Uptake coefficient

The model results were compared with the HOx concentrations measured by the FAGE (Fluorescence Assay by Gas Expansion) technique during four days of clean Southern Ocean marine boundary layer (MBL) air. The models overestimated OH concentrations by about 10% on two days and about 20% on the other two days. HO2 concentrations were measured during two of these days and the models overestimated the measured concentrations by about 40%. Better agreement with measured HO2 was observed by using data from several MBL aerosol measurements to estimate the aerosol surface area and by increasing the HO2 uptake coefficient to unity. This reduced the modelled HO2 overestimate by 40%, with little effect on OH, because of the poor HO2 to OH conversion at the low ambient NOx concentrations. 

The accommodation coefficients for OH and HO2 in our model are parameterised as temperature dependent accommodation coefficients (Gratpanche et al., 1996) in Table 3, with no account taken of the surface characteristics. There are a few papers reporting uptake coefficients for both OH and HO2 with lower limits quoted for the HO2 coefficients due to experimental limitations, giving rise to a low confidence in current experimental values for HO2 (Cooper and Abbatt, 1996 Hanson et al., 1992). The impact of reactions on aerosol on HO2 concentrations in the remote atmosphere could be significant if the uptake coefficient was greater than 0.1, and could dominate if it was close to unity (Saylor, 1997). 

Fig. 8. Effect on [HO2] of changing uptake coefficient and RASA (15-16 February).

Changing the HO2 uptake coefficient and the RASA had little effect on [OH], because the recycling of OH from HO2... 

Gratpanche, F., Ivanov, A., Devolder, P, Gershenzon, Y., and Saw-erysyn, J.-P Uptake coefficients of OH and HO2 radicals on material surfaces of atmospheric interest, 14 International Symposium on Gas Kinetics, Leeds, September, 1996. 

An accurate nasal model must also account for the airflow rate and the concentration of the inspired gas. Aharonson et al, conclusively demonstrated that the uptake coefficient," or average mass-transfer coefficient, over the entire nose for acetone, ozone, sulfur dioxide, and ether increased with increasing airflow rate. 

Let us now return briefly to the question of the relationship between net reactive uptake coefficients measured in laboratory systems, where the liquid films are generally quite thick, and particles in the atmosphere, which can be quite small and hence effectively have thin liquid films. A measure of the distance from the interface in which the reaction occurs is the diffuso-reactive length, l, which is defined as... 

FIGURE 5.18 Correction factors for the measured uptake coefficient, -ymcsls, as a function of the ratio of the diffuso-reactive length (/) to the droplet radius (a) (adapted from Hanson et al., 1994). 

Another important factor to recognize is that the net uptake coefficient determined using Knudsen cells may not represent the true uptake or trapping of the gas by the surface if reevaporation into the gas phase occurs, which must be taken into account in such cases. In principle, the mass accommodation coefficient is the... 

For gas-liquid combinations with relatively small uptake coefficients ( 10 4-10-7), longer interaction times between the gas and liquid are needed than can be obtained with the falling-droplet apparatus. These are provided in a bubble apparatus, a typical example of which is shown in Fig. 5.24. The gas of interest as a mixture with an inert carrier gas is introduced as a stream of bubbles into the liquid of interest. The interaction time is varied by moving the gas injector relative to the surface. The composition of the gas exiting the top of the liquid is measured as a function of the interaction time (typically 0.1-1 s), e.g., by mass spectrometry. The interaction time is limited by the depth in the liquid at which the bubbles are injected and their buoyancy. Longer interaction times and better control over them have been achieved using a modified apparatus in which the bubbles are generated and transported horizontally (Swartz et a.l., 1997). 

You are using a Knudsen cell with a circular reactive surface of diameter 4.9 cm. If you anticipate that the uptake coefficient for a reaction of interest is 0.01, what diameter should the aperture to the mass spectrometer be to observe a drop in the reactant signal of 50% on exposure to the reactive surface ... 

Aikin, A. C and W. D. Pesnell, Uptake Coefficient of Charged Aerosols—Implications for Atmospheric Chemistry, Geophys. Res. Lett., 25, 1309-1312(1998). 

The first order loss rate of a gaseous species with mean speed co on a surface A per unit volume V is defined in tenns of the "uptake coefficient", y, by... 

The measured uptake coefficient y is a convolution of the processes which affect the rate of gas uptake. Uptake coefficients have been measured for various PSC surfaces and sulfuric acid solutions using a variety of experimental methods, and have been interpreted in tenns of physico-chemical models to yield reactivities and solubilities of gaseous species. More most cases the total experimental uptake coefficient ym can be represented in tenns of a resistance model, whereby the resistances l/ym and 1/y are associated with reaction and liquid solubility [42] Quantitative values for y will be discussed later in section 2.3.4. 

It is possible to calculate the net uptake coefficient for small sulfuric aerosols in the atmosphere, and thus derive the effectiveness of direct chlorine activation, from basic physical and chemical parameters [43]. The net uptake coefficient for the atmosphere, y, can be expressed by ... 

