| As an important primary particle in the atmosphere,mineral dust particles have attracted much attention because of their large emission,varied composition and sensitivity to environmental factors.Mineral dust particles are an important medium for the heterogeneous reactions of trace gases in the atmosphere,which contribute to the secondary generation of atmospheric particles,and can affect the global climate through long-distance transport from inland to remote oceans.In recent years,the frequent occurrence of dust weather has made the heterogeneous phase reaction between mineral dust particles and trace gases become a research hotspot in atmospheric chemistry.Although the transformation mechanism and kinetics of heterogeneous reactions have been extensively investigated in laboratory studies,the interaction mechanism of trace gases on the surface of mineral dust under complex atmospheric conditions remains unclear.Therefore,this work has deeply explored the heterogeneous reactions of atmospheric trace gases on the surface of mineral dust particles,aiming to provide a new understanding of the heterogeneous generation mechanism of atmospheric particles under actual environmental conditions.Specifically,the heterogeneous reactions of inorganic acid gases(SO2,NO2),organic acid gases(formic acid,acetic acid and acrylic acid)and alkaline gases(NH3)on the surface of typical mineral dust particles(TiO2,α-Fe2O3,kaolin particles KGa-2)were simulated by insitu Diffuse Reflection Infrared Fourier Transform Spectroscopy(DRIFTS).Through the online qualitative characterization of products in the reaction process and the quantitative characterization of NH4+,NO3-and other salts on the surface of particulate matter after the reaction,the heterogeneous reaction mechanism of a single trace gas on the surface of mineral dust particles and the interaction mechanism between complex trace gases(SO2 and acrylic acid,NH3 and organic acid gas,NH3 and inorganic acid gas)were analyzed.The influence of complex atmospheric conditions(light,relative humidity)on heterogeneous reactions was also investigated.The following were investigated,and related results were obtained:(1)Infrared spectroscopic study of heterogeneous reactions of SO2 on the surfaces of single component mineral dust particles.First,under dark conditions,the main product of the heterogeneous reaction of SO2 on the TiO2 surface is the S(Ⅳ)species.The TiO2 particles are excited under light,and the photogenerated electron-hole pairs promote the oxidation of SO2,triggering S(Ⅳ)oxidation to S(Ⅵ).When RH≈37%,water vapor inhibits the heterogeneous reaction of SO2,but light and water vapor have little effect on the reaction of acrylic acid.Secondly,the interaction between SO2 and acrylic acid on particulate matter is mainly through competition for active sites,and the pre-adsorption process promotes competition.Research results show that under dark conditions,regardless of the presence of water vapor,acrylic acid will significantly inhibit the heterogeneous reaction of SO2,while SO2 has almost no impact on the heterogeneous reaction of acrylic acid.On the contrary,SO2 inhibits the reaction of acrylic acid under light conditions,and the generated H2SO4 will reduce the pH of the surface of the particles,causing the strong acid to replace the weak acid,thereby accelerating the conversion of acrylic acid salt to acrylic acid.(2)Infrared spectroscopic study of heterogeneous reactions of NH3 on the surfaces of single component mineral dust particles.The heterogeneous reaction of NH3 on α-Fe2O3 particles can generate a stable NH3-Fe complex,promoting the dissolution of α-Fe2O3 particles,and the transformation of the valence state of iron ions on the surface of the particles leads to the oxidation of NH3 to NO3-.In the reaction process,light promotes the formation of NH4+and NO3-and the dissolution of α-Fe2O3 particles.Formic acid can strongly inhibit the heterogeneous reaction of NH3,while acetic acid and acrylic acid can promote the conversion of NH3 to NH4+.The deacidification of organic acids occupied most of the photoactive sites,resulting in a significant reduction of NH4+ and NO3-under light conditions.In addition,organic acids can inhibit the dissolution of iron oxide by weakening the complexation of NH3.About the influence of NH3 on organic acids,since the dipole moment of acrylic acid(2.47 D)is larger than those of formic acid(1.41 D)and of acetic acid(1.74 D),the heterogeneous uptake of formic acid and acetic acid is dominated by molecular polarity.Moreover,under light conditions,the uptake of the three organic acids in the mixed experiment was enhanced compared to that under dark conditions.(3)Infrared spectroscopic study of heterogeneous reactions of NH3 on the surfaces of mixed components mineral dust particles.SO2 and NO2 promote the conversion of NH3 to NH4+,but because the NH3-Fe complex generated in the separate reaction makes it difficult for NH3 to desorb during the purge process,SO2 and NO2 both promote the generation of NH4+under light conditions.For the heterogeneous oxidation of NH3,on α-Fe2O3 particles,compared to the indivudual reaction of NH3 SO2 will compete with NH3 for photogenic carriers under light conditions,causing the generation of NO3-to be inhibited.In the reaction involving NO2,NH3 inhibits the heterogeneous reaction of NO2,but the generation of NO3-increases under light,indicating that in the mixed reaction,in addition to the heterogeneous generation of NO2-the heterogeneous oxidation of NH3 promoted by light is also an important source of NO3-.Due to the low proportion of α-Fe2O3 and TiO2,two photoactive substances,the effect of light is limited.Therefore,the generation of NO3～and the dissolution of α-Fe2O3 particles were not observed in any experiment with KGa-2 particles.The effect of water vapor on the heterogeneous reactions of SO2 and NO2 with NH3 in the range of RH=0%-70%was studied.On α-Fe2O3 particles,the formation of NH4+and NO3-in the reaction of NH3 alone is negatively correlated with RH.The addition of SO2 promotes the production of NH4+at low RH by enhancing the interaction with NH3,which results in maximum uptake of NH3 at RH=15%,and futher decreases as RH continues to rise.On this basis,the reaction on synthetic KGa-2 particles was further studied,and it was found that the formation of NH4+ was inhibited by water vapor,and the addition of SO2 promoted the uptake of NH3,reaching the maximum uptake in the range of RH=15-30%.In addition,the reaction of NO2 also weakens with the increase of RH.Therefore,in the mixed reaction,although the conversion of NH3 to NH4+ is promoted by the addition of NO2,most of the active sites are still occupied by water vapor,and the generation of NH4+ and NO3-are negatively correlated with RH. |
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