Laboratory measurements of NO and NO2 depositions onto soil and cement surfaces

000+6981/78/1201-2315 sCn.00~

Armospheric Emirmmenr Vol. 12,pp.2315-2319. 0 Perpmon Press Ltd. 1978. Printed in Greal Britain

LABORATORY MEASUREMENTS OF NO AND NO2 DEPOSITIONS ONTO SOIL AND CEMENT SURFACES* HENRY S. JUDEIKIS and ANTHONY G. WREN

Chemistry and Physic8 Laboratory, The Ivaq A. Getting Laboratories, The Aerospace Corporationt, El Segundo, California, U.S.A. (First received 6 March 1978 and in final form 25 May 1978) Abstract - Laboratory measurements have been made of the deposition of NO and NO, onto selected soil and cement surfaces. Experimental results yielded deposition velocities of -0.1-0.2 cm s-l for NO and w 0.3-0.8 cm s- ’ for NO, over freshly prepared surfaces. Deposition was largely irreversible and decreased with time (exposure to NO or NO*). The latter observations indicate a finite capacity of these surfaces for removal of the gaseous species. However, additional experiments carried out in this study suggest that surface activity in the environment can be regenerated, in the case of NO*, but not NO, by interaction with atmospheric ammonia. The results further suggest, that for both NO and NOz, surfaces can be reactivated by precipitation washing away soluble surface reaction products. The lower deposition rates and capacities, and more limited regeneration of surface activity observed for NO, relative to NO,, suggest that uptake of NO by ground level surfaces in the environment will be considerably less important than that of NO,.

I. INTRODUCTION

Knowledge of deposition rates of gaseous air pollutants onto ground-level surfaces is essential in determining their atmospheric lifetimes and transport properties. Much interest ‘1has been accorded to

measurement of the deposition rates of sulfur dioxide onto various surfaces (Judeikis and Stewart, 1976; Garland, 1977 and references therein) in view of the environmental concern about this species. Similar concern is growing over the health and environmental effects attributable to NO, N02, and their chemical reaction products. However, only a limited amount of data exists on the deposition rates of nitrogenous air pollutants onto ground level surfaces. Here we report on laboratory measurements of the deposition of NO and NO2 onto selected soil and cement surfaces of interest. Deposition velocities obtained in this study are compared to other values reported in the literature for nitrogenous air pollutants. Saturation effects were observed that would limit the ability of the surfaces examined here to remove atmospheric NO and NOz. Regeneration of the activity of saturated surfaces is considered, and interaction of such surfaces with ammonia or pre-

* This work has been supported, in part, by Grant Number R-802687-02, from the Environmental Protection Agency. The contents do not necessarily reflect the views and policies of the Environmental Protection Agency, nor does mention of trade names or commercial product8 constitute endorsement or recommendation for use. t Reprint requests should be sent to the authors at The Aerospace Corporation, P.O. Box 92957, Los Angeles, CA 90009, U.S.A.

cipitation washing away soluble surface reaction products are shown to be potentially viable atmospheric processes. II. EXPERIMENTAL

The apparatus used in these experiments has been described elsewhere (Jude&is and Stewart, 1976) and will be only briefly nientioned here. The apparatus is a cylindrical flow reactor consisting of two concentric Pyrex cylinders, the inner one, which is -48 cm long, being coated with the solid of interest. The coating begins about one third the distance down the tube from the inlet end, and continues to the end of the tube. This allows for full development of laminar flow before the gas reaches the solid. This method of coating the cylinder permitted us to use fully developed laminar flow models to analyze the data. The outer cylinder was uncoated. The experimental procedure allowed a homogeneous gasphase mixture containing trace amounts of the reactant gas (NO or NO*) to be passed through the cylinder. The trace species would then diffuse to the wall where it was removed by deposition on the solid surface. This removal of the trace gas I@ to concentration gradients of the species along both the cylinder axis and in the radial dimension. The axial concentration gradient was measured directly using a system of small probes whose intakes were centcred along the cylinder axis. These probes fed into a multiport rotary valve and then into the ionizer of a quadrupole mass spectrometer. The smooth surfaces and laminar flow condition8 used in these experiments permitted us to analyze data using a model that specifically accounted for mass transport by diffusion and flow (Judeikis and Stewart, 1976). The deposition velocity was determined utilizing the boundary condition -D

