Uptake of pyrene by NaCl, NaNO3, and MgCl2 aerosol particles

Photoelectric charging experiments measure heterogeneous uptake coefficients for pyrene on model marine aerosol particles, including NaCl, NaNO(3), and MgCl(2). The analysis employs a multilayer kinetic model that contains adsorption and desorption rate constants for the bare aerosol surface and for pyrene-coated surfaces. First coating the aerosol particles with a pyrene layer and following the desorption using both t-DMA and photoelectric charging yields the desorption rate constants. Separate experiments monitor the increase in surface coverage of initially bare aerosol particles after exposure to pyrene vapor in a sliding-injector flow tube. Analyzing these data using the multilayer model constrained by the measured desorption rate constants yields the adsorption rate constants. The calculated initial heterogeneous uptake coefficient, γ(0)(295 K), is 1.1 × 10(-3) for NaCl, 6.6 × 10(-4) for NaNO(3), and 6.0 × 10(-4) for MgCl(2). The results suggest that a free energy barrier controls the uptake rate rather than kinematics.     

Homogeneous ice nucleation from aqueous inorganic/organic particles representative of biomass burning: water activity, freezing temperatures, nucleation rates

Homogeneous ice nucleation plays an important role in the formation of cirrus clouds with subsequent effects on the global radiative budget. Here we report on homogeneous ice nucleation temperatures and corresponding nucleation rate coefficients of aqueous droplets serving as surrogates of biomass burning aerosol. Micrometer-sized (NH(4))(2)SO(4)/levoglucosan droplets with mass ratios of 10:1, 1:1, 1:5, and 1:10 and aqueous multicomponent organic droplets with and without (NH(4))(2)SO(4) under typical tropospheric temperatures and relative humidities are investigated experimentally using a droplet conditioning and ice nucleation apparatus coupled to an optical microscope with image analysis. Homogeneous freezing was determined as a function of temperature and water activity, a(w), which was set at droplet preparation conditions. The ice nucleation data indicate that minor addition of (NH(4))(2)SO(4) to the aqueous organic droplets renders the temperature dependency of water activity negligible in contrast to the case of aqueous organic solution droplets. The mean homogeneous ice nucleation rate coefficient derived from 8 different aqueous droplet compositions with average diameters of ～60 μm for temperatures as low as 195 K and a(w) of 0.82-1 is 2.18 × 10(6) cm(-3) s(-1). The experimentally derived freezing temperatures and homogeneous ice nucleation rate coefficients are in agreement with predictions of the water activity-based homogeneous ice nucleation theory when taking predictive uncertainties into account. However, the presented ice nucleation data indicate that the water activity-based homogeneous ice nucleation theory overpredicts the freezing temperatures by up to 3 K and corresponding ice nucleation rate coefficients by up to ～2 orders of magnitude. A shift of 0.01 in a(w), which is well within the uncertainty of typical field and laboratory relative humidity measurements, brings experimental and predicted freezing temperatures and homogeneous ice nucleation rate coefficients into agreement. The experimentally derived ice nucleation data are applied to constrain the water activity-based homogeneous ice nucleation theory to smaller than ±1 order of magnitude compared to the predictive uncertainty of larger than ±6 orders of magnitude. The atmospheric implications of these findings are discussed.     

Heterogeneous reactivity of chlorine atoms with sodium chloride and synthetic sea salt particles

The uptake of chlorine atoms on sodium chloride (NaCl) and synthetic sea salt (SSS) particles was studied using a discharge flow reactor coupled to a molecular beam mass spectrometer. The reactive surfaces were prepared by coating the inner surface of the reactor using two different methods: either by depositing size-selected particles on halocarbon wax or by spray depositing thin films using a constant output atomizer. The observed uptake coefficients of Cl˙ on NaCl particles are γ(NaCl)(Cl) ≈ 2 × 10(-2) for size-selected particles or γ(NaCl)(Cl) ≈ 5 × 10(-2) for thin films and for SSS particles γ(SSS)(Cl) ≈ 4 × 10(-3). Heterogeneous recombination of Cl atoms to Cl(2) molecules was observed for the two solid surfaces. The study was performed over the temperature range 258 to 353 K. The temperature dependence of the uptake was observed and the heat of adsorption of Cl˙ on NaCl particles was estimated at Q(ads) = 63 kJ mol(-1) assuming an Eley-Rideal mechanism. The role of surface adsorbed water has also been shown. The atmospheric implications of these findings are discussed briefly.     

