Observational Study On The Impact Of VOCs And Aerosols On Photochemical Processes In The Lower Tropospher

With the enactment of Air Pollution Prevention and Control Action Plan in 2013,China’s air quality has improved significantly.Concentrations of pollutants,such as PM_2.5,decreased dramatically while those of ozone increased,accompanying with more frequent ozone pollution events in China,which have become a new challenge for China.Ozone in the lower troposphere is mainly generated by photochemical reactions,and its concentration is closely related to VOCs emissions and aerosol extinction effects.Besides,there are significant diurnal variations and vertical exchange of matter and energy in the lower troposphere,which make important effects on the concentration of ozone and its precursors.Therefore,it is of great significance to clarify the vertical structure of photochemical components and its impact factors in the lower troposphere for understanding the photochemical formation mechanism of ozone pollution.Hence,through vertical observational data analysis,this study clarified the vertical distribution of ozone,aerosols,VOCs in the lower troposphere of Nanjing and revealed the influence of VOCs on ozone profiles.Besides,observational constrained numerical simulations were used to disentangle the effects of different types of aerosols on vertical structures of ozone.Our results could provide theoretical basis for controlling ozone pollution.Firstly,based on the long-term surface and short-term vertical observations,the variation characteristics,vertical distribution characteristics of PM_2.5,BC and Ozone in Nanjing have been studied.From 2015 to 2020,the surface PM_2.5 concentration decreased significantly,and the BC concentration fluctuated,while ozone concentration increased.The concentrations of PM_2.5 and BC were high in winter and low in summer,and the diurnal variations were bimodal.The concentration of ozone shows a seasonal variation characteristic of high in summer and low in winter,and the diurnal variations shows a unimodal distribution.Ozone profiles can be divided into uniform increase,great gradient in the low level and great gradient in the high level.The concentration of VOCs decreased with the increase of height,and alkanes account for the largest proportion at each height,followed by aromatics,halohydrocarbons,alkenes and acetylene.The VOCs accumulated in the lower layer in the morning and evening with a great gradient and the proportion of alkane increased.The VOCs profiles were uniform in the afternoon.Affected by the surrounding air masses from different urban functional areas,the concentrations and components proportions of VOCs at each height were different.Three main types of aerosol profiles were revealed:homogenous profiles（HO）,negative gradient profiles（NG）,and positive gradient profiles（PG）,respectively.HO and NG were related to the diurnal evolution of the BL.PG can be subdivided into positive gradient in the low level（PG1）and positive gradient in the high level（PG2）,which were caused by surrounding emissions from high stacks and regional transport,respectively.Secondly,through the vertical observation and analysis of VOCs in the lower troposphere in Nanjing in autumn 2020,the effects of VOCs components,reactivity,aging degree and ozone formation potential（OFP）on the vertical distribution of ozone were studied.The results showed that The VOCs were classified according to the reactivity of different organic molecules with OH,and ALK4 was the most abundant VOCs species in the lower troposphere,and ALK4,ARO2 and OLE1 made greater contributions to photochemical ozone generation in the daytime.ARO2 made the greatest contributions to the SOA generation in the nighttime.Affected by atmospheric junction,alkanes and alkenes dominated the photochemical reactions in the mixing layer,and alkanes and aromatics were the dominant species in the stable boundary layer and residual layer.On the whole,the atmosphere in the northern suburbs of Nanjing was aged obviously,but the degree was lower in the mixing layer which is more conductive to ozone formation.The contributions of high reactive VOCs components to OFP were occurred in the low layer,and ethene and propene were the representative components.Alkane made greater contribution to the photochemical generation of ozone in the upper atmosphere,which reflects higher aging and the characteristics of regional atmospheric.Aromatics were the primary precursors for SOA generation.Limiting chemical plant emissions,improving vehicle emission standards and fuel quality are the key to improve ozone and SOA pollution in Nanjing.Finally,using field vertical measurements and observation-based model simulations,this study revealed the effects of different types of aerosols on photochemical ozone profiles in the lower troposphere.In terms of observation,absorbing aerosols were negative correlated with ozone in the low layer and weakened ozone generation.Scattering aerosols and ozone had positive correlations in the high layer.A sufficient number of aerosols can increase the gradient of ozone and downward shortwave radiation above the boundary layer,and stimulated ozone formation in the upper layer.It is determined from the perspective of observation that the aerosol extinction effects are the key factors affecting the photochemical ozone profiles in clear days by excluding the effects of NO titration,heterogeneous reaction and turbulent exchange.Comparing the BC radiative forcing（RF）and heating rate（HR）calculated using actual BC profiles and surface BC concentrations,RF calculated using surface BC concentrations led to an overestimate in NG profiles and an underestimate in PG profiles.The vertical distributions of BC HR exhibited clear sensitivity to BC profile types.NG profiles had a large HR in the low level and PG profiles had a large HR in the high level.In the terms of models,absorbing aerosols weakened upward shortwave radiation and J[NO₂],and then inhabited the photochemical ozone throughout the entire BL.A similar inhibition effect occurred within the low layer under sufficient scattering aerosols.In contrast,scattering aerosols enhanced upward shortwave radiation and J[NO₂],consequently promoting photochemical ozone generation in the upper boundary layer.Besides,the ratio of BC to PM_2.5 does not change the effect of aerosol on the mechanism of photochemical ozone generation.