0$ =

v,c

which equates the diffusive flux of the trace species to the wall, -D(&/ar), to its heterogeneous removal rate, V,+T. The molecular diffusivity of the trace. specie-s in the gaseous mixture is given by D ; c and (&/&) are the concentration and

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HENRYS. JUDEIKBand ANTHONYG. WREN

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radial concentration gradient of the trace species at the wall respectively, and V, is the deposition velocity. Deposition velocities obtained in this manner are independent of mass transport and reflect the rates of heterogeneous removal of the trace species by interactions with the surface. As such, these deposition velocities represent maximum values that would be obtained under turbulent atmospheric conditions in the environment. Experimental determinations were made at ambient temperature (20 - 25°C) and at total pressures of - 100 Torr. Sub-ambient pressures were used to avoid obtaining results that were limited by mass transport in the laboratory system (Judeikis and Stewart, 1976). Typical reaction mixtures contained -10% oxygen with the balance made up of nitrogen, water vapor (0 or 95% r.h.) and traces of NO or NOI. Flow rates were in the range of 8-14cm3s-‘, with linear velocities of -0.5 cm s-‘. Resultant Reynolds numbers were CM. All gases were reagent grade. Solids used were representative sandy loam and adobe clay soils taken from the Los Angeles area (O-20cm layer), and commercial bagged cement. In several experiments, coatings prepared from powdered wood charcoal or reagent grade MnO,, PbO, or

A&O3 were also exposed to NOz. Coated cylinders were prepared using aqueous suspensions of the various solids as described earlier (Judeikis and Stewart, 1976). The coated cylinders were subsequently air dried, and then dried in vacuum (- low4 Torr). Experiments conducted with h~idifi~ reaction mixtures led to some uptake of water vapor and surface wetting. Average tbicknesses of the solid coatings were - 1 mm with surface roughnesses of a few tenths of a millimeter. Cylinders were cleaned with soap and water between experiments and thoroughly rinsed and dried. In several experiments, blank runs were carried out on the clean, uncoated cylinders. The soil and cement samples were tested for their pH in water solutions. An Orion gel-filled combination pH electrode, model 91-05, and a Beckman model 3500 digital pH meter were used for this purpose. The electrode was calibrated using the two-buffer standardixation recommended by Orion with Beckman pH 7.00 and pH 10.00 standard buffers. Solid samples were finely ground with a mortar and pestle and mixed with distilled water, pH 7.00. The suspension was allowed to settle and readings were made directly with the electrode. Results are shown in Table 1 for samples prior to NO or NO1 exposure. In the case of the commercial oement, similar results were obtained for the uncured material and for cured material crushed to a fine powder. Mass spectrometry was employed for analysis of NO and NO,. The instrument used was a modified E.A.I. Ouad 200 operating in the positive ion mode. The NO: and NG+ peaks were used for NO, and NO, respectively. We found that the mass spectrometer’s sensitivity to NO: was greatly reduced because of the decomposition of NO: in the ionizer and lens region of the spectrometer to NO+ and 0. This decreased the spectrometer’s sensitivity to -10 ppm NO,. Decomposition of NO+ was not a problem and lower concentrations of this species could be used. Typical partial pressure-s of NO2 and NO in the reaction mixtures were - 10-40 millitorr, and - l-3 mTorr, respectively. In several experiments solids were analyzed for surface reaction products. In these experiments, the coated cylinders were removed from the reaction chamber upon completion of the experiment, rinsed with distilled water, and filtered, and Table I. Soil and cement pH measurements Material

PH

Adope clay soil Sandy loam soil Commercial cement

7.9 8.2 11.4

the filtrate analyzed for NO; (i.e. NO; or NO;). A wet chemical procedure (Feigl, 1958) or nitrate ion electrode (Beckman, Model No. 39618) employing the manufacturer’s recommended procedures was used for NO; analysis, while wet chemical techniques were employed for NO; analysis (Fe&l, 1958). The latter test gives a positive result for both NO; and gaseous NO2 dissolved in solution. Consequently, the NO; test cannot distinguish between adsorbed NO, and NO;. An additional uncertainty in the analyses of NO; lies in the possibility that NO; could be formed from adsorbed NO2 during the washing procedures.