Uptake of SO2 on HOBr-ice surfaces

The uptake of SO2 on HOBr-treated ice surfaces has been studied using a flow reactor coupled with a differentially pumped quadrupole mass spectrometer at 190-240 K. The initial uptake coefficient was determined as a function of HOBr surface coverage, theta(HOBr), on the ice. The uptake coefficients increase as the HOBr coverage increases. The uptake coefficient can be expressed as gamma(t) = k(h)theta(HOBr), where k(h) = 1.5 x 10(-19) molecules(-1) cm(-2) at 191 K and k(h) = 6.4 x 10(-21) molecules(-1) cm(-2) at 210 K and theta(HOBr) is in the range of 8 x 10(13) to 1.2 x 10(15) molecules cm(-2). The effects of temperature and film thickness on the uptake coefficients of SO2 by the HOBr-treated ice films were also studied. The activation energy E(a) of SO(2) on HOBr-ice surfaces is approximately -81 +/- 8 kJ/mol in the 190-215 K range. Kinetic results were interpreted in terms of the Eley-Rideal mechanism. This study suggests that the uptake of SO2 on ice/snow surfaces is enhanced by the presence of HOBr near the ice surface. The implication for atmospheric chemistry is that HOBr-ice surfaces may not provide a significant pathway to oxide S(IV) in the boundary layer due to both lower uptake coefficient and smaller HOBr surface coverage at T > 220 K.     

Mass accommodation of H2SO4 and CH3SO3H on water-sulfuric acid solutions from 6% to 97% RH

The uptake of H2SO4 and CH3SO3H onto particles composed of water and sulfuric acid was studied in a laminar flow reactor at atmospheric pressure. Their first-order gas-phase loss rate coefficients were determined using a chemical ionization mass spectrometer. Relative humidity was varied from 6% to 97% at 295-297.5 K. The mass accommodation coefficient, alpha, was found to be close to unity for both species. These findings show that alpha does not limit particle growth rates resulting from H2SO4 and CH3SO3H uptake. Diffusion coefficients in N2 for these two species are also reported and a significant dependence upon relative humidity was seen for H2SO4 but not for CH3SO3H. Last, production of small particles was observed due to the presence of SO2 in particle chargers. Formation of these particles can be significantly reduced by adding an OH scavenger such as propane.     

Kinetic study of the reaction of OH with CH2I2

Flash photolysis (FP) coupled to resonance fluorescence (RF) was used to measure the absolute rate coefficients (k(1)) for the reaction of OH(X(2)Π) radicals with diiodomethane (CH(2)I(2)) over the temperature range 295-374 K. The experiments involved time-resolved RF detection of the OH (A(2)Σ(+)→X(2)Π transition at λ = 308 nm) following FP of the H(2)O/CH(2)I(2)/He mixtures. The OH(X(2)Π) radicals were produced by FP of H(2)O in the vacuum-UV at wavelengths λ > 120 nm. Decays of OH radicals in the presence of CH(2)I(2) are observed to be exponential, and the decay rates are found to be linearly dependent on the CH(2)I(2) concentration. The results are described by the Arrhenius expression k(1)(T) = (4.2 ± 0.5) × 10(-11) exp[-(670 ± 20)K/T] cm(3) molecule(-1) s(-1). The implications of the reported kinetic results for understanding the atmospheric chemistry of CH(2)I(2) are discussed.     

