Measured Uptake Coefficients Research Articles

Uptake of Hg0(g) on TiO2, Al2O3, and Fe2O3 Nanoparticles: Importance in Atmospheric Chemical and Physical Processes.

Mineral dust aerosols play an important role in tropospheric chemistry and aerosol-cloud interaction processes. Yet, their interactions with gaseous elemental mercury (Hg0(g)) are not currently well understood. Using a coated-wall flow tube (CWFT) reactor, we measured the uptake of Hg0(g) on some common components of mineral dust aerosols, including TiO2, Al2O3, and Fe2O3, and the effects of irradiation (dark, visible and UV-A) and relative humidity (<2% to 60%) on the uptake kinetics. Under UV-A irradiation (320-400 nm) in dry air, we measured uptake coefficients (γ) equal to >1 × 10-3 and (3 ± 1) × 10-6 on TiO2 and Al2O3, respectively. Under visible light irradiation (380-700 nm), Hg0(g) uptake was only observed on TiO2, with γ = (4 ± 3) × 10-4. Raising the relative humidity inhibited the uptake on both TiO2 and Al2O3, and the uptake coefficient at 60% RH for TiO2 under UV-A irradiation was lower by ca. 3 orders of magnitude than dry conditions. Furthermore, we observed that water vapor induced the desorption of two distinct fractions from Hg-exposed surfaces via the displacement of weakly, physisorbed Hg and the photocatalyzed reduction of chemisorbed Hg. Based on the uptake coefficients from this report, we estimate that heterogeneous interactions with mineral dust may be significant under conditions with low relative humidity (<30%) and high dust loading masses. We herein discuss the implication of this study on understanding the life cycle analysis of atmospheric mercury in nature.

Unexpectedly High Indoor HONO Concentrations Associated with Photochemical NO2 Transformation on Glass Windows.

Nitrous acid (HONO) is an important gaseous pollutant contributing to indoor air pollution because it causes adverse health effects and is the main source of hydroxyl radicals (OH). Here, we present direct measurements of HONO produced through light-induced heterogeneous reactions of NO2 with grime adsorbed on glass window. The uptake coefficients of NO2 [γ(NO2)] on the glass plates from the kitchen increased markedly from (2.3 ± 0.1) × 10-6 at 0% RH to (4.1 ± 0.5) × 10-6 at 90% RH. We report a significant quantity of daytime HONO produced in the kitchen, compared to the living room and bedroom. Kinetic modeling suggests that phase state and bulk diffusivity play important roles in the NO2 uptake; the best fit to the measured uptake coefficients is obtained with fixed diffusion coefficients. Photon scattering may be occurring at the surface of the films, leading to enhanced photon-excitation rates of polycyclic aromatic hydrocarbons. By taking these effects into account, the results from this study indicate that the HONO yields obtained in this study can explain the missing HONO in the photochemical models describing the indoor air chemistry.

Bed flow photoreactor experiments to assess the photocatalytic nitrogen oxides abatement under simulated atmospheric conditions

Small scale bed flow photoreactor experiments were performed to assess the photocatalytic performance of cement-based TiO2-containing materials for NOx reduction through the determination of kinetic parameters under variation of the experimental conditions (relative humidity, flow rate, mixing ratio and light intensity) and monitoring of potential reaction products in the gas phase and the aqueous extract of the surface.The results clearly demonstrated the general potential of the tested material to photocatalytically remediate gas phase NOx by conversion into nitrite and nitrate as identified reaction products at the surface. The measured uptake coefficients for NO and NO2 under atmospheric relevant conditions were in the range of 5 × 10−5 with a corresponding surface deposition velocity of about 0.5 cm s−1. However, it became also clear that the photocatalytic activity is in part significantly dependent on the experimental conditions. The relative humidity and the mixing ratio of the air pollutant were identified as the most important parameters. In addition, under certain conditions, a renoxification process can occur.The comprehensive results of the present study are discussed in detail to develop recommendations for a possible future application of this technique to improve urban air quality.

Uptake of Gaseous Alkylamides by Suspended Sulfuric Acid Particles: Formation of Ammonium/Aminium Salts.

