Emily A. Weitkamp

Rethinking Organic Aerosols: Semivolatile Emissions and Photochemical Aging

Most primary organic-particulate emissions are semivolatile; thus, they partially evaporate with atmospheric dilution, creating substantial amounts of low-volatility gas-phase material. Laboratory experiments show that photo-oxidation of diesel emissions rapidly generates organic aerosol, greatly exceeding the contribution from known secondary organic-aerosol precursors. We attribute this unexplained secondary organic-aerosol production to the oxidation of low-volatility gas-phase species. Accounting for partitioning and photochemical processing of primary emissions creates a more regionally distributed aerosol and brings model predictions into better agreement with observations. Controlling organic particulate-matter concentrations will require substantial changes in the approaches that are currently used to measure and regulate emissions.

Constraining Particle Evolution from Wall Losses, Coagulation, and Condensation-Evaporation in Smog-Chamber Experiments: Optimal Estimation Based on Size Distribution Measurements

A goal of secondary organic aerosol (SOA) experiments performed in smog chambers is to determine the condensation of SOA onto suspended particles. Complicating the calculation of the condensation rate are uncertainties in particle wall-loss rates. Wall-loss rates generally depend on particle size, turbulence in the bag, the size and shape of the bag, and particle charge. In analyzing smog-chamber data, some or all of the following assumptions are commonly made regarding the first-order wall-loss rate constant: (a) that it is constant during an experiment; (b) that it is constant between experiments; and (c) that it is not a strong function of particle size for the relatively narrow size distributions in smog chamber experiments. Each of these assumptions may not be justified in some circumstances. We present the development and evaluation of the Aerosol Parameter Estimation (APE) model. APE is an inverse model that solves the aerosol general dynamic equation to determine best estimates for the size-dependent condensation rate and size-dependent wall-loss rate as a function of time. Size distribution measurements from a Scanning Mobility Particle Sizer (SMPS) provide time boundary conditions that constrain the general dynamic equation. The APE model is tested using data from a smog chamber experiment with dry ammonium sulfate particles in which no condensation occurred. Finally, we assess the variability in predicted SOA production between different wall-loss correction methods for relatively-fast-chemistry limonene-ozonolysis experiments and relatively-slow-chemistry toluene-oxidation experiments. In the fast limonene experiments, wall-loss correction methods agree within 10% for SOA production, and in the slow toluene experiments, wall-loss correction methods disagree up to a factor of 2.

Laboratory measurements of the oxidation kinetics of organic aerosol mixtures using a relative rate constants approach

[1] Organic aerosols in the atmosphere are exposed to oxidants, but the oxidation kinetics are largely unknown. We investigate the decay of organic species in laboratory-generated organic aerosols exposed to atmospherically relevant ozone concentrations in a smog chamber. The experiments were conducted using five different organic aerosols, varying in complexity from three to twelve components. These mixtures include alkenoic acids, alkanoic acids, alkanedioic acids, n-alkanes, and sterols and are designed to simulate meat cooking emissions. A relative rate constants approach was used to compare reaction rates of individual organic species and to compare the reaction rates of the aerosol species to gas phase tracers. Significant decay was observed for all species (except for the n-alkanes) in at least one of the experimental systems. By relating the decomposition of condensed phase alkenoic acids to gas phase alkenes, we show that the reaction rate constants of oleic acid and palmitoleic acid evolve as the aerosol is processed, decreasing by a factor of ∼10 over the course of a 4-hour experiment. The decay rate constants of cholesterol, oleic acid, and palmitic acid all depend strongly on aerosol composition, with more than an order of magnitude change in the effective rate constants depending on mixture composition. Effects of aerosol composition are likely to be even more significant in atmospheric aerosol, where particle compositions are highly variable. The data presented here indicate these mixture effects are complicated, making it difficult to extrapolate from simple laboratory systems to atmospherically relevant conditions.