Ultra-slow water diffusion in aqueous sucrose glasses

We present measurements of water uptake and release by single micrometre-sized aqueous sucrose particles. The experiments were performed in an electrodynamic balance where the particles can be stored contact-free in a temperature and humidity controlled chamber for several days. Aqueous sucrose particles react to a change in ambient humidity by absorbing/desorbing water from the gas phase. This water absorption (desorption) results in an increasing (decreasing) droplet size and a decreasing (increasing) solute concentration. Optical techniques were employed to follow minute changes of the droplet's size, with a sensitivity of 0.2 nm, as a result of changes in temperature or humidity. We exposed several particles either to humidity cycles (between ～2% and 90%) at 291 K or to constant relative humidity and temperature conditions over long periods of time (up to several days) at temperatures ranging from 203 to 291 K. In doing so, a retarded water uptake and release at low relative humidities and/or low temperatures was observed. Under the conditions studied here, the kinetics of this water absorption/desorption process is controlled entirely by liquid-phase diffusion of water molecules. Hence, it is possible to derive the translational diffusion coefficient of water molecules, D(H(2)O,) from these data by simulating the growth or shrinkage of a particle with a liquid-phase diffusion model. Values for D(H(2)O)-values as low as 10(-24) m(2) s(-1) are determined using data at temperatures down to 203 K deep in the glassy state. From the experiment and modelling we can infer strong concentration gradients within a single particle including a glassy skin in the outer shells of the particle. Such glassy skins practically isolate the liquid core of a particle from the surrounding gas phase, resulting in extremely long equilibration times for such particles, caused by the strongly non-linear relationship between concentration and D(H(2)O). We present a new parameterization of D(H(2)O) that facilitates describing the stability of aqueous food and pharmaceutical formulations in the glassy state, the processing of amorphous aerosol particles in spray-drying technology, and the suppression of heterogeneous chemical reactions in glassy atmospheric aerosol particles.     

Hydroxyl radical at the air-water interface

Interaction of the hydroxyl radical with the liquid water surface was studied using classical molecular dynamics computer simulations. From a series of scattering trajectories, the thermal and mass accommodation coefficients of OH on liquid water at 300 K were determined to be 0.95 and 0.83, respectively. The calculated free energy profile for transfer of OH across the air-water interface at 300 K exhibits a minimum in the interfacial region, with the free energy of adsorbtion (DeltaGa) being about 1 kcal/mol more negative than the hydration free energy (DeltaGs). The propensity of the hydroxyl radical for the air-water interface manifests itself in partitioning of OH radicals between the bulk water and the surface. The enhancement of the surface concentration of OH relative to its concentration in the aqueous phase suggests that important OH chemistry may be occurring in the interfacial layer of water droplets, aqueous aerosol particles, and thin water films adsorbed on solid surfaces. This has profound consequences for modeling heterogeneous atmospheric chemical processes.     

Temperature dependence of the mass accommodation coefficients of 2-nitrophenol, 2-methylphenol, 3-methylphenol, and 4-methylphenol on aqueous surfaces

The uptake of 2-nitrophenol, 2-methylphenol, 3-methylphenol, and 4-methylphenol on aqueous surfaces was investigated between 278 and 303 K, using the wetted-wall flow tube technique coupled with UV absorption spectroscopic detection. The uptake coefficients gamma were found to be independent of the aqueous phase composition and of the gas-liquid contact times. In addition, the uptake coefficients and the derived mass accommodation coefficients alpha show a negative temperature dependence in the temperature range studied. The mass accommodation coefficients decrease from 5.2 x 10(-3) to 8.3 x 10(-4), from 5.0 x 10(-3) to 3.1 x 10(-4), from 6.7 x 10(-3) to 7.3 x 10(-4), and from 1.2 x 10(-2) to 5.9 x 10(-4) for 2-nitrophenol, 2-methylphenol, 3-methylphenol, and 4-methylphenol, respectively. These results are used to discuss the incorporation of these species into the liquid using the nucleation theory. These data combined with the Henry's law constants were used to estimate the partitioning of the phenolic compounds between gaseous and aqueous phases and the corresponding atmospheric lifetimes under clear sky (tau(gas)) and cloudy conditions (tau(multiphase)) have then been derived.     