III. REXJL’IS

Results obtained for NO and NO1 deposition over selected soil and cement samples are given in Table 2. The results from the blank runs with the uncoated Pyrex cylinder indicate that gas phase reactions of NO and NOz with oxygen @chuck and Stephens, 1969) or water (Kaiser and Wu, 1977 and references therein) were small compared to heterogeneous removal of these species by the soil and cement samples. Initial deposition velocities, measured over freshly prepared soil and cement samples, were the same, to within experimentai error, for dry and humidified mixtures. Similar results were also obtained for mixtures containing 0 and 10% oxygen. These results indicate that deposition rates are independent of the oxygen concentration and relative humidity of the reaction mixture. With time (NO or NOz exposure) deposition velocities gradually decreased and ultimately approached zero, suggesting that these surfaces have a finite capacity for NO and NO, removal. These decreases in deposition velocities and their approach to zero were noted for both dry and humidified reaction mixtures; however, the decreases were much more gradual with h~idi~~ gases. The latter results, which indicate higher capacities for NO and NOz removal in humidified systems, are in contrast to results obtained for initial deposition velocities which were essentially independent of relative humidity. Quantitative measurements of capacities were not carried out. However, they were of the order of several g mm2 for Table 2. Measured deposition velocities* Deposition velocity, ems-’ Solid Uncoated Pyrex cylinder Sandy loam soil Adobe clay soil Cement?

NO

NO,

s 0.003 0.19 0.13 0.21

90.003 0.60 0.77 0.32

* Values are averages of initial deposition velocities generally from three separate freshly prepared coatings of each material. Three separate detonations were made on the uncoated Pyrex cylinder for both NO and NO,, the cylinder being cleaned between experiments. Variations between and within given experiments indicate uncertainties (standard deviations) of f 30%. t Cured.

NO and NO2 depositions onto soil and cement NO2 uptake from humidified gas mixtures, and about an order of magnitude less for NO uptake from h-id&d mixtures. For actual environmental SUF faces, greater depths, porosities. and water content could lead to even higher capacities. Selected exp&ments indicated that deposition of NO and NO* onto the soil and cement surfaces was, for the most part, irreversible. In experiments conducted with NO dc~sition over cement, and NO, deposition over both soils and cement, the surfaces wereexposed to humidified reaction mixtures until the deposition velocities decreased to ~0.003 cm s-l. The NO or NOz component flows in the gas stream were then terminated. This led to a rapid decay of the NO or NO, concentrations in the flowing gas stream. Measured decay constants were characteristic of the flow velocities. Much longer decay constants would be expected for reversible processes since the flowing gas stream would be fed by NO or NO1 desorbing frbm these, surfaces upon termination of the component flows of these species. In several of these experiments, surfaces exposed to saturation, or partial saturation levels of NO or NO, were subsequently subjected to overnight evacuation (_ low4 torr) or exposure to gas streams devoid of NO or NO* for periods of one hour or more. Neither treatment led to any substantial restoration of surface activity. For example, an adobe clay sample was exposed to a humidified gas mixture containing NOz until the deposition velocity decreased to * 3% of its initial value. The sample was then exposed to the same gas mixture, but without NO,, for 1 h. The deposition velocity me&sured after the latter, treatment was N 4% of the initial value, indicating a minimal restoration of activity. Analyses of reaction products were carried out for selected samples exposed to NOz. The complexity of the soil and cement samples, and potential analytical. interferences, precluded determinations for adsorbed nitrates and nitrites (NO;). However, we did find that NO* deposited onto MnOz, PbO, A1203, and charcoal was initially quantitatively converted to both adsorbed nitrates and nitrites, the dominant species depending upon the particular material. Thus MnO, and charcoal yielded primarily NO;, while PbO gave mostly NO;. Samples of Al,O, from two different sources yielded different results. Elemental and X-ray analyses of these materials were slightly different. The first material, characterized by X-ray analysis as Al(O with traces (0.~2-0.08%) of Si, Fe, and Na, yielded primarily NO;. Analyses of the second material, which gave primarily NO; on reaction with NO,, gave the same basic X-ray pattern with several added (unidentified) peaks and higher concentrations of Si, Fe, and Na (O.i-0.3%). With time (NO, exposure), the conversion of NO, to adsorbed NO; became less than quantitative and gaseous NO began to form. This conversion of NO2 to NO was also observed by Hill (1971) over an alfalfa canopy. With continued exposure we found that the