Amides represent an important class of nitrogen-containing compounds in the atmosphere that can in theory interact with atmospheric acidic particles and contribute to secondary aerosol formation. In this study, uptake coefficients (γ) of six alkylamides (C1 to C3) by suspended sulfuric acid particles were measured using an aerosol flow tube coupled to a high resolution time-of-flight chemical ionization mass spectrometer (HRToF-CIMS). At 293 K and < 3% relative humidity (RH), the measured uptake coefficients for six alkylamides were in the range of (4.8-23) × 10-2. A negative dependence upon RH was observed for both N-methylformamide and N,N-dimethylformamide, likely due to decreased mass accommodation coefficients (α) at lower acidities. A negative temperature dependence was observed for N,N-dimethylformamide under < 3% RH, also consistent with the mass accommodation-controlled uptake processes. Chemical analysis of reacted sulfuric acid particles indicates that alkylamides hydrolyzed in the presence of water molecules to form ammonium or aminium. Our results suggest that multiphase uptake of amides will contribute to growth of atmospheric acidic particles and alter their chemical composition.

Effect of soot microstructure on its ozonization reactivity

Large uncertainty among the measured uptake coefficients of O(3) on soot highlights the importance of the sources and chemical structures of soot samples in this reaction. Soot samples with different microstructures were prepared by combusting n-hexane under controlled conditions. Their reactivities to O(3) were further investigated using in situ Raman spectroscopy. The fuel/oxygen ratio in the combustion experiments not only affected the diameter of the primary particles, but also influenced the micro-chemical structure of soot. Average diameters of soot particles decreased with the decreasing fuel/oxygen ratio. Compared to the "fuel-rich" flame soot, the "fuel-lean" flame soot showed lower structural uniformity with higher disordered carbon content at the graphene layer edges (D1 band) and the surface graphene layers (D2 band) and the amorphous carbon content (D3 band). This disordered carbon was identified as the reactive component for the ozonization of both the "fuel-rich" and "fuel-lean" flame soot samples. The kinetics study demonstrated that the disordered carbon at the surface graphene layers was more active than that at the graphene layer edges in one sample, and the reactivity of these two microstructures types to O(3) in the "fuel-rich" flame soot was higher than that in the "fuel-lean" flame soot.

Reactive Uptake of N2O5 by Aerosols Containing Dicarboxylic Acids. Effect of Particle Phase, Composition, and Nitrate Content

Reactive uptake coefficients for loss of N(2)O(5) to micron-size aerosols containing oxalic malonic, succinic, and glutaric acids, and mixtures with ammonium hydrogen sulfate and ammonium sulfate, are presented. The uptake measurements were made using two different systems: atmospheric pressure laminar flow tube reactor (Cambridge) and the Large Indoor Aerosol Chamber at Forschungszentrum Juelich. Generally good agreement is observed for the data recorded using the two techniques. Measured uptake coefficients lie in the range 5 x 10(-4)-3 x 10(-2), dependent on relative humidity, on particle phase, and on particle composition. Uptake to solid particles is generally slow, with observed uptake coefficients less than 1 x 10(-3), while uptake to liquid particles is around an order of magnitude more efficient. These results are rationalized using a numerical model employing explicit treatment of both transport and chemistry. Our results indicate a modest effect of the dicarboxylic acids on uptake and confirm the strong effect of particle phase, liquid water content, and particulate nitrate concentrations.

Light induced conversion of nitrogen dioxide into nitrous acid on submicron humic acid aerosol

Abstract. The interactions of aerosols consisting of humic acids with gaseous nitrogen dioxide (NO2) were investigated under different light conditions in aerosol flow tube experiments at ambient pressure and temperature. The results show that NO2 is converted on the humic acid aerosol into nitrous acid (HONO), which is released from the aerosol and can be detected in the gas phase at the reactor exit. The formation of HONO on the humic acid aerosol is strongly activated by light: In the dark, the HONO-formation was below the detection limit, but it was increasing with the intensity of the irradiation with visible light. Under simulated atmospheric conditions with respect to the actinic flux, relative humidity and NO2-concentration, reactive uptake coefficients γrxn for the NO2→HONO conversion on the aerosol between γrxn &lt;10−7 (in the dark) and γrxn=6×10−6 were observed. The observed uptake coefficients decreased with increasing NO2-concentration in the range from 2.7 to 280 ppb and were dependent on the relative humidity (RH) with slightly reduced values at low humidity (&lt;20% RH) and high humidity (&gt;60% RH). The measured uptake coefficients for the NO2→HONO conversion are too low to explain the HONO-formation rates observed near the ground in rural and urban environments by the conversion of NO2→HONO on organic aerosol surfaces, even if one would assume that all aerosols consist of humic acid only. It is concluded that the processes leading to HONO formation on the Earth surface will have a much larger impact on the HONO-formation in the lowermost layer of the troposphere than humic materials potentially occurring in airborne particles.