Partitioning of metals between the aqueous phase and suspended insoluble material in fog droplets

This paper discusses the partitioning of metals (K, Na, Ca, Mg, Al, Cu, Fe, Pb and Zn) between the aqueous phase and the suspended insoluble material in fog samples collected in the Po Valley during two extensive fields campaigns. Metals represent on average 11% of the mass of suspended insoluble matter, while the main component is carbon (both organic carbon, OC = 35%, and black carbon, BC = 8%). The unaccounted suspended matter mass is very high, on average 46%, and is attributable to non metallic species, such as O and N and of Si. The principal metals in the insoluble suspended fraction are Fe and Al (2-5%), while the contributions of other metals (Na, Mg, Cu, Pb and Zn) are lower than 1%. Ca and K exhibited high blank values and could not be detected above blank detection limit threshold. The main components in the aqueous phase are NO3- (34%), WSOC (23%), SO4(2-) (18%) and NH4+ (19%), while trace metals and remaining cations and anions accounted for less than 1% of solute mass. The main dissolved trace metals in fog droplets are Zn, Al and Fe, while the main metallic cations are Na and Ca. Fe and Al are the only metals preferentially distributed in the suspended insoluble matter of fog droplets (partitioning ratio respectively 37% and 33%). All other metals are mostly dissolved in the aqueous phase (mean partitioning ratios of Mg, Pb, Zn, Cu and Na are 69%, 70%, 77%, 81% and 87%). These findings are in agreement with literature data on metal speciation in cloud and rain samples. The dependence of partitioning ratios on pH is investigated for the different metals, with only Al showing a clear partitioning ratio decrease with increasing pH. Conversely, the other metals show no dependence or a complex and highly variable behaviour. The partitioning ratio of iron (mean 37%) observed in the Po Valley fog samples is much higher than the water extractable iron in aerosol particles (typically 1-2 %): this fact can be explained by differences in the aerosol sources and composition among sites and by chemical processes in the aqueous phase, such as complexation and redox reactions involving organic ligands (oxalate, or other organic acids as humic-like organic matter) which may promote Fe solubility.     

Mass spectral evidence that small changes in composition caused by oxidative aging processes alter aerosol CCN properties

Oxidative processing (i.e., "aging") of organic aerosol particles in the troposphere affects their cloud condensation nuclei (CCN) activity, yet the chemical mechanisms remain poorly understood. In this study, oleic acid aerosol particles were reacted with ozone while particle chemical composition and CCN activity were simultaneously monitored. The CCN activated fraction at 0.66 +/- 0.06% supersaturation was zero for 200 nm mobility diameter particles exposed to 565 to 8320 ppmv O3 for less than 30 s. For greater exposure times, however, the particles became CCN active. The corresponding chemical change shown in the particle mass spectra was the oxidation of aldehyde groups to form carboxylic acid groups. Specifically, 9-oxononanoic acid was oxidized to azelaic acid, although the azelaic acid remained a minor component, comprising 3-5% of the mass in the CCN-inactive particles compared to 4-6% in the CCN-active particles. Similarly, the aldehyde groups of alpha-acyloxyalkylhydroperoxide (AAHP) products were also oxidized to carboxylic acid groups. On a mass basis, this conversion was at least as important as the increased azelaic acid yield. Analysis of our results with K?hler theory suggests that an increase in the water-soluble material brought about by the aldehyde-to-carboxylic acid conversion is an insufficient explanation for the increased CCN activity. An increased concentration of surface-active species, which decreases the surface tension of the aqueous droplet during activation, is an interpretation consistent with the chemical composition observations and K?hler theory. These results suggest that small changes in particle chemical composition caused by oxidation could increase the CCN activity of tropospheric aerosol particles during their atmospheric residence time.     

Kinetics of N(2)O(5) hydrolysis on secondary organic aerosol and mixed ammonium bisulfate-secondary organic aerosol particles

