Micro-FTIR Study Of Heterogeneous Reaction Kinetics Of Atmospheric Particle Surface

Atmospheric particles play key roles in a variety of environmental issues including sky visibility,air quality,climate change and public health along with the aggravation of the air pollution.The research on the heterogeneous reaction of particle surface can explain the mechanism of haze formation from the micro perspective and provide the basic data for the air pollution.In this paper,microscopic Fourier transform infrared（micro-FTIR）was used to in situ monitor the micro-sized particles and measure kinetics of heterogeneous reaction of atmospheric trace gases with the organics or inorganic components of particle.Moreover,we highlighted the influence of relative humidities（RHs）.The main results were as follows:1.Ozone initiated heterogeneous oxidations of oleic acid（OA）,linoleic acid（LA）,and linolenic acid（LOA）micron-sized single droplets were investigated.The pseudo-first-order rate constant(k_app)and the overall uptake coefficient（γ）were obtained by quantitatively estimating the changes in absorbance area of the C=O stretching band at 1710 cm^-1,which was assigned to the carboxyl group of the reactant.And the effect of surface,which was derived from the ozone concentration and particle size effects on reaction kinetics,played an important role during the reaction.The organics showed the positive correlation between double bonds and reaction rate.In the view of RH effect,both k_app andγwere strongly enhanced by about a factor of three for the LOA/O₃ reaction system as the RH increased from⁰%to 83%.For the LA/O₃ reaction system,k_app andγincreased less one time with the increasing RH from⁰%to 85%.However,the k_app andγof the OA/O₃ reaction system were independent of the RH changes.Moreover,the various hygroscopicities of the three acids and corresponding products lead to different reactivities.2.The heterogeneous reactions of soot particles with ozone and the mixture of ozone and SO₂ were studied as a function of RHs.The reactions were followed in real time using micro-FTIR spectrometer to obtain kinetic data.The results showed that the ketone（C=O）group was the main product of the O₃/soot reaction,and the sulfate was identified on the surface of soot particles in the presence of O₃/SO₂.Both reactions were sensitive to RHs and surrounding water significantly promoted the proceeding of the heterogeneous reactions.The pseudo-first-order rate constant increased from 3.2×10^-44 s^-1 to 7.1×10^-44 s^-1with increasing RH in the range of 1%to 82%for the O₃/soot reaction.When O₃ and SO₂existed simultaneously during the reaction,the reaction rates and uptake coefficients were all enhanced by an order of magnitude as the RH increased from 1%to 83%.The possible reaction mechanisms were speculated from the infrared analysis and theoretical calculation.The high production of the oxygen-containing function groups and sulfate on soot surface was of highly hydrophilic,which played a key role in climate changes and would be more toxic and harmful to human health under humid environments.3.The RHs effects on heterogeneous reactions of TiO₂ particles with O₃/SO₂ and SO₂/NO₂ were investigated.On the basis of change trend of sulfate product characteristic peak intensity over time,the reactions were distributed into initial stage and steady stage.For the initial stage,with increasing RHs from 3%to 84%,theγvalues for the reactions of TiO₂ particles with O₃/SO₂ and SO₂/NO₂ were increased five and six times,respectivly.4.We first studied the kinetics of heterogeneous reactions of micron-sized MgCl₂ and ZnCl₂ single droplets with O₃/SO₂.By comparison of infrared characteristic peaks changes in the process of reactions and the corresponding sulfate droplets,we speculated that the MgCl₂ and ZnCl₂ react with O₃/SO₂ to form sulfate and release HCl.Moreover,the values ofγvs droplet sizes and RHs for the two reactions were also investigated,the two reaction systems were all exhibited obvious sizes and RHs dependences.Theγvalues increased from 2.64×10^-5 to 6.59×10^-5 with increasing RH in the range of 63%to 92%for the reaction of MgCl₂ with O₃/SO₂ reaction.When the RHs increased from 55%to 85%,theγvalues increased from 4.37×10^-5 to 8.20×10^-5 for the reaction of ZnCl₂ with O₃/SO₂ reaction.Thus water gases significantly promoted theγvalues of the heterogeneous reactions.The uptake ability of ZnCl₂ was stronger than that of MgCl₂.