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Black carbon is an important constituent of atmospheric aerosol particle matter (PM) with significant effects on the global radiation budget and on human health. The soot particle aerosol mass spectrometer (SP-AMS) has been developed and deployed for real-time ambient measurements of refractory carbon particles. In the SP-AMS, black carbon or metallic particles are vaporized through absorption of 1064 nm light from a CW Nd:YAG laser. This scheme allows for continuous "soft" vaporization of both core and coating materials. The main focus of this work is to characterize the extent to which this vaporization scheme provides enhanced chemical composition information about aerosol particles. This information is difficult to extract from standard SP-AMS mass spectra because they are complicated by extensive fragmentation from the harsh 70 eV EI ionization scheme that is typically used in these instruments. Thus, in this work synchotron-generated vacuum ultraviolet (VUV) light in the 8-14 eV range is used to measure VUV-SP-AMS spectra with minimal fragmentation. VUV-SP-AMS spectra of commercially available carbon black, fullerene black, and laboratory generated flame soots were obtained. Small carbon cluster cations (C(+)-C5(+)) were found to dominate the VUV-SP-AMS spectra of all the samples, indicating that the corresponding neutral clusters are key products of the SP vaporization process. Intercomparisons of carbon cluster ratios observed in VUV-SP-AMS and SP-AMS spectra are used to confirm spectral features that could be used to distinguish between different types of refractory carbon particles. VUV-SP-AMS spectra of oxidized organic species adsorbed on absorbing cores are also examined and found to display less thermally induced decomposition and fragmentation than spectra obtained with thermal vaporization at 200 °C (the minimum temperature needed to quantitatively vaporize ambient oxidized organic aerosol with a continuously heated surface). The particle cores tested in these studies include black carbon, silver, gold, and platinum nanoparticles. These results demonstrate that SP vaporization is capable of providing enhanced organic chemical composition information for a wide range of organic coating materials and IR absorbing particle cores. The potential of using this technique to study organic species of interest in seeded laboratory chamber or flow reactor studies is discussed.

Heterogeneous reactions of oleic acid aerosol particles with ozone are studied below 1% relative humidity. The particles have inert polystyrene latex cores (101-nm diameter) coated by oleic acid layers of 2 to 30 nm. The chemical content of the organic layer is monitored with increasing ozone exposure by using an aerosol mass spectrometer. The carbon-normalized percent yields of particle-phase reaction products are 20−35% 9-oxononanoic acid, 1−3% azelaic acid, 1−3% nonanoic acid, and 35−50% other organic molecules (designated as CHOT). There is approximately 25% evaporation, presumably as 1-nonanal. To explain the formation of CHOT molecules and the low yields of azelaic and nonanoic acids, we suggest a chemical mechanism in which the Criegee biradical precursors to azelaic acid and nonanoic acid are scavenged by oleic acid to form CHOT molecules. These chemical reactions increase the carbon-normalized oxygen content (z/x) of the CxHyOz layer from 0.1 for unreacted oleic acid to 0.25 after high ozone expo...

We report here the first experimental determination of the mass accommodation coefficient (α) of H217O(g) on aqueous sulfuric acid solutions. The uptake of 17O-labeled gas-phase water was measured as a function of temperature (250−295 K) and acid concentration (50−82 wt % H2SO4) using a droplet train flow reactor. The mass accommodation coefficient exhibits a negative temperature dependence and increases with increasing H2SO4 concentration. For 50 wt % sulfuric acid solution, the mass accommodation coefficient ranges from 0.50 ± 0.05 at 250 K to 0.41 ± 0.07 at 278 K, and for 70 wt % solution, it ranges from 0.69 ± 0.07 at 252 K to 0.54 ± 0.05 at 295 K. The dependence of the mass accommodation coefficient on the sulfuric acid composition correlates with surface coverage of water. The uptake coefficient of D2O was measured to be unity on 70 wt % H2SO4, independent of temperature between 263 and 293 K. The D2O uptake coefficient is larger than the mass accommodation coefficient for H217O because D−H isotope ...

With use of a droplet train apparatus, the uptake by 1-octanol of gas-phase α-pinene, γ-terpinene, p-cymene, and 2-methyl-2-hexanol was measured as a function of relative humidity and temperature (263−293 K). 1-Octanol was selected as a surrogate for hydrophobic oxygenated organic compounds found in tropospheric aerosols. The mass accommodation coefficients (α) of γ-terpinene, p-cymene, and 2-methyl-2-hexanol obtained from these measurements exhibit negative temperature dependences. The upper and lower values of α at 265 and 290 K, respectively, are as follows: for γ-terpinene 0.12 and 0.076; for p-cymene 0.20 and 0.097; and for 2-methyl-2-hexanol 0.25 and 0.11. The uptake of α-pinene is solubility limited, yielding values for the product HDl1/2 for α-pinene in 1-octanol (H = Henry's law constant, Dl = liquid-phase diffusion coefficient). With use of estimated values of Dl the Henry's law constant is obtained as ln H (M/atm) = −(6.59 ± 1.16) + (3.80 ± 0.31) × 103/T. The presence of water vapor does not a...

