Characteristics And Sources Of Low-volatility Vapors In Urban Beijing And Their Contribution To Secondary Aerosol

Low-volatility vapors are important contributors to atmospheric nucleation and the formation of secondary aerosol,and thus,they can influence air quality,human health and global climate indirectly.Within the continental boundary layer,sulfuric acid is the most important precursor for new particle formation,and oxygenated organic molecules（OOMs）are the main precursors for the growth of secondary organic aerosol（SOA）.Currently,the nationwide insufficient measurements of those two low-volatility vapors hamper the understanding of particle formation and subsequent growth.Beijing is one of the seven megacities in China with significant anthropogenic emissions.The characteristics of sulfuric acid and OOMs in urban Beijing can to some extent represent those in other megacities in China.In this thesis,based on the long-term observations of sulfuric acid and OOMs in urban Beijing,we analyzed the temporal variations of their concentrations and compositions,explored their main sources,and evaluated their roles in atmospheric nucleation and the formation of secondary aerosol.The main contents are as follows:According to the observation and analysis of sulfuric acid,it was found that the concentration of sulfuric acid during daytime was 0.1–1.7×10⁷molec cm^-3,usually reaching the highest in May and September and declining to the lowest from November to February of the next year.From 2019 to 2021,sulfuric acid concentration generally declined.Daytime sulfuric acid is mainly produced by the oxidation of SO₂by OH radical,and mainly lost by condensing onto particle surface（CS）.Based on this source-loss balance,three daytime sulfuric acid proxies using OH-SO₂-CS,UVB-SO₂-CS and UVB-SO₂-PM_2.5(expressed by Pro_OH-CS,Pro_UVB-CS,and Pro_UVB-PM2.5respectively)were proposed.Since they reflect the real chemical productions and physical losses of ambient sulfuric acid,both their expressions and embedded parameters were able to be applied to other sites,and they had better performance than numerical regression proxies in estimating sulfuric acid concentration.Among them,the three parameters used in Pro_UVB-PM2.5have been widely measured at most sites and thus it has potential for wider application.During wintertime,the nighttime concentration of sulfuric acid was 1.0–3.0×10⁶molec cm^-3.Under clean and some mildly polluted conditions,the oxidation of SO₂by Stabilized Criegee Intermediates and OH radical from the ozonolysis of alkenes was the main source of nighttime sulfuric acid.Furthermore,sulfuric acid was the main precursor for the formation of sub-3nm particles during both day and night.The mechanism is most likely sulfuric acid-amine-water nucleation.According to the measurement by NO₃^--based Chemical Ionization Mass Spectrometer（abbreviated as NO₃^--CIMS）,total OOM concentration in urban Beijing was 1.3×10⁷–2.2×10⁸molec cm^-3.This concentration was comparable with those in other urban and suburban areas but was significantly lower than in forested areas.Around 1000 OOM molecules were identified by NO₃^--CIMS in each season.Regarding the chemical composition,most OOMs contain 5–10 carbon atoms,3–7 effective oxygen atoms,0–2 nitrogen atoms and 0–6 values of double bond equivalence.To distinguish the sources of OOMs,the positive matrix factorization method was first used.It retrieved six non-nitrophenol OOM factors,including photochemical factor,local pollution factor,low-molecular-weight OOM factor,local aging pollution factor,transport pollution factor,and mixing factor.These factors were driven by the atmospheric physical and chemical processes,such as the transport of air mass and the variations in oxidants,but were not precursor-indicative.To address this issue,a new workflow was developed together with collaborators,which successfully classified OOMs into four sources,including isoprene,monoterpene,aromatic and aliphatic OOMs.Both aromatic（29–41%）and aliphatic（26–41%）OOMs contributed to large fractions of total OOMs in four seasons,and isoprene OOMs also accounted for up to 33%in summer.In fact,the compositions of OOMs in Beijing and Nanjing were quite similar in summer,which indicates that the characteristics and sources of OOMs in Beijing likely represent those in typical urban areas in China.The aerosol growth model showed that OOMs contributed 25–66%to SOA throughout the year.Among them,aromatic（46–62%）and aliphatic（14–32%）OOMs had large contributions to SOA in all four seasons,and the contribution of monoterpene OOMs to SOA was also non-negligible（8–12%）.Based on the analysis of OOMs from NO₃^--CIMS,the performance of Vocus Proton Transfer Reaction Mass Spectrometer（abbreviated as Vocus hereafter）in OOM measurement was further investigated,and the consistency and complementarity of the two instruments for OOM measurement were explored.In terms of molecular composition,Vocus shows high sensitivity to non-nitrogen-containing OOMs with 1–3 effective oxygen atoms,while NO₃^--CIMS has optimal sensitivities for OOMs with effective oxygen number larger than 3.As for volatility,Vocus is good at measuring intermediate-volatility and volatile organic compounds,while NO₃^--CIMS is good at measuring extremely low-volatility,low-volatility and semi-volatile organic compounds.Therefore,through the combined measurements of Vocus and NO₃^--CIMS,we have the potential to measure OOMs with full oxygen content and volatility range.For the common 75 OOMs with good reliability measured by both Vocus and NO₃^--CIMS,only 15 of them showed good correlations in time variations,which indicates that they are the same compounds.Due to the immaturity of Vocus in OOM quantification,the concentrations of OOMs measured by Vocus were 1–4 orders of magnitude higher than NO₃^--CIMS.The upper and lower limits of OOM concentrations from Vocus still differed by～71 times even after corrections.Therefore,an accurate quantification of OOMs from Vocus is the key technical issue that needs to be solved in the future.