Nicole C. Shantz

Particle formation and growth from ozonolysis of α‐pinene

Observations of particle nucleation and growth during ozonolysis of α-pinene were carried out in Calspans 600 m3 environmental chamber utilizing relatively low concentrations of α-pinene (15 ppb) and ozone (100 ppb). Model simulations with a comprehensive sectional aerosol model which incorporated the relevant gas-phase chemistry show that the observed evolution of the size distribution could be simulated within the accuracy of the experiment by assuming only one condensable product produced with a molar yield of 5% to 6% and a saturation vapor pressure (SVP) of about 0.01 ppb or less. While only one component was required to simulate the data, more than one product may have been involved, in which case the one component must be viewed as a surrogate having an effective SVP of 0.01 ppb or less. Adding trace amounts of SO2 greatly increased the nucleation rate while having negligible effect on the overall aerosol yield. We are unable to explain the observed nucleation in the α-pinene/ozone system in terms of classical nucleation theory. The nucleation rate and, more importantly, the slope of the nucleation rate versus the vapor pressure of the nucleating species would suggest that the nucleation rate in the α-pinene/ozone system may be limited by the initial nucleation steps (i.e., dimer, trimer, or adduct formation).

In-cloud oxidation of SO2 by O3 and H2O2: Cloud chamber measurements and modeling of particle growth

Controlled cloud chamber experiments were conducted to measure particle growth resulting from the oxidation of SO2 by O3 and H2O2 in cloud droplets formed on sulfuric acid seed aerosol. Clouds were formed in a 590 m3 environmental chamber with total liquid water contents ranging from 0.3–0.6 g m−3 and reactant gas concentrations <10 ppbv for SO2 and H2O2 and <70 ppbv for O3. Aerosol growth was measured by comparison of differential mobility analyzer size distributions before and after each 3–4 min cloud cycle. Predictions of aerosol growth were then made with a full microphysical cloud model used to simulate each individual experimental cloud cycle. Model results of the H2O2 oxidation experiments best fit the experimental data using the third-order rate constant of Maass et al. [1999] (k = 9.1 × 107 M−2 s−1), with relative aerosol growth agreeing within 3% of measured values, while the rate of Hoffmann and Colvert [1985] produced agreement within 4–9%, and the rate of Martin and Damschen [1981] only within 13–18%. Simulation results of aerosol growth during the O3 oxidation experiments were 60–80% less than the measured values, confirming previous results [Hoppel et al., 1994b]. Experimental results and analyses presented here show that the SO2 - O3 rate constants would have to be more than 5 times larger than currently accepted values to explain the measured growth. However, unmeasured NH3 contamination present in trace amounts (<0.2 ppb) could explain the disagreement, but this is speculative and the source of this discrepancy is still unknown.

Chamber measurements of CI depletion in cloud‐processed sea‐salt aerosol

Cl− depletion in sea-salt CCN was measured in chamber experiments after cloud formation in the presence of SO2 and O3. In each experiment, two successive clouds were formed on a sea-salt aerosol generated from the nebulization of filtered seawater, while chamber SO2 (g) and O3 (g) concentrations ranged from 4.8–8.3 ppb and 69–75 ppb. Particle growth from cloud processing reactions was measured with a differential mobility analyzer (DMA) immediately after cloud dissipation, and particles activated from dry radii of 0.02–0.03 μm showed growth to 0.08 and 0.12 μm, with total aerosol mass increases of 7.0 and 2.4 μg m−3 (assuming a particle density of 2.4 g cm−3). Particle growth was greater than that predicted by standard SO2-O3 oxidation kinetics and cannot be accounted for by the buffering capacity of the sea-salt CCN. Micro-orifice impactor (MOI) measurements of inorganic ions in the postcloud aerosol size distributions show, within analytical error, a 1:1 displacement of Cl− with nss - SO42−, until the SO42− is in excess and the Cl− is completely depleted at the smaller CCN sizes. Calculations of Cl− depletion, based upon the input size distribution and assuming particle growth via sulfate addition, show greater Cl− depletion on smaller size particles than the measured values. However, there is agreement within the uncertainty of the measured values.

Optical Properties of Aerosol Particles over the Northeast Pacific

Abstract In July 2002, atmospheric aerosol measurements were conducted over the northeast Pacific Ocean as part of the Subarctic Ecosystem Response to Iron Enhancement Study (SERIES). The following aerosol quantities were measured: particle number size distribution, particle scattering and backscattering coefficients at three wavelengths, particle absorption coefficient at one wavelength, and size-segregated particle chemical composition. Using Mie theory to calculate the aerosol particle scattering and absorption coefficients from the size distribution and chemical measurements, closure with the measured optical coefficients is not attained. Discrepancies between the calculated and measured scattering and backscattering coefficients are largely a result of the fact that the nephelometer measures scattering only between 7° and 170°. Over 90% of the total scattering and 50% of the backscattering in this study was not measured by the nephelometer because of the missing forward-scattering (0°–7°) and backsca...