The kinetics of the hydrolysis reaction of N(2)O(5) on secondary organic aerosol (SOA) produced through the ozonolysis of α-pinene and on mixed ammonium bisulfate-SOA particles was investigated using an entrained aerosol flow tube coupled to a chemical ionization mass spectrometer. We report room temperature uptake coefficients, γ, on ammonium bisulfate and SOA particles at 50% relative humidity of 1.5 × 10(-2) ± 1.5 × 10(-3) and 1.5 × 10(-4) ± 2 × 10(-5), respectively. For the mixed ammonium bisulfate-SOA particles, γ decreased from 2.6 × 10(-3) ± 4 × 10(-4) to 3.0 × 10(-4) ± 3 × 10(-5) as the SOA mass fraction increased from 9 to 79, indicating a strong suppression in γ with the addition of organic material. There is an order-of-magnitude reduction in the uptake coefficient with the smallest amount of SOA material present and smaller additional reductions with increasing aerosol organic content. This newly coated organic layer may either decrease the mass accommodation coefficient of N(2)O(5) onto the particle or hinder the dissolution and diffusion of N(2)O(5) into the remainder of the aerosol after it has been accommodated onto the surface. The former corresponds to a surface effect and the latter to bulk processes. The low value of the uptake coefficient on pure SOA particles will likely make N(2)O(5) hydrolysis insignificant on such an aerosol, but atmospheric chemistry models need to account for the role that organics may play in suppressing the kinetics of this reaction on mixed organic-inorganic particles.     

Temperature induced formation of particle coated non-spherical droplets

Herein we offer a simple method to produce non-spherical emulsion droplets stabilized by freshly formed Mg(OH)(2) nanoparticles (MPs). The non-spherical degree of droplets as a function of experiment conditions was investiged and the origins of the presence of non-spherical droplets were discussed. The results of optical microscope images show that stable spherical droplets can be fused into non-spherical at given aging temperature. It is also recognized that particle concentration, oil/water ratio and aging time significantly affect droplet fusion and excess particles that are not adsorbed on the oil/water interface are helpful in restraining droplet fusion. Based on the TEM, XRD and Fluorescence confocal microscopy results, the origins of droplet fusion are inferred from the presence of vacant holes in the particle layer. Because of Oswald ripening, particles on droplet surfaces grow larger than the freshly precipitated ones under a given aging temperature. The growth of particles results in the reduction of total cover area of particle layer and thus creates vacant holes in the particle layer which would cause partial coalescence of droplets once they collide. Thus, these findings can offer a simple alternative to obtain a large amount of non-spherical emulsion droplets but also can help the preparation of non-spherical colloid particles.     

Infrared optical constants of crystalline sodium chloride dihydrate: application to study the crystallization of aqueous sodium chloride solution droplets at low temperatures

Complex refractive indices of sodium chloride dihydrate, NaCl·2H(2)O, have been retrieved in the 6000-800 cm(-1) wavenumber regime from the infrared extinction spectra of crystallized aqueous NaCl solution droplets. The data set is valid in the temperature range from 235 to 216 K and was inferred from crystallization experiments with airborne particles performed in the large coolable aerosol and cloud chamber AIDA at the Karlsruhe Institute of Technology. The retrieval concept was based on the Kramers-Kronig relationship for a complex function of the optical constants n and k whose imaginary part is proportional to the optical depth of a small particle absorption spectrum in the Rayleigh approximation. The appropriate proportionality factor was inferred from a fitting algorithm applied to the extinction spectra of about 1 μm sized particles, which, apart from absorption, also featured a pronounced scattering contribution. NaCl·2H(2)O is the thermodynamically stable crystalline solid in the sodium chloride-water system below the peritectic at 273.3 K; above 273.3 K, the anhydrous NaCl is more stable. In contrast to anhydrous NaCl crystals, the dihydrate particles reveal prominent absorption signatures at mid-infrared wavelengths due to the hydration water molecules. Formation of NaCl·2H(2)O was only detected at temperatures clearly below the peritectic and was first evidenced in a crystallization experiment conducted at 235 K. We have employed the retrieved refractive indices of NaCl·2H(2)O to quantify the temperature dependent partitioning between anhydrous and dihydrate NaCl particles upon crystallization of aqueous NaCl solution droplets. It was found that the temperature range from 235 to 216 K represents the transition regime where the composition of the crystallized particle ensemble changes from almost only NaCl to almost only NaCl·2H(2)O particles. Compared to the findings on the NaCl/NaCl·2H(2)O partitioning from a recent study conducted with micron-sized NaCl particles deposited onto a surface, the transition regime from NaCl to NaCl·2H(2)O is shifted by about 13 K to lower temperatures in our study. This is obviously related to the different experimental conditions of the two studies. The partitioning between the two solid phases of NaCl is essential for predicting the deliquescence and ice nucleation behavior of a crystalline aerosol population which is subjected to an increasing relative humidity.     