Measurement of Effective Knudsen Diffusion Coefficients for Powder Beds Used in Heterogeneous Uptake Experiments

The effective Knudsen diffusion coefficients for characteristic oxide powder beds used in heterogeneous uptake experiments have been measured using countercurrent diffusion and transient pressure drop techniques. Room-temperature thermal-velocity-normalized effective Knudsen diffusion coefficients are found to lie in the 0.15 to 0.35 microm range for magnesium silicate, aluminum oxide, and iron oxide powder beds. Measured values are compared with theoretical estimates and are consistent with low bed tortuosities (below 3) expected for media with open porosity above 0.5. The impact of uncertainties in effective diffusion coefficients on corrections of measured uptake coefficients is discussed. The value of careful uptake measurements in both the low and high sample mass limits is reinforced, as this allows uptake corrections independent of explicitly measured or estimated diffusion coefficient values. It is suggested that correction procedures requiring tortuosity values greater than 3 are suspect.

Heterogeneous Uptake of 8−2 Fluorotelomer Alcohol on Liquid Water and 1-Octanol Droplets

The heterogeneous uptake of the 8-2 fluorotelomer alcohol, F(CF2)8CH2CH2OH, on liquid water surfaces over the temperature range 256-273 K and on 1-octanol surfaces over the temperature range 264-295 K has been investigated with a droplet train flow reactor. The uptake coefficient on water droplets is zero within the error of the measurement (+/-0.01) and is independent of droplet temperature. In contrast, significant uptake onto 1-octanol is observed. Measured uptake coefficients for 1-octanol showed a negative temperature dependence, varying from 0.034 +/- 0.005 (1sigma) at 295 K to 0.103 +/- 0.009 at 264 K. The measured uptake coefficients on 1-octanol were independent of gas-liquid contact time, for typical contact times varying between 3 and 15 ms, and independent of the 8-2 fluorotelomer alcohol gas-phase concentration, indicating that the uptake coefficients are equivalent to mass accommodation coefficients. The uptake coefficients on 1-octanol were also independent of relative humidity. These results show that the uptake of FTOHs on or into the aqueous component of cloud/fog droplets or aqueous aerosol particles is not likely to be an important atmospheric sink for these compounds. In these experiments, 1-octanol was used as a model compound for organic-containing atmospheric particles. The larger uptake coefficient measured for 1-octanol surfaces indicates that FTOH partitioning to organic-containing cloud/fog droplets and aerosol particles may be an atmospheric loss mechanism.

Laboratory Investigation of Heterogeneous Interaction of Sulfuric Acid with Soot

The internal mixing state of soot with sulfuric acid is believed to significantly impact the optical, cloud-forming, and chemical properties of soot-containing aerosols, but little is known about the interaction between soot and sulfuric acid. We report the first measurements of the uptake of H2SO4 on three types of soot generated from methane, hexane, and kerosene combustion. H2SO4 loss on soot is found to be irreversible. The measured uptake coefficients are 0.018 +/- 0.007 for kerosene soot, 0.012 +/- 0.006 for methane soot, and 0.0076 +/- 0.0016 for hexane soot at a total pressure of 1-2 Torr and 298 K assuming a geometric surface area, likely corresponding to the upper limits. Additional experiments using the differential mobility analysis and Fourier transform infrared spectroscopy techniques are carried out to further characterize the interaction of H2SO4 with soot. The results suggest that uptake of H2SO4 takes place efficiently on soot particles, representing an important route to convert hydrophobic soot to hydrophilic aerosols.

