Heterogeneous Mechanisms Of Typical Long-Chain Oxygenated Volatile Organic Compounds In The Atmosphere

With the development of society and economy,oxygenated volatile organic compounds（OVOCs）,a class of typical volatile organic compounds（VOCs）,are emitted into the atmosphere in large quantities via various natural and anthropogenic sources.The large emission of OVOCs leads to air pollution which limits the development of economy and influences human health.With a wide range of sources and components,OVOCs possess high reactivity and have been proved to be significant precursors for SOA via heterogeneous chemistry,contributing to particle formation.Until now,studies of heterogeneous chemistry mainly focused on small molecule weight carbonyls（e.g.,methylglyoxal and glyoxal）and alcohols（e.g.,methanol and ethanol）while the heterogeneous mechanisms for long-chain OVOCs are still lacking.Hence,to systemically investigate the heterogeneous chemistry of typical long-chain OVOCs is necessary and meaningful.In this work,octanal（OAL）and 2,4-hexadienal（HAL）,1-butanol（BOL）and 1-decanol（DOL）were chosen as the represents of long-chain carbonyl compounds and alcohols,respectively,to investigate their heterogeneous chemistry.By quantum chemical methods and kinetic calculation,full reaction systems were constructed and the influence of functional groups（i.e.,C=C bond）was also discussed which clarified the heterogeneous mechanisms of long-chain aldehydes and alcohols and the contribution to SOA.The major conclusions are presented as follows:（1）Under the acidic condition,the direct hydration of OAL/HAL is thermodynamically hindered owing to the large energy barriers while the protonation of OAL/HAL is easy to occur to form diols.These diols then further transform to carbenium ions after protonation and dehydration.In OAL and HAL reaction systems,there are three possible pathways to form oligomers including aldol condensation,hemiacetal reactions and oligomerization mediated by carbenium ions.Owing to the large barriers and small rate constants,the aldol condensation and hemiacetal reactions are hard to occur,and the contribution to SOA is neglectable.The carbenium ion-mediated oligomerization occurs between carbenium ions and diols with rate constants of～10⁹ M^-1 s^-1,which forms various dimers and dominates the SOA formation.With C=C bonds,HAL undergoes extra intramolecular cyclization,which synergistically contributes to SOA with oligomerization,indicating that functional groups significantly affect the heterogeneous mechanisms.（2）The initial reactions of BOL/DOL include H₂O-elimination,SO₂/SO₃-addition and protonation.H₂O-elimination and SO₂/SO₃-addition possess large energy barriers and small rate constants while the protonation is barrierless with a high branching ratio（branching ratioГ>99%）,which dominates the initial reactions.Carbenium ions are formed after the protonation and dehydration of BOL/DOL,which then undergo three pathways including HSO₄^-/SO₄^2--addition to yield organosulfates（OSAs）,HSO₃^-/SO₃^2--addition to yield organosulfites（OSIs）and oligomerization with parent diols to yield dimers.At 298 K,the rate constants of barrierless formation of OSA/OSI mediated by carbenium ions are（4.53～5.05）×10⁹ M^-1 s^-1 and theГof OSAs/OSIs is constant（83.8%）over the temperature range of 216-298 K,which indicates that OSAs/OSIs are the dominant products of BOL/DOL.OSAs and OSIs exhibit hygroscopicity because of their high solubility and polarity,indicating the contribution to SOA via the growth of nanoparticles.However,dimers possess no reactive groups and could to further oligomerize and thus the contribution to SOA is neglectable.