Solubility of acetic acid and trifluoroacetic acid in low-temperature (207-245 k) sulfuric acid solutions: implications for the upper troposphere and lower stratosphere

The solubility of gas-phase acetic acid (CH(3)COOH, HAc) and trifluoroacetic acid (CF(3)COOH, TFA) in aqueous sulfuric acid solutions was measured in a Knudsen cell reactor over ranges of temperature (207-245 K) and acid composition (40-75 wt %, H(2)SO(4)). For both HAc and TFA, the effective Henry's law coefficient, H*, is inversely dependent on temperature. Measured values of H* for TFA range from 1.7 × 10(3) M atm(-1) in 75.0 wt % H(2)SO(4) at 242.5 K to 3.6 × 10(8) M atm(-1) in 40.7 wt % H(2)SO(4) at 207.8 K. Measured values of H* for HAc range from 2.2 × 10(5) M atm(-1) in 57.8 wt % H(2)SO(4) at 245.0 K to 3.8 × 10(8) M atm(-1) in 74.4 wt % H(2)SO(4) at 219.6 K. The solubility of HAc increases with increasing H(2)SO(4) concentration and is higher in strong sulfuric acid than in water. In contrast, the solubility of TFA decreases with increasing sulfuric acid concentration. The equilibrium concentration of HAc in UT/LS aerosol particles is estimated from our measurements and is found to be up to several orders of magnitude higher than those determined for common alcohols and small carbonyl compounds. On the basis of our measured solubility, we determine that HAc in the upper troposphere undergoes aerosol partitioning, though the role of H(2)SO(4) aerosol particles as a sink for HAc in the upper troposphere and lower stratosphere will only be discernible under high atmospheric sulfate perturbations.     

Infrared optical constants of highly diluted sulfuric acid solution droplets at cirrus temperatures

Complex refractive indices for supercooled sulfuric acid solution droplets in the mid-infrared spectral regime (wavenumber range 6000-800 cm(-1)) have been retrieved for acid concentrations ranging from 33 to 10 wt % H2SO4 at temperatures between 235 and 230 K, from 36 to 15 wt % H2SO4 at temperatures between 225 and 219 K, and from 37 to 20 wt % H2SO4 at temperatures between 211 and 205 K. The optical constants were derived with a Mie inversion technique from measured H2SO4/H2O aerosol extinction spectra that were recorded during controlled expansion cooling experiments in the large coolable aerosol chamber AIDA of Forschungszentrum Karlsruhe. The new data sets cover a range of atmospherically relevant temperatures and compositions in the binary sulfuric acid/water system for which infrared refractive indices have not been published so far, namely, the regime when supercooled H2SO4/H2O solution droplets at T < 235 K are subjected to an environment that is supersaturated with respect to the ice phase. With increasing ice supersaturation, the H2SO4/H2O aerosol particles will continuously dilute by the uptake of water vapor from the gas phase until freezing of the solution droplets eventually occurs when the acid concentration has dropped below a critical, temperature-dependent threshold value. With the aid of the new measurements, the homogeneous freezing process of supercooled H2SO4/H2O solution droplets at cirrus temperatures can be quantitatively analyzed by means of Fourier transform infrared spectroscopy, thereby overcoming a major drawback from previous studies: the need to use complex refractive indices that were measured at temperatures well above 235 K to deduce the composition of the low-concentrated H2SO4/H2O aerosol particles. As in the case of the complex refractive indices for sulfuric acid solutions with acid concentrations greater than 37 wt % H2SO4, the new low-temperature optical constants for highly diluted droplets also reveal significant temperature-induced spectral variations in comparison with the refractive indices for higher temperatures, which are associated with a change in the equilibrium between sulfate and bisulfate ions.     

