The Variation Characteristics Of Atmospheric Free Radicals And Pollutants In Lanzhou

Atmospheric free radicals are very important part of the atmospheric environment.They have strong oxidation capacity and dominate the removal&conversion of sulfur dioxide,nitrogen dioxide and volatile organic compounds in the atmosphere.Additionally,their oxidation-induction capacity is the core driving force for the formation of secondary fine particle pollutants.Therefore,studyi ng the source,distribution and the trend of hydroxyl radicals in the atmospheric environment is of great significance for exploring the formation mechanism of secondary particles.In this work,the driving mechanism of OH radical oxidation and add uction is taken as the core.Studying the variation characteristics of OH radicals in the atmosphere and the trends of relavant pollutants in Lanzhou can help to understand and explore t he mechanism of secondary particle explosive increase;moreover,this work can provide scientific theoretical basis for secondary fine particle pollution control.1.The vertical distribution of OH radical concentration and particulate matter concentration（PNC）research was carried out at the observation point of the Qilihe District of Lanzhou City（36°3’22.7"N,103°46’36.3"E）.The sampling points are arranged on the platforms 3m,15m,30m and 45m above the ground.The results show that the OH radicals have the highest concentration（6.51×10⁶/cm³）at the 3m platform and the lowest OH concentration（2.42×10⁶/cm³）at the 30m platform during the monitoring period..In the year of 2018,the quarterly average concent ration of OH radical:Autumn（2.89×10⁶/cm³）>Spring（2.66×10⁶/cm³）>Summer（2.58×10⁶/cm³）>Winte r（2.34×10⁶/cm³）;among the 12 months,the free radical concentration in January was the lowest（2.22×10⁶/cm³）,and the free radical concentration in October was the highest（3.44×10⁶/cm³）.The correlation research between temperature,humidity and free radical concentration indicated that the correlation coefficient between temperature and free radical concentration Poisson reached 0.4820 in July;the correlation coefficient of humidity and free radical concentratio n Poisson reached0.2169 in November.2.Under the condition of OH radical oxidation-induction,inorganic salts（Na₂SO₄,（NH₄）₂SO₄,Na HSO₄,Na NO₃,KNO₃,NH₄NO₃,Na Cl and NH₄Cl）,gasoline generator exhaust gas,organic mixture（acetaldehyde,benzaldehyde and ethyl benzoat）by experimental kinetic simulation is to study the effects of the particle number concentration and mass concentration in the chamber environment and explore the mechanism and major factor in the process of secondary particle explosive growth.In the presence of a certain concentration of OH radicals,the total mass accumulation of sulfate seeds in the chamber for 2 hours is much higher than that of other inorganic salt seeds（Na NO₃,KNO₃,NH₄NO₃,Na Cl and NH₄Cl）.Total particulate matter cumulative mass with 8 kind of inorganic salts mixture seeds.for 2 hours is more than twice that in on condition to the OH radical-free oxidation-inducing.After the sulfur dioxide and the organic mixture were dispersed into the chamber,the secondary particles gradually formed.The first fine particle number（PNC）peak appeared at about 6 hours after the start of the experiment;subsequently,the PNC fluctuated.The highest peak reached when the accumulation time arrive the 16th hour.Under the condition of OH radical oxidation induction,the gasoline generator was used to simulate automobile exhaust.The research found that the contribution of automobile exhaust to fine particulate matter（DP<0.3um）in the atmospheric environment ca n not be ignored.The particle size of automobile exhaust（Dp<0.3um）account for over 80%.3.By use of PM_2.5,PM₁₀,sulfur dioxide,nitrogen dioxide,ozone and carbon monoxide online monitoring data at the observation point（36°3’22.7"N,103°46’36.3"E）from 2017 to 2018,we compared and analyzed the month and seasonal variation characteristics.The results showed that the pollutants near the observation point appearred lower concentration in summer and autumn,but the concentration in spring and winter is higher.Ozone pollution is heavy in the end of spring and summer;the trend of OH and ozone is relatively close,OH peak concentration appearred in May and October;VOCs are significantly higher in summer than in other seasons.The change trend of the concentration of fine particles in 2018 is consistent with that of PM_2.5,which is higher in spring and winter,and the particle size is slightly different.In the study of the correlation between ozone,NO₂,VOCs and OH radicals,O₃ has the strongest correlation with OH radicals.The correlation coefficient of Poisson between the two in early May reached 0.7009.Under special weather conditions,the concentration of fine particles explosive increase within 2-3 hours.And then a period of time later,there was a cliff-like decline and returned to the normal concentration range.The correlation between the concentration of particulate matter and OH radicals in this process showed that the smaller the particle size,the stronger the correlation between PNC and OH radical concentration,and the correlation coefficient between PNC and free radical concentration of 0.2-0.3um particles reaches 0.3548.