Modelling the impact of noctilucent cloud formation on atomic oxygen and other minor constituents of the summer mesosphere

Abstract. The formation, evolution and eventual sublimation of noctilucent clouds (NLC) may have a significant effect on the odd oxygen and hydrogen chemistry of the high latitude summer mesosphere. Three mechanisms are considered here: the direct uptake of atomic oxygen on the surface of the ice particles; the redistribution of water vapour, which changes the photochemical source of odd hydrogen species; and the direct photolysis of the ice particles themselves to produce odd hydrogen species in the gas phase. A 1-D photochemical model is employed to investigate the potential importance of these mechanisms. This shows, using the recently measured uptake coefficients of O on ice, that the heterogeneous removal of O on the surface of the cloud particles is too slow by at least a factor of 5x103 to compete with gas-phase O chemistry. The second and third mechanisms involve the solar Lyman-α photolysis of H2O in the gas and solid phase, respectively. During twilight, Lyman-α radiation is severely attenuated and these mechanisms are insignificant. In contrast, when the upper mesosphere is fully illuminated there is a dramatic impact on the O profile, with depletion of O at the base of the cloud layer of close to an order of magnitude. A correspondingly large depletion in O3 is also predicted, while H, OH, HO2 and H2O2 are found to be enhanced by factors of 3-5. In fact, rocket-borne mass spectrometer measurements during summer have revealed local H2O2 enhancements in the region of the clouds. Rocket-borne measurements of atomic O and O3 profiles in the presence of mesospheric clouds in the daytime are highly desirable to test the predictions of this model and our understanding of the genesis of mesospheric clouds.

Uptake and reaction of atmospheric organic vapours on organic films

Films composed in whole or in part of organic compounds represent an important atmospheric interface. Urban surfaces are now known to be coated with a film ("grime") whose chemical composition somewhat resembles that of urban atmospheric aerosols. Such films may act as media in which atmospheric trace gases may be sequestered (leading to their removal from the gas phase); they may also act as reactive media, either as a "solvent" or as a source of reagents. Organic coatings on aqueous surfaces are also important, not just on ocean and lake surfaces ("biofilms") but also on the surfaces of fogwaters and atmospheric aerosol particles. We have initiated experimental uptake studies of trace gases into simple proxies for urban organic films using two techniques: a Knudsen cell effusion reactor and a laser-induced fluorescence method. We will discuss our first results on non-reactive uptake of organic compounds by organic films we use as proxies for urban grime coatings. In general, the measured uptake coefficients appear to track the octanol-air partition coefficients, at least qualitiatively. We have also measured the kinetics of reactions between gas-phase ozone and small polycyclic aromatic hydrocarbons (PAHs), when these are adsorbed at the air-aqueous interface or incorporated into an organic film. Reactions at the "clean" air-water interface and at a coated interface consisting of a monolayer of various amphiphilic organic compounds all follow a Langmuir-Hinshelwood mechanism, in which ozone first adsorbs to the air-aqueous interface, then reacts with already adsorbed PAH. By contrast, the reaction in the pure organic film occurs in the bulk phase. Under some circumstances, heterogeneous oxidation of PAHs by ozone may be as important in the atmosphere as their gas phase oxidation by OH.

Nitrogen dioxide multiphase chemistry: Uptake kinetics on aqueous solutions containing phenolic compounds

The uptake coefficients of NO2 on aqueous solutions containing guaiacol, syringol and catechol were determined over the pH range from 1 to 13 using the wetted wall flowtube technique. The measured uptake coefficients were used to determine the rate coefficients for the reaction of the physically dissolved NO2 with the neutral and deprotonated forms of phenolic compounds listed above. These organic compounds are ubiquitous not only in biomass burning plumes but also in soils, where they form part of the building blocks of humic acids. The NO2 uptake kinetics on solutions containing guaiacol, syringol or catechol were observed to be strongly pH dependent with uptake coefficients increasing from below 10(-7), under acidic conditions, to more than 10(-5) at pH values above 10. This behaviour illustrates the difference of reactivity between the neutral phenolic species and the phenoxide ions. The corresponding second order rate coefficients were typically observed to increase from 10(5) M(-1) s(-1) for the neutral compounds to a minimum of 10(8) M(-1) s(-1) for the phenoxide ions.