Laser tweezers Raman study of optically trapped aerosol droplets of seawater and oleic acid reacting with ozone: implications for cloud-droplet properties

In this communication we report the first use of the Raman laser tweezers technique to trap and hold a mixed droplet of oleic acid and water at atmospheric pressure for 30 min, oxidize the oleic acid on the droplet, follow the decay of reactants and the growth of chemical products using Raman spectroscopy, and monitor the growth in size of the droplet as it becomes more hydrophilic. We demonstrate that the oxidation of organic films on water droplets could have large climatic effects in the atmosphere. We show that cloud-droplet growth and activation of cloud condensation nuclei (to become cloud droplets) is retarded by the presence of an organic film and that chemical oxidation of this film would allow a cloud droplet to grow, reducing cloud albedo by inducing precipitation, and would allow a cloud condensation nucleus to grow to a cloud droplet, thus forming a cloud and increasing the albedo of the Earth.     

The heterogeneous interaction of Br(2P3/2) and Br(2P1/2) with surfaces of Teflon™ and polycrystalline nickel

Pulses of Br(²P_3/2) and Br(²P_1/2) (=Brast;) have been exposed to Teflon (PTFE) and to polycrystalline Ni surfaces in a Knudsen cell. The Br and Br* atom densities have been measured as a function of time using [3 + 2] Resonance Enhanced Multiphoton Ionization (REMPI) at 461.9 and 459.1 nm, respectively, and an absolute calibration of the sum of the density of Br and Br* on Teflon at ambient temperature has been measured to result in identical values within experimental error for both Br and Br*, namely γ(Br) = (5.6 ± 1.5) · 10^?5, if an appropriate correction for the radiative lifetime of Br* of 0.77 s^?1 is applied. The uptake coefficients for Br and Br* on polycrystalline Ni seem to be identical: γ(Br) = γ(Br*) = (5.6 ± 1.8) · 10^?3 and independent of temperature in the range 295 to 500 K. A possible exception is the value for γ(Br*) of 2.3 · 10^?3 at T = 295 K which seems to be significantly lower than the remainder of the uptake data. In the temperature range 500 to 700 K the uptake coefficients for both Br and Br* can be expressed as . The system has a small positive activation energy in the range 3.3 to 4.5 kcal/mol. Br* seems to be less reactive than Cl* with respect to surface deactivation on poly Ni by a factor of six. In analogy to Cl the present system is characterized by kinetic complications in conjunction with the reversible surface poisoning of bromine, both atomic and molecular, on surfaces of Teflon (PTFE) and poly Ni that leads to the decrease of Br and Br* uptake with increasing exposure. © 1995 John Wiley & Sons, Inc.     

Observation of the binary coalescence and equilibration of micrometer-sized droplets of aqueous aerosol in a single-beam gradient-force optical trap

The binary coalescence of aqueous droplets has been observed in a single-beam gradient-force optical trap. By measuring the time-dependent intensity for elastic scattering of light from the trapping laser, the dynamics of binary coalescence have been examined and the time scale for equilibration of a composite droplet to ambient conditions has been determined. These data are required for modeling the agglomeration of aqueous droplets in dense sprays and atmospheric aerosol. Elastic-light scattering from optically trapped particles has not been used previously to study the time-resolved dynamics of mixing. It is shown to offer a unique opportunity to characterize the binary coalescence of aqueous droplets with radii from 1 to 6 μm. The study of this size regime, which cannot be achieved by conventional imaging methods, is critical for understanding the interactions of droplets in the environment of dense sprays.     

Uptake of methacrolein into aqueous solutions of sulfuric acid and hydrogen peroxide

Multiphase acid-catalyzed oxidation by hydrogen peroxide has been suggested to be a potential route to secondary organic aerosol (SOA) formation from isoprene and its gas-phase oxidation products, but the kinetics and chemical mechanism remain largely uncertain. Here we report the first measurement of uptake of methacrolein into aqueous solutions of sulfuric acid and hydrogen peroxide in the temperature range of 253-293 K. The steady-state uptake coefficients were acquired and increased quickly with increasing sulfuric acid concentration and decreasing temperature. Propyne, acetone, and 2,3-dihydroxymethacrylic acid were suggested as the products. The chemical mechanism is proposed to be the oxidation of carbonyl group and C═C double bonds by peroxide hydrogen in acidic environment, which could explain the large content of polyhydroxyl compounds in atmospheric fine particles. These results indicate that multiphase acid-catalyzed oxidation of methacrolein by hydrogen peroxide can contribute to SOA mass in the atmosphere, especially in the upper troposphere.