Multiphase acid-catalyzed oxidation by hydrogen peroxide has been suggested to be a potential route to secondary organic aerosol formation from isoprene and its gas-phase oxidation products, but the lack of kinetics data significantly limited the evaluation of this process in the atmosphere. Here we report the first measurement of the uptake of isoprene, methacrylic acid and methyl methacrylate into aqueous solutions of sulfuric acid and hydrogen peroxide. Isoprene cannot readily partition into the solution because of its high volatility and low solubility, which hinders its further liquid-phase oxidation. Both methacrylic acid and methyl methacrylate can enter the solutions and be oxidized by hydrogen peroxide, and steady-state uptake was observed with the acidity of solution above 30 wt.% and 70 wt.%, respectively. The steady-state uptake coefficient of methacrylic acid is much larger than that of methyl methacrylate for a solution with same acidity. These observations can be explained by the different reactivity of these two compounds caused by the different electron-withdrawing conjugation between carboxyl and ester groups. The atmospheric lifetimes were estimated based on the calculated steady-state uptake coefficients. These results demonstrate that the multiphase acid-catalyzed oxidation of methacrylic acid plays a role in secondary organic aerosol formation, but for isoprene and methyl methacrylate, this process is not important in the troposphere.
A cavity ring down spectroscopy instrument was introduced and designed for measuring extinction efficiencies of pure and mixing aerosols in this paper.Through averaging 500 individual waveforms,the minimal detectable aerosol extinction coefficient of 8.4 × 10-7 m-1 was achieved.By the test results using the NaCl particles,we concluded that this system could measure the extinction efficiencies of an aerosol with an uncertainty less than 3% under laboratory controlled experimental conditions.The refractive indices of different aerosols were retrieved through comparing the measured extinction efficiencies of each aerosol type with which predicted by Mie theory.Aerosols composed of ammonium sulphate and succinic acid with different weight ratios were used to create a model of mixed aerosols using these two materials,whose extinction efficiencies and complex refractive indices were derived.The refractive indices of the mixed aerosols were also calculated by various optical mixing rules.We found that all the molar refraction/absorption mixing rule,the volume ratio linear rule,and Maxwell-Garnett rule did provide comparable results,of which the volume ratio linear rule gave a slightly worse fit than the others.
Heterogeneous reactions of nitrogen dioxide （NO2） on soils collected from Dalian （S 1） and Changsha （S2） were investigated over the relative humidity （RH） range of 5%-80% and temperature range of 278-328 K using a horizontal coated-wall flow tube. The initial uptake coefficients of NO2 on S2 exhibited a decreasing trend from （10 ± 1.3） × 10-8 to （3.1 ± 0.5） x 10-8 with the relative humidity increasing from 5% to 80%. In the temperature effect studies, the initial uptake coefficients of S1 and S2 decreased from （10± 1.2） × 10-8 to （3.8 ± 0.5） × 10-8 and from （16± 2.2） × 10-8 to （3.8 ±0.4） × 10-8 when temperature increased from 278 to 288 K for S1 and from 278 to 308 K for S2, respectively. As the temperature continued to increase, the initial uptake coefficients of S1 and S2 returned to （7.9 ± 1.1）× 10-8 and （20 ± 3.1） × 10-8 at 313 and 328 K, respectively. This study shows that relative humidity could influence the uptake kinetics of NO2 on soil and temperature would impact the heterogeneous chemistry of NO2.
