Homogeneous And Interfacial Behavior Of Typical Unsaturated Carbonyl Compounds And Its Environmental Impact

Unsaturated carbonyl compounds are a kind of oxygenated volatile organic compounds(OVOCs)with wide sources,high reaction activity,and interface preference.They play a significant role in the atmospheric budget of ·OH and O3,organic acids,secondary organic aerosols(SOA),and radiative forcing in the atmosphere.As an important part of carbonyl compounds,the tropospheric characteristics of most unsaturated ketones/aldehydes are seriously ignored,and the transformation/degradation mechanism and the SO A correlation of them are not fully understood.More importantly,unsaturated ketones/aldehydes show a strong interface connection.Their heterogeneous and multiphase reactions at air-solid and airaqueous interfaces have been proved to cause the double increase of SOA which has a great threat to human health and climate change.Exploring the gas and aqueous properties of unsaturated ketones/aldehydes,especially its interface heterogeneous chemistry,is of great significance to fully evaluate the environmental impact of OVOCs,master the effects of the interface environment,and more accurately and completely grasp the composition of SOA,so as to further predict and control the adverse effects of SOA.In this paper,unsaturated ketones/aldehydes are selected as model compounds to explore the homogeneous and heterogeneous effects of carbonyl compounds by computational chemistry methods.The research contents include the basic exploration of gas/aqueous-phase oxidation of unsaturated ketones/aldehydes,the adsorption and ozonolysis of unsaturated ketone at mineral particle interface,the properties of MVK and MACR at the air-water interface,and the absorption and reaction of MACR on acidic nanoparticles.The main contents of this paper are summarized as follows:(1)Gas-phase reactions of a series of unsaturated ketone/aldehyde with ozoneWith the basic mechanism and kinetics of gas-phase reaction as a start,the degradation mechanism and rate constant of O3 with a series of unsaturated ketones/aldehydes were first studied.The results show that the reaction of unsaturated ketones/aldehydes with O3 has a similar mechanism:the formation and decomposition of primary ozonides,and the bimolecular and self-decomposition reaction of formed Criegee intermediate RR’COO.In addition to some aldehydes and diketones,some organic peroxides are also the main products of these reactions.The reactions of the seven RR’COO with HCOOH and H2O are the easiest,followed by the reactions with NO and HCHO.The total rate constants decrease with the increase of altitude.Compared with the middle and low troposphere,the gas-phase reaction of these carbonyl compounds with O3 has less effect on the higher troposphere.(2)Adsorption and ozonolysis of unsaturated ketones on SiO2 mineral particlesBased on the gas-phase reaction properties,the gas-solid heterogeneous adsorption and the reaction of the four unsaturated ketones mentioned above was simulated.The results show that SiO2 owns better adsorption performance of these ketones,which represents a good sinking of atmospheric OVOCs.The heterogeneous reactions of these ketones onto the surface of mineral aerosols may compete with the corresponding gas-phase reactions.Organic hydroperoxides and LMW carbonyl compounds generated in these reaction processes may change the properties of aerosols,which have essential impacts on tropospheric chemistry and climate.The presence of SiO2 could accelerate the reactions of unsaturated ketones with O3 without changing their degradation mechanism.It also should be emphasized the promoting effect of SiO2 clusters to CH2OO+H2O,which may explain the high product yields of HMHP and H2O2 detected on SiO2 particles in the experiment.This study enables us to further understand the gas-solid heterogeneous pollution mechanism of OVOCs,and emphasizes the important role of mineral particles in the troposphere.(3)The degradation of MVK by·OH and ·Cl in the aqueous particlesTaking MVK as the model compound,the aqueous-phase chemistry of MVK with·Cl and·OH was studied,and the solvent effect was also discussed by comparing the gas-phase reaction.A new pathway for the formation of alkoxyl radicals from peroxyl radicals is presumed.The reaction activity of alkoxyl radicals takes the order of β-scission>with O2>with H2O>Russell mechanism.The aqueous environment shows the negative solvent effect on these two systems,and the "polarization" of the proportion of the addition and abstraction reactions is intensified by the solvent effect.Although the rate constants kMVK-Cl>kMVK-·OH>kMVK-03,·OH-induced MVK reaction is still the dominant sink of MVK.If the effect of ·Cl is ignored,the atmospheric lifetimes of MVK will be overestimated by 12%and even more.Low volatility oxygenated products produced in the studied reactions may be an important bridge between unsaturated ketone and aqSOA.An increased eco-toxicity can occur from MVK to its degradation products in the·Cl-MVK system.This work mainly focused on the aqueous phase properties of unsaturated carbonyl compounds,and compared the reaction mechanism and product information of unsaturated ketones with different oxidants(O3,·Cl and·OH).(4)The adsorption and reaction properties of MVK and MACR at the air-water interfaceAir-water interfaces are ubiquitous in nature,as manifested in the form of the surfaces of oceans,lakes,and atmospheric aqueous aerosols.The properties of MVK and MACR(the significant precursors of Criegee intermediates and SOA)at the air-water interface were studied from a theoretical perspective.The free energy wells of MVK and MACR occur at the air-water interface,and their absorption probabilities are 71%and 67%,respectively.Repulsion dominates the interactions between MVK/MACR and water molecules in the bulk region,while attraction is dominant at the interface.The most likely reaction scenario of O3initiated MVK/MACR reaction in the troposphere is also determined for the first time.Based on the molecular dynamics simulation results,the activity sequence of MVK+O3 is given in four different environments by the DFT method:air-water interface,mineral clusters interface,bulk solution,and homogeneous gas.The interfacial water molecule can catalyze the reaction of MVK with O3,and the rate constant at the air-water interface is～6 times larger than that on the mineral surface model.Compared with mineral particles,aqueous particles play a more significant role in modifying the reaction properties of atmospheric organic species.(5)The uptake and heterogeneous reaction of MACR with the acidic nanoparticleConsidering the moderate acidity of aerosols,the formation of SOA through acidcatalyzed heterogeneous reactions has become a recent concern.Results show that the aqueous nanoparticle interface,especially when it is an acidic nanoparticle interface,is a perfect area to adsorb and accommodate MACR.The occurrence percentage of MACR on the interface is more than 70%.With the increase of SA concentration,the first solvation shell changed from only water to a mixture of S A and water,which facilitates the heterogeneous hydration reaction of MACR.Compared with the neutral nanoparticle interface,the acidic nanoparticle interface exhibits a better ability to uptake and accommodate gaseous carbonyl species.Moreover,SA can catalyze the hydration reaction of MACR inside the aqueous media,and the resulting oligomers contribute to the formation and growth of SOA.The hydration reaction indirectly promotes the continuous adsorption of MACR at the acidic nanoparticle interface.The acidcatalyzed reactions have an important impact on environmental chemistry,such as cloud SOA formation,within the range of about 2-6 km.Through the above research,we have achieved a systematic study of unsaturated carbonyl species in gas-phase,aqueous-phase,air-solid interface,air-water interface and air-acid interface,which provides ideas for the systematic exploration of homogeneous and interfacial behavior of tropospheric pollutants,and proves the important role of heterogeneous chemistry in the formation of tropospheric SOA.