With use of a droplet train apparatus, the mass accommodation coefficients (α) of gas-phase HCl, HBr, HI, and CH3COOH were measured for 1-octanol to probe the nature of the hydrophobic organic surface as a function of relative humidity and temperature (263−283 K). In the absence of water vapor, α for both HBr(g) and HI(g) is unity, independent of temperature. The mass accommodation coefficients for acetic acid and HCl are smaller, about 0.3 for acetic acid and 0.01 for HCl at 273 K, displaying negative temperature dependence. The value of α for acetic acid is independent of relative humidity. However, values of α for HBr, HI, and HCl change dramatically as a function of relative humidity. As the relative humidity increases, the α values for HBr and HI decrease, and α for HCl increases. At a relative humidity of about 50%, α for all three species converges to that on pure water. A model is proposed to explain these unexpected results.

The second-order rate constant (k2) for the aqueous-phase reaction Cl2(aq) + OH- → HOCl + Cl- was determined by measuring the uptake of gas-phase chlorine into water in a bubble train flow reactor. In making these measurements, we avoided some of the difficulties encountered in earlier studies. Data were obtained at 275, 293, and 303 K, yielding k2 = (1.3 ± 0.5) × 108 M-1 s-1, (6 ± 2) × 108 M-1 s-1, and (8 ± 3) × 108 M-1 s-1, respectively. The atmospheric relevance of the chlorine hydrolysis reaction is discussed.

Organic compounds are a significant component of tropospheric aerosols. In the present experiments, ethylene glycol was selected as a surrogate hydrophilic organic compound for a study of surface properties of organic liquid−aqueous solutions. The mass accommodation coefficient of gas-phase HCl and HBr on ethylene glycol−water surfaces has been measured as a function of water mole fraction (0−1) and temperature. The gas uptake studies were performed under liquid−vapor equilibrium conditions using a droplet train flow reactor. The mass accommodation coefficient (α) for HCl on pure ethylene glycol increases from about 0.40 ± 0.06 at 303 K to 0.79 ± 0.12 at 258 K. Such negative temperature dependence for α has been observed in studies of gas uptake by aqueous surfaces. The HBr mass accommodation coefficient on ethylene glycol is near unity independent of temperature in the range studied. The mass accommodation coefficient on the ethylene glycol−water solution is well represented by α = αH2OX(s)H2O + αEG(1 − ...

The simultaneous uptake of gas-phase DMS−O3 mixtures by water as a function of temperature (274−300 K) was studied in a bubble train flow reactor. The uptake data yielded the second-order rate constant for the aqueous-phase reaction DMS + O3 → DMSO + O2. The reaction products were established with NMR analysis. The reaction rate constant k2 was measured at four temperatures: 274, 283, 293, and 300 K, yielding (5.1 ± 2.0) × 108 M-1 s-1, (5.9 ± 2.0) × 108 M-1 s-1, (8.6 ± 3.6) × 108 M-1 s-1, and (11 ± 4.5) × 108 M-1 s-1 respectively. It is noteworthy that the magnitude of the aqueous reaction rate constant exceeds the corresponding gas-phase rate by a factor of about 106. The atmospheric importance of the aqueous-phase DMS/O3 reaction is discussed.

The rate constant for the reaction of HO2 with O3 was measured using a turbulent flow reactor with tunable diode laser absorption detection of HO2. Two separate methods were used to determine k1 from the pseudo-first-order decay of HO2 in excess O3. The isotopic labeling method used H18O2 in excess 16O3 to avoid reformation of the reactant. The OH scavenger method used trifluorochloroethylene to remove the OH product and prevent re-formation of HO2. The turbulent flow reactor allowed measurements at temperatures between 297 and 197 K at total pressures from 80 to 175 Torr, spanning a wide range of stratospheric conditions. The temperature-dependent rate constant is given by the three-term expression k1(T) = {(103 ± 57) exp[−(1323 ± 160)/T] + 0.88} × 10-15 cm3 molecule-1 s-1, reflecting its non-Arrhenius behavior at low temperatures. These results demonstrate that the catalytic destruction of lower stratospheric O3 by HOx radicals (OH and HO2) may proceed faster than current models predict. The rate consta...

