₁/C₂ decreased with the increase of the initial oxalic acid concentration,and its value is in the range of 0.54 to 2.29,which means that not all secondary sources have a C₁/C₂ ratio greater than 1.Oxalic acid was the most abundant dicarboxylic acid in offline observations,and the secondary formation of atmospheric particulate matter at the two observation points dominated.,Oxalic acid was mainly produced by malonic acid and glyoxal in both places.The oxidants that acted in the two places during the day were the·OH generated by NO₂ and O₃ pathways.Moreover,the oxidation reactions at night in Beijing were mainly under the action of NO₃·radicals;while those at night in Gucheng were mainly under the·OH produced by the NO₂pathway.The correlation between oxalic acid and water-soluble iron was significantly correlated during the day and night,confirming the possibility of oxalate-iron complexes participating in the atmospheric photooxidation reaction.The atmospheric environment during the day was more conducive to the generation of active oxygen,and the oxidation potential of the two regions showed daytime>nighttime.In addition,Fe₂(C₂O₄)₃ complexes in the atmosphere in the Beijing was very likely to be present in large quantities and may participate in photochemical oxidation;while the oxidation potential in Gucheng had the most significant impact on the water-soluble iron,and followed by Fe₂(C₂O₄)₃ complexes.Therefore,the water-soluble iron emitted in the atmosphere has the significant impact on human health,also the existence of Fe₂(C₂O₄)₃ complexes oxidation system had the great impact on human health at the same time.