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ratio of NO produced in the gas phase to adsorbed NO; gradually increased until saturation levels were approached, .at which time both the NO gas and adsorbed NO; reaction products decreased. Overall, in samples exposed to saturated levels of NO1, approximately one third of the NO2 removed from the gas stream was converted to adsorbed NO;, with the remaining’ two thirds forming gaseous NO. Results q~litatively consistent with those described above for the oxi+le and charcoal samples were obtained from analyses of gas mixtures containing NO,, exposed to a cement surface. Initially, only NO, loss from’the gas stream was observed. With time (NO2 exposure) the formation of gaseous NO was observed, as determined from the increase in the NO+ to NO; ratio from the mass spectrometer over that obtained from pure NO1. Integration over the entire exposure to NO, up to saturation indicated that - 50% of the NO2 lost was converted to gaseous NO, with the remainder presumably forming adsorbed NO;. Experiments were also conducted in which solids, exposed to saturation levels of NO or NOz, were subsequently exposed to gaseous ammonia and then reexposed, to NO or NO*. The purpose of these experiments was to determine if the reactivity of a saturated surface could be restored after interaction with ammonia, which is a common tropospheric cpnstituent that has, been shown to be effective in regenerating activities of surfaces exposed to saturation levels of another acidic gas, SO2 (Judeikis and Stewart, 1976). The ammonia treatment was found to be effective in restoring the activity of samples exposed to saturation levels of NO,. For example, a sandy loam sample was exposed to NO, until the deposition velocity of NO, was reduced to ~0.3% of its initial value. Exposure to gaseous ammonia, equivalent to 5% of the amount of NO, required to saturate the surface, on a molar basis, restored the deposition velocity to 10% of its initial value. Further exposure to NH,, equivalent to 170% of the MO, saturation exposure on a molar basis, led to restoration of the deposition velocity for NO, to its initial value, to within experimental error. Qualitatively similar results were obtained for an adobe clay sample exposed to saturation levels of NO, and then to ammonia. The ammonia treatment was not found to be effective in restoririg the activity of samples exposed to saturation levels of NO. For adobe clay and cement samples exposed to NO until the deposition velocities decreased to < 0.003 cm SC’, no measurable restoration of activity was observed for subsequent amnionia exposures np to 3 and 40 times the equivalent molar NO exposures, respectively. Additional experiments were conducted to determine if soluble surface reaction products could be washed away to rejuvenate surface activity. In these experiments, cement samples were exposed to saturation levels of NO, or NO, subsequently removed from the reactor and the surfaces lightly rinsed with

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HENRY S. JUDEIKISand ANTHONY G. WREN

distilled water. The samples were then put back into the reactor for measurement of deposition velocities. For the sample exposed to NO,, the latter measurement yielded a deposition velocity equal to 60% of its initial value. In the case of the sample exposed to saturation levels of NO, the water rinse led to a measured deposition velocity equal to 50% ofits initial value. We also conducted an experiment to determine if a surface, saturated with respect to NO uptake, would still be active toward NO,. In this experiment, a cement surface was saturated by exposure to a humidified mixture containing NO (V, decreased from its initial value to <0.003 cm s-l). The sample was subsequently exposed to a humidified mixture containing NO, with the result that NO, uptake did occur with a deposition velocity of 0.043 cm s- ‘. IV. DISCUSSION