Reactive uptake coefficients for heterogeneous reaction of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt; with submicron aerosols of NaCl and natural sea salt

Abstract. The kinetics of uptake of gaseous N2O5 on submicron aerosols containing NaCl and natural sea salt have been investigated in a flow reactor as a function of relative humidity (RH) in the range 30-80% at 295±2K and a total pressure of 1bar. The measured uptake coefficients, γ, were larger on the aerosols containing sea salt compared to those of pure NaCl, and in both cases increased with increasing RH. These observations are explained in terms of the variation in the size of the salt droplets, which leads to a limitation in the uptake rate into small particles. After correction for this effect the uptake coefficients are independent of relative humidity, and agree with those measured previously on larger droplets. A value of γ=0.025 is recommended for the reactive uptake coefficient for N2O5 on deliquesced sea salt droplets at 298K and RH&gt;40%.

Mass Accommodation Coefficients of Phenol, 2-Nitrophenol, and 3-Methylphenol over the Temperature Range 278−298 K

The reactive uptake of phenol, 2-nitrophenol, and 3-methylphenol (m-cresol) was measured in a vertical wetted-wall flow reactor over the temperature range 278−298 K using bromine as an aqueous phase scavenger. First-order decays in gas-phase concentration as a function of increased gas−liquid contact time in the reactor were monitored by UV absorption downstream of the contact zone. Mass accommodation coefficients, α, were derived from measured uptake coefficients by correcting for limitations to mass transfer from radial gas-phase diffusion. Temperature-dependent expressions fitted to the data yielded values of α that decrease from 3.7 × 10-2 to 6.6 × 10-3 for phenol, 1.5 × 10-2 to 1.1 × 10-3 for 2-nitrophenol, and 1.0 × 10-2 to 5.1 × 10-3 for m-cresol over the range 278 K to 298 K. (Estimated overall uncertainty in α values of ∼±30%). These are the first published accommodation data for the latter two aromatic species. The thermodynamic data derived from the values of α were interpreted in terms of the ...

Heterogeneous reactions of NO2 and HNO3 on oxides and mineral dust: A combined laboratory and modeling study

This study combines laboratory measurements and modeling analysis to quantify the role of heterogeneous reactions of gaseous nitrogen dioxide and nitric acid on mineral oxide and mineral dust particles in tropospheric ozone formation. At least two types of heterogeneous reactions occur on the surface of these particles. Upon initial exposure of the oxide to NO2 there is a loss of NO2 from the gas phase by adsorption on the particle surface, i.e., NO2(g) → NO2(a). As the reaction proceeds, a reduction of gaseous NO2 to NO, NO2 (g) → NO (g) is found to occur. Initial uptake coefficients γ0 for NO2 on the surface of these particles have been measured at 298 K using a Knudsen cell reactor coupled to a mass spectrometer. For the oxides studied, α,γ‐Al2O3, α,γ‐Fe2O3, TiO2, SiO2, CaO, and MgO, γ0 ranges from &lt;4×10−10 for SiO2 to 2×10−5 for CaO with most values in the 10−6 range. For authentic samples of China loess and Saharan sand, similar reactivity to the oxides is observed with γ0 values of 2×10−6 and 1×10−6, respectively. For HNO3 the reactivity is 1–2 orders of magnitude higher. Using these laboratory measurements, the impact of heterogeneous reactions of NO2 and HNO3 on mineral dust in tropospheric ozone formation and on O3‐precursor relationships is assessed using a time‐dependent, multiphase chemistry box model. Simulations with and without heterogeneous reactions were conducted to evaluate the possible influence of these heterogeneous reactions on ambient levels. Results show that values of the initial uptake for NO2 and HNO3, adjusted for roughness effects, must be greater than 10−4 to have an appreciable impact on NOx, HNO3, and O3 concentrations for the conditions modeled here. Thus the measured uptake coefficients for NO2 on dry surfaces are just below the lower limit to have an impact on the photochemical oxidant cycle, while the heterogeneous reactivity of HNO3 is sufficiently large to have an effect. Under conditions of high mineral dust mass loadings and/or smaller size distributions the importance of these reactions (both NO2 and HNO3) is expected to increase.

