Mixing State Of Black Carbon-Containing Aerosol And Nitrate In Zhengzhou

Black carbon（BC）plays an important role in the radiation balance of atmospheric environment and climate change.The freshly emitted black carbon particles gradually mix with other chemical components such as sulfate,nitrate and organics during the aging process.This process significantly changes their particle size,hygroscopicity,optical properties and chemical environment of the black carbon particles.Many studies have reported the effects of mixing state and the core-shall structure of aged BC particles on their morphology,light absorption and hygroscopicity.These studies focus on the mixing process of sulfate and organics with BC.In recent years,with the improvement of atmospheric environmental quality,the proportion of sulfate in the atmospheric fine particles of the North China Plain has decreased significantly and the proportion of nitrate has increased,which directly and/or indirectly affect the mixing state of BC particles.Although some field observations have reported that the internal mixing state of BC and nitrate has increased,these studies have not systematically evaluated the overall evolution trend of the mixing state of black carbon during different seasons and under different atmospheric environments.The dominant factors of BC mixing with nitrate and the subsequent consequences on the physical and chemical properties of BC particles are not clear yet.In this study,single particle aerosol mass spectrometer（SPAMS）was used to study the chemical composition and mixing state of black carbon single particles in the three seasons of spring,summer and winter in Zhengzhou in 2018.We carried out chemical classification based on the mass spectra characteristics of single particles,discussed the types and mixing states of BC particles in different seasons,and focused on the seasonal distributions and evolution of nitrate-containing BC particles.The PM_2.5 concentration,temperature,relative humidity,black carbon mass concentration,nitrate concentration and the absorbing （?）ngstr（?）m exponents（AAE）were used to analyze the influencing factors of the mixing process of BC with nitrate.In addition,we also systematically studied the mixing state of BC and nitrate during two haze episodes in spring,and explained the internal relationship between the black carbon mixing state and the haze evolution.The main conclusions are as follows:（1）The highest number fraction（N_f）of black carbon in spring in Zhengzhou was BC-nitrate（BC-N,52%）,followed by BC-nitrate-sulfate（BC-NS,22%）.In summer,the N_f of BC-NS particles was the highest（28%）followed by BC-N particles（23%）.In winter,BCOC particles（41%）were the most abundant species,and BC-N particles（31%）and BC-NS particles（29%）accounted for the same proportion.Nitrate-containing black carbon particles accounted for 74%,51%and 60%of the total black carbon particles in three seasons,indicating that the internal mixing state of nitrate and BC has become the major state of aged BC particles.In addition,the N_f of fresh black carbon particles in the three seasons are 4%,17%and 0,respectively.This suggests that the aging state of black carbon particles in winter was the heaviest,and the mixed package of organics,nitrate and black carbon was the main factor of black carbon aging.（2）The number concentration of BC-N particles increased significantly during the period of 6:00-10:00 in the morning in spring and summer,while the concentration in the afternoon showed a downward trend.The fluctuation range was greater in summer,and there was no diurnal change in winter.These observations suggest that the mixing state of black carbon and nitrate was affected by both the vehicle emissions and the ambient temperature.It is only found in summer that the number concentration of nitrate-containing black carbon particles and the content of nitrate decrease significantly as the increase of temperature.At the same time,we also found that the number concentration of nitrate-containing black carbon particles and the nitrate content increased significantly with the increase of relative humidity in three seasons.These findings indicate that changes in relative humidity have an effect on the mixing process of black carbon and nitrate in three seasons,while temperature only affect the mixing of nitrate with BC particles under high temperature in summer.In addition,the nitrate mass concentration and the number of nitrate-containing black carbon particles were well correlated in spring and summer,but there was no correlation in winter.Combined with the characteristics of the N_f of nitrate-containing black carbon particles in three seasons,it shows that the change of chemical composition in fine particles is a key factor affecting the aging state of black carbon particles.（3）During the two typical haze episodes in spring,the number concentration and N_f of BC-N particles increased significantly with the increase of the PM_2.5concentration in the environment.This was consistent with the mixing characteristics of nitrate on black carbon particles and the evolution of haze.The increase in the relative humidity and the concentration of nitrate were important reasons for the increase in the N_f of black carbon and nitrate.AAE decreased significantly as the increase of N_f of BC-N particles and the decrease of N_f of BCOC particles,indicating that the change in the internal mixing state of black carbon and nitrate may have an important impact on the overall optical properties of black carbon particles.In summary,we systematically analyzed the composition and mixing states of black carbon single particles in three seasons of spring,summer and winter in Zhengzhou,and found that the internal mixing of black carbon with nitrate has been the dominant pathway in the aging process of black carbon.Among them,the ambient temperature,relative humidity,and the chemical composition of PM_2.5 were important factors that affect the aging process of BC particles.The internal mixing state of black carbon and nitrate is associated with the evolution of haze pollution,and affects the optical properties of black carbon particles during the haze period.The results of this work help to further clarify the characteristics of the mixing state of black carbon particles in the process of continuous improvement of the atmospheric environment,and provide data support for the comprehensive assessment of the optical properties and climate effects of the current black carbon particles.