Laboratory Study On Hygroscopicity And Optical Properties Of Submicron Particles In Ambient Air

The hygroscopic properties of the atmospheric aerosol play a crucial role in atmospheric chemistry and climate change. Denoted as ability of water uptake, hygroscopic growth will dramatically alter the size, phase state and optical properties of the atmospheric aerosol particles, thus directly affects the air quality, visibility, radiation forcing and climate change. Since the sources, chemical composition, size and mixing state of atmospheric aerosols are distinctly difference, thus, close attention has been paid to study the relationship between the hygroscopic properties and chemical composition, size and mixing state of atmospheric aerosols.In this paper, the impacts of RH and particle size on hygroscopic properties of several salts particles,(NH4)2SO4, NaCl, Na2SO4, NaNO3, NH4Cl and NH4NO3included, are investigated using H-TDMA technique. And the iso-GF curves are put forward to differentiate the impacts of size-effect and RH. In addition, the hygroscopicity and optical properties of alkyaminium sulfates are investigated by using H-TDMA system coupled with CRDS and Nephelometer. Meantime, the hygroscopic and optical properties of soot reaction with H2SO4and triethylamine (TEA) are also measured using these instruments.The present studies mainly accomplished four parts of work as follows:The first part focused on the paricle size effects on the hygroscopicity of non-volatile inorganic salts. When RH is higher than the deliquescence point (DRH), the GFs of (NH4)2SO4, NaCl, Na2SO4and NaNO3aerosols increase with Do throughout the investigated size range. Further analyses reveal the GFs increase pronouncedly for particles in the range of20-100nm, and the increase trend relatively slows down for particles in the range of100-200nm. This phenomenon was mainly caused by Kelvin effect. In this study, to distinguish the dominant impact on GFs from size-effect and RH, iso-GF curves with Do and RH as the coordinates are put forward. Above DRH, the GFs of (NH4)2SO4, NaCl, Na2SO4, are more sensitive to particle size for particles smaller than60nm, while the GFs are more sensitive to RH for particles larger than80nm. For NaNO3aerosols, at80%≤RH≤86%, the GFs are easier to increase with Do for particles smaller than60nm, while they are easier to increase with RH for particles larger than80nm. However, at20%≤RH≤70%, it is noted that the GFs are more sensitive to the RH than to Do even though the particles are smaller than60nm.The second part focused on the paricle size and RH effects on the hygroscopicity and evaporation of NH4Cl and NH4NO3aerosols, and discusses the evaporation mechanism of ammonium salts aerosols. When RH is higher than the DRH, the GFs of NH4Cl and NH4NO3aerosols increase with Do throughout the investigated size range. Further analysis indicates that the GFs increase pronouncedly for particles in the range of40-100nm, and the increase trend relatively slows down for particles in the range of100-200nm. This phenomenon was mainly caused by Kelvin effect. The iso-GF curves reveal that the GFs of NH4Cl and NH4NO3particles are more sensitive to Do than RH both at RH below and above its DRH due to their unique volatility properties, which are quite different with the performances of (NH4)2SO4, NaCl, Na2SO4and NaNO3aerosols. In addition, the proposed evaporation mechanism of NH4Cl and NH4NO3aerosols derived in this study suggests that solid NH4X(S)(X represent Cl-and NO3-) equilibrates to its precursors NH3(g) and HX(g) at a stable condition. Increases of RH can alter the chemical equilibrium, i.e., NH4X{s)(?)NH3(g)+HX(g), by converting NH3(g) and HX(g) into NH3·nH2O and HX·nH2O, which will accelerate the evaporation of NH4X(s).The third part focused on the hygroscopicity and optical properties of alkylaminium sulfates. In this study, Ethylaminium sulfate (EAS), Diethyaminium sulfate (DEAS) and Triethylaminium sulfate (TEAS) do not exhibit the deliquescence phenomenon and show continuous growing with RH increasing. The closely hygroscopic growth curves of those three alkylaminium sulfates indicate the hygroscopicity of alkylaminium sulfates are ethyl group number independent. The ZSR model results reveal that, when the RH less than80%, the hygroscopic growth of the mixing aerosol (DEAS and (NH4)2SO4) are enhanced markedly with increasing of the fraction of DEAS. While this enhance trend slows down when the RH higher than80%. The optical results shows the extinction and scattering cross section are decreased when RH changed from2%to30%, which may be caused by the change of particle phase with RH incrasing. While as the RH further increases, the extinction and scattering cross section will increases since the particle size of EAS、DEAS and TEAS are significantly increased during the hygroscopic growth process.The fourth part focused on the impact of heterogeneous reaction on the hygroscopicity and optical properties of soot aerosols. Soot particles exposed to sulfuric acid vapor (Soot+SA) exhibit a marked enhance in hygroscopicity, and the hygroscopic growth curve shows a continuous growing with RH increasing. While the hygroscopic growth curve of Soot reaction with H2SO4and TEA (Soot+TEAS) exhibit an obviously deliquescence point and the DRH occurs between70%and85%. In addition, the GF at85%RH of Soot+TEAS larger than Soot+SA indicated that the TEA will further enhance the hygroscopic growth of soot particles. The TEM measurements revealed a marked change in morphology of the particles:the TEAS will be form and enclosure the soot particles during the heterogeneous reaction. This morphology change will significantly affect the optical properties of soot particles. In this study, we found the absorption cross section of soot at5%RH was decreases during the heterogeneous reaction process, this was mainly caused by the TEAS enclosure of Soot aerosols. While at85%RH, the absorption cross section is increased due to the water content increases of the hygroscopic particles.