Unexpected large uptake of O3 on sea salts and the observed Br2 formation

Uptake coefficients of O3 on various model compounds of sea salts at ambient temperature were measured using a Knudsen cell reactor coupled to a differentially pumped quadrupole mass spectrometer. Uptake of O3 was below the detection limit (&lt;1×10−5) for pure alkali metal bromides, whereas reactive uptake with Br2 formation was observed on two types of synthetic sea salts (Aqua Ocean and Instant Ocean) and commercial natural sea salt. Measured uptake coefficients were 10−2–10−3, (1.4±1.1) × 10−3 and (9.7±4.6) × 10−4 for Aqua Ocean, Instant Ocean and natural sea salt, respectively. These uptake coefficients were about three orders of magnitude larger than that on bulk bromide solution estimated from known liquid phase reactions. The results imply that minor components enhance reactivity of O3 on synthetic and natural sea salts. This heterogeneous reaction is potentially an important Br source in the marine boundary layer on a global scale as well as in the polar troposphere.

The uptake of SO2 on synthetic sea salt and some of its components

The uptake of sulfur dioxide (SO2) on synthetic sea salt (SSS) and its components, NaCl and MgCl2•6H2O, was studied at 298 K using a Knudsen cell interfaced to a quadrupole mass spectrometer. Significant uptake on dried salts was not observed, placing upper limits on the uptake coefficients, γ, of &lt; 1 × 10−4 for NaCl, &lt; 5 × 10−4 for MgCl2•6H2O, and &lt; 8 × 10−5 for SSS. However, SSS and MgCl2•6H2O that had not been dried before use showed significant uptake of SO2. The magnitude of the uptake depended strongly on the exposure time and the amount of water desorbing. Initially, the measured uptake coefficients for SO2 on SSS were as high as 0.09, but they rapidly decreased below 10−2 with a t−1/2 dependence as expected for approach to the equilibrium saturation concentration in an aqueous solution. The decreasing uptake coefficient slowly approaches zero over hours, consistent with reactions in a water layer with species such as CaCO3. The products of the reaction were shown by diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) to include low solubility metal sulfites. These studies show that uptake of SO2 on sea salt particles, even below their deliquescence/effluorescence points, can be treated as if it is into an aqueous salt solution.

HONO Solubility and Heterogeneous Reactivity on Sulfuric Acid Surfaces

The solubility of HONO in various wt % sulfuric acid solutions was measured. HONO becomes increasingly more soluble in high acid wt % solutions, probably resulting from the protonation of HONO to give H2ONO+, followed by dissociation and association to give NO+HSO4- (nitrosyl sulfuric acid). The reaction of HONO with HCl on 50, 60, 67, and 70 wt % sulfuric acid solutions was also studied. The measured uptake coefficients could not be explained simply by the occurrence of a bulk phase reaction, and a surface effect is found to contribute to the uptake. The reaction of HONO with HCl was measured on sulfuric acid particles, and a small reacto-diffusive length further supports a surface reaction. From extrapolating our measured uptake values to stratospheric conditions, the formation of ClNO from the heterogeneous reaction of HONO and HCl is concluded to be an insignificant process in the volcanically unperturbed stratosphere.

Heterogeneous reactions in sulfuric acid aerosols: A framework for model calculations

A framework for applying rates of heterogeneous chemical reactions measured in the laboratory to small sulfuric acid aerosols found in the stratosphere is presented. The procedure for calculating the applicable reactive uptake coefficients using laboratory‐measured parameters is developed, the necessary laboratory‐measured quantities are discussed, and a set of equations for use in models are presented. This approach is demonstrated to be essential for obtaining uptake coefficients for the HOCl + HCl and ClONO2 + HCl reactions applicable to the stratosphere. In these cases the laboratory‐measured uptake coefficients have to be substantially corrected for the small size of the atmospheric aerosol droplets. The measured uptake coefficients for N2O5 + H2O and ClONO2 + H2O as well as those for other heterogeneous reactions are discussed in the context of this model. Finally, the derived uptake coefficients were incorporated in a two‐dimensional dynamical and photochemical model thus for the first time the HCl reactions in sulfuric acid have been included. Substantial direct chlorine activation and consequent ozone destruction is shown to occur due to heterogeneous reactions involving HCl for volcanically perturbed aerosol conditions at high latitudes. Smaller but significant chlorine activation also is predicted for background aerosol loadings at extreme high latitudes, suggesting chlorine activation can occur on background sulfuric acid aerosol in these regions. The coupling between homogeneous and heterogeneous chemistry is shown to lead to important changes in the concentrations of various reactive species. The basic physical and chemical quantities needed to better constrain the model input parameters are identified.