A variety of experimental techniques have been used for the determination of uptake coefficients and especially Knudsen cells and flow tubes have found most application [42]. Knudsen cells are low-pressure reactors in which the rate of interaction with the surface (solid or liquid) is measured relative to the escape through an aperture, which can readily be calibrated, thus putting the gas-surface rate measurement on an absolute basis. Usually, a mass spectrometer detection system monitors the disappearance of reactant species, as well as the appearance of gas-phase products. The timescale of Knudsen cell experiments ranges from a few seconds to h lindens of seconds. A description of Knudsen cell applied to low temperature studies is given [66,67]. 

Wall-coated flow tube reactors have been used to study the uptake coefficients onto liquid and solid surfaces. This method is sensitive over a wide range of y (10" to 10 1). For liquids this method has the advantage that the liquid surface is constantly renewed, however if the uptake rate is fast, the liquid phase becomes saturated with the species and the process is limited by diffusion within the liquid, so that corrections must be applied [70,72,74]. Many experiments were designed to investigate the interaction of atmospheric species on solid surfaces. In this case the walls of the flow tube were cooled and thin films of substrate material were frozen on the wall. Most of the reaction probabilities were obtained from studies on flow tubes coated with water-ice, NAT or frozen sulfate. Droplet train flow tube reactors have used where liquid droplets are generated by means of a vibrating orifice [75]. The uptake of gaseous species in contact with these droplets has been measured by tunable diode laser spectroscopy [41]. 

Tables on the "reaction probalility or "uptake coefficient" have been summarized for various heterogeneous reactions in a recent review article [87], and by the IUPAC [88] and NASA-JPL [86] evaluation teams. For the purpose of this article, a rough comparison is made of the uptake rates for the reactions (1) to (5) on the different type surfaces. Three major type of surfaces have been considered a) NAT, or Type I PSC, b) Water ice, or Type II PSC and c) sulfuric acid aerosol, which is normally a liquid surface generally composed of 60-80 wt % H,S04 and 40-20 wt % H,0 also considered is the solid form SAT (sulfuric acid tetrahydrate) with a composition of 57.5 wt % H,S04. The importance of chlorine activation on sulfuric acid solutions has been demonstrated in a recent article [89]. Halogen activation on seasalt material will shortly be reviewed as part of the tropospheric processes.

The uptake coefficient on liquid sulfuric acid is a strong function on the water activity, in analogy to the hydrolysis of CIONO, and therefore depends upon the composition of the mixture [92]. It was suggested [93] that the CIONO, uptake due to reaction with HC1 is dependent on both bulk and surface concentrations of HC1 y varies by more than two orders of magnitude (0.3>y>10°), and depends strongly on the HC1 partial pressure... 

The hydrolysis of C10N02 (reaction 2) is expected to be important for chlorine activation if and when the HQ reservoir becomes depleted via reaction (1) and (4). The uptake coefficient on NAT surfaces ranges between 7x1 O 3 and 4x10 [90,94]. The reaction probability however depends on the partial HjO pressure. The uptake coefficient y over pure water-ice is very high and is of the order of 0.3. Older measurements have indicated much lower values, which are explained by the use of too large QONO, concentrations. Recently, a value of 0.03 was obtained at ice temperatures ranging 75-140 K [95]. 

The uptake coefficient on liquid sulfuric acid is due to QONO, hydrolysis and has been shown to depend strongly on the composition. It was indicated that y depends on the HjO activity of the mixture [93]. A detailed model for applying the laboratory uptake coefficient for this reaction to the small aerosol composition found in the stratosphere has been developed [43,96]. 

Hydrolysis of N205 on sulfuric acid represents a very efficient channel for nitrogen deactivation. Measurements using large 100-pm droplet trains [75,96] and submicron sulfuric acid aerosols [73,77] indicate high uptake probalilities (y = 0.1), without strong dependence on f SO, concentration or temperature. The data were fitted into an uptake model [96], Uptake coefficients over on water-ice (y = 0.02), SAT ((y = 0.006) and NAT (y= 0.0003) are much suppressed [86,90]. 

The uptake coefficients are summarized [86]. On NAT-like or water-rich NAT substrates the uptake coefficient is 0.1 to 0.2 in the temperature range 191-202 K, but decreases two orders of magnitude for conditions of low relative humidity, corresponding to HNO,-rich NAT substrates. For water-ice surfaces, values of y are... 

The uptake coefficients for reactions involving other bromine containing compounds are generally high. The y-values for reactions (13), (16) to (18) on sulfate aerosol and water-ice substrates are in the order of 0.1 to 0.3. Reactions involving BrONO, on sulfuric acid solutions occur with near unity efficiency, except at concentrations >70 wt %, where ydrops to 0.1 at 85 wt % [99-101]. 

The uptake coefficients on various surfaces are listed [86]. On NAT-like substrates y is large, near 0.2. On water-ice substrates, y is even larger, on the order of 0.3. The uptake coefficient of HO on liquid sulfuric acid bulk solutions decreases with increasing activity of the mixture [41,82]. The solubility of HC1 in those mixtures is the controling factor. The uptake on sulfuric acid droplets was recently measured [102], The mass accommodation coefficient a was found to be inversely proportional to temperature and increases from 0.06 at 184 K to 1.0 at - 230 K. The uptake of HOC1 on water-ice in the... 

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