Uptake of gas-phase NO2 by water was studied in both the droplet train and bubble train flow reactors. Aqueous surface reaction of NO2(g), reported previously in the literature, is not substantiated by these studies. The uptake of NO2(g) is a function of Henry's law coefficient (HNO2) and the second-order NO2(aq)−NO2(aq) hydrolysis reaction rate coefficient (k2), in the form HNO2(k2)1/2. The NO2(aq)−NO2(aq) hydrolysis rate coefficient is defined by: −d[NO2(aq)]/dt = 2k2[NO2(aq)]2. The coupled nature of the uptake makes it difficult to obtain reliable separate values for the two parameters H and k2. Literature values for these parameters vary by as much as a factor of 5. With the bubble train apparatus it was possible to separate clearly the effects of solubility and reaction on NO2 gas uptake. From these measurements and analysis of literature values, recommended values of these parameters at 293 K are HNO2 = (1.4 ± 0.2) × 10-2 M atm-1 and k2 = (3.0 ± 0.9) × 107 M-1 s-1. At 276 K, our best estimates are ...

The uptake of gas-phase ammonia by aqueous surfaces was measured as a function of temperature, gas liquid interaction time, and pH in the range 0−13. Uptake measurements at low pH yielded values of the mass accommodation coefficient (α) as a function of temperature. The mass accommodation coefficient increases as the temperature decreases, from 0.08 at 290 K to 0.35 at 260 K. Time dependence of the uptake yielded values for the Henry's law constant. Uptake measurements at high pH indicate that an ammonia surface complex is formed at the interface. Codeposition studies in which an aqueous surface, initially at pH = 4, was simultaneously exposed to both gas-phase ammonia and SO2 were also performed. In such a codeposition experiment, the species entering the liquid neutralize each other and as a result the uptake of each species is enhanced. Modeling calculations indicate that the uptake of each species is in accord with bulk liquid-phase kinetics.

The uptake of gas-phase ammonia by sulfuric acid surfaces was measured as a function of temperature (248−288 K), gas−liquid interaction time (2−15 ms), and acid concentration (20−70 wt % H2SO4) using a droplet train apparatus. The uptake coefficient increases as a function of acid concentration and reaches unity at about 55 wt % H2SO4. The increased NH3 uptake in acid solution is apparently due to reaction between NH3 and H+ at the gas−liquid interface. The results yielded parameters required to model the reaction of NH3 with H+ at the gas−liquid interface. These uptake experiments were expanded to include a detailed study of gas transport to a moving train of droplets. An analysis of previous sulfuric acid aerosol neutralization experiments shows that the uptake of ammonia by ternary NH3−H2SO4−H2O solutions is significantly lower than that by fresh binary H2SO4−H2O solutions. At typical tropospheric water and ammonia vapor concentrations, NH3 uptake coefficients need to be included in detailed microphysi...

The experimental determination of the mass accommodation coefficient of H2SO4 vapor on aqueous sulfuric acid and the gas-phase diffusion coefficient of H2SO4 vapor in N2/H2O at 303 K is reported. The measurements were carried out under laminar flow conditions in a coated wall tubular flow reactor coupled to a chemical ionization mass spectrometer for gas-phase detection. Wall loss rates of H2SO4 vapor, from which both the mass accommodation coefficient and the gas diffusion coefficient were determined, were measured as a function of total reactor pressure, water vapor concentration, and sulfuric acid vapor concentration. The observed wall loss rate coefficient depends linearly on the inverse of the total reactor pressure (0.54−10 Torr) and is independent of the aqueous sulfuric acid composition over the range 73−98 wt %, which was varied by the addition of water vapor. A kinetic model based on the additivity of kinetic resistances that couples gas-phase diffusion and mass accommodation to the measured H2S...

The gas-phase reaction of SO3 with H2O and the heterogeneous reaction of SO3 with H2O−H2SO4 surfaces have been studied in a fast flow reactor coupled to a chemical ionization mass spectrometer (CIMS) for species detection. The gas-phase reaction was studied under turbulent flow conditions over the pressure range from 100 to 760 Torr N2 and the temperature range from 283 to 370 K. The loss rate of SO3 was measured under pseudo-first-order conditions; it exhibits a second-order dependence on water vapor concentration and has a strong negative temperature dependence. The first-order rate coefficient for the SO3 loss by gas-phase reaction shows no significant pressure dependence and can be expressed as kI(s-1) = 3.90 × 10-41 exp(6830.6/T)[H2O]2 where [H2O] is in units of molecule cm-3 and T is in Kelvin. The overall uncertainty of our experimentally determined rate coefficients is estimated to be ±20%. At sufficiently low SO3 concentrations (