Aerosol formation and growth in atmospheric organic/NOx systems

[摘要] Secondary atmospheric aerosols are formed by gas-to-particle conversion of condensible vapors produced by reactions of primary species such as organics, NOx, SO2, and NH3. The rates and mechanisms leading to organic aerosol formation are the least well understood aspect of secondary atmospheric aerosols. Gas-phase measurements of organics, NOx, O3, and measurements of particle formation and growth have been made in smog chamber experiments to determine the total aerosol yields of the photochemical oxidation of various organics. Measurements of size distribution dynamics reveal the competition between nucleation and condensation, allowing estimation of the physical properties of the aerosol formed and the likelihood that a particular organic forms aerosol in the atmosphere.A new scanning electrical mobility spectrometer (SEMS) was developed to monitor aerosol size distribution dynamics. The measurement of particle size distributions using electrical mobility has been significantly accelerated using a new mode of operating mobility instruments. Rather than changing the electric field in discrete steps to select particles in a given mobility range, the electric field is scanned continuously. The particles are classified in a time-varying electric field, but for an exponential ramp in the field strength, there remains a one-to-one correspondence between the time a particle enters the classifier and the time it leaves. By this method, complete scans of mobility with as many as 100 mobility measurements have been made in 30 seconds using a differential mobility classifier with a condensation nuclei counter as a detector.Outdoor smog chamber experiments have been performed to determine the aerosol forming potential of selected C7- and C8- hydrocarbons in sunlight-irradiated hydrocarbon NOx mixtures. Measured aerosol size distributions were used to determine the rates of gas-to-particle conversion and to study the effects of the addition of SO2 and/or NH3 on aerosol formation and growth. The average aerosol yields by mass for the hydrocarbons studied were: methylcyclohexane9.2% 1-octene 4.2% toluene 18.6% n-octane<0.001% Addition of SO2 to the organic/NOx systems led to an early nucleation burst and subsequent rapid growth of the newly formed aerosols. In the presence of NH3, the gas-to-particle conversion rate of the organic/NOx system was enhanced perhaps due to the formation of NH4NO3 or the reaction of NH3 with carboxylic acids. Sustained particle formation was observed when both SO2 and NH3 were present, presumably a result of (NH4)2SO4 formation. We have estimated the complexity of the 1-octene aerosol and identified 5-propyl furanone as a major component of the aerosol.Aerosol dynamics that were observed in the outdoor smog chamber experiments are simulated by numerical solution of the aerosol general dynamic equation. The vapor source generation rate was estimated directly from the experimental measurements assuming a single surrogate condensing species for each hydrocarbon studied. Sensitivity analysis of the simulated aerosol dynamics to various input parameters revealed that the physical properties of the condensing vapor are important in determining the interplay between nucleation and condensation while the vapor source generation rate is the only factor that determines the eventual total amount of vapor converted to aerosol. The simulations suggest that over 99% of the mass of condensible vapor is converted to aerosol by condensation even when a significant burst of nucleation occurs.

[发布机构] University:California Institute of Technology;Department:Engineering and Applied Science

[关键词] Environmental Science and Engineering