Deposition velocities of -0.3-0.8 cm s- ’ obtained for NO2 over freshly prepared surfaces in this study are slightly lower than those previously determined for SO, (Judeikis and Stewart, 1976; Garland, 1977 and references therein). The latter values lie in the range of -0.3-2.5 cm s-l, with most values centered around N 1 cm s-i. Deposition velocities determined here for NO (_ 0.1-0.2 cm s- ‘) over freshly prepared surfaces are lower than those of either NO2 or SO1. Abeles et al. (1971) observed NO, uptake by soil, while Hill (1971) measured uptake of both NO and NO, by an alfalfa canopy. The latter author found uptake rates corresponding to deposition velocities of 0.1 and 1.9 cm s- 1for NO and N02, respectively. The value for NO agrees well with those measured here, while the value for NO, is considerably higher than ours. In separate experiments with another reactive gas, molecular chlorine, Hill (1971) found that the uptake rate of this species by wet soil for a canopy height of -0cm was a factor of four lower than the chlorine uptake rate for a canopy height of N 40 cm, the height used in the NOz measurements. If this same ratio were applicable to N02, then the value for a zero canopy height, which would more closely resemble our experiments would be -0.5cm s-l, well within the range of values reported here. Recently Brice et al. (1977) observed diurnal variations of atmospheric NzO which they attributed to deposition of this species onto ground level surfaces. From their study, they arrived at a deposition velocity of 0.0125 cm s- i for N,O. This value is considerably lower than those obtained here for NO and NO,, which is not surprising when the lower chemical reactivity of N,O compared to NO and NO, is taken into consideration. The results obtained for NO1 in this study are qualitatively similar, in many respects, to these previously obtained for SO, deposition on related surfaces (Judeikis and Stewart, 1976). Thus, for both NO, and S02, deposition occurs with high initial rates that

gradually decrease on prolonged exposure to the reactive gas and ultimately approach zero. Additional similarities pertain to the rejuvenation of the surface activity of spent samples by exposure to ammonia or water rinsing of the saturated surfaces. There are, however, some significant differences between the two studies. Whereas SOZ was quantitatively converted to adsorbed surface reaction products during the entire SO2 exposure in the previous study, NO* was quantitatively converted to adsorbed products only during the initial periods of exposure. Subsequently, gaseous NO began to form; an effect, as noted above, previously observed by Hill (1971) for NOz deposition over an alfalfa canopy. Another difference is related to the ordering of the cement and soils with respect to their activity toward NOz and SO2 removal. The uptake of SOZ by various European soils was found to increase with increasing pH (Payrissat and Beilke, 1975), as might be expected on the basis of diminishing reaction rates at lower pHs, attributed to sulfuric acid formation (e.g. Junge and Ryan, 1958 ; McKay, 1971). Our earlier results for SO2 (Judeikis and Stewart, 1976) are consistent with this suggestion in that we found various cements to be more reactive than soils by factors of two to four. Although surface pHs were not determined in the latter study, we expect they were comparable to those indicated here in Table 1, since the materials in both sets of experiments were obtained from similar sources. Effects attributable to surface pH might also be expected for NOz interactions with those solids, in view of the acidic products formed by NO, reactions (Kaiser and Wu, 1977 and references therein). As noted in Table 2, however, the cement samples, with their higher pH, were found to be a factor of two less reactive than the lower pH soils. Another possibility for this difference between the SOZ and NOz results could be complexing with metal ions on the surface that promote heterogeneous oxidation (e.g. Freiberg, 1975). The limited kinetic data obtained from this study, and the complexities of the surfaces involved do not permit us to draw any definite conclusions regarding the difference between the NO, and SO, results. The lower deposition rates for NO compared to NO, observed here, as well as the lower capacities, suggest a much lower reactivity for NO with environmental ground level surfaces. Moreover, although water rinsing of the cement surface exposed to saturation levels of NO did restore activity, ammonia exposures of saturated adobe clay and cement surfaces did not. Even in the case of a cement surface exposed to saturation levels of NO, we found that this surface was still active toward NO2 removal. These observations further tend to indicate that NO-surface interactions will be quantitatively less important than NO1-surface reactions. The implication of the results obtained in this study is that removal of reactive oxides of nitrogen (NO + NO,) by deposition on ground level surfaces in the environment will occur primarily through

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NO and NO, depositions onto soil and cement

NOz-surface interactions. Although these species are emitted primarily as NO, the NO is converted to NOz in the atmosphere, which is subsequently photolyzed back to NO (Schuck and Stephens, 1969). The NO to NOZ ratio is determined primarily by the balance of atmospheric oxidant and sunlight levels. In polluted urban atmospheres, where high oxidant levels lead to high NO2 concentrations at certain times of day, removal of NO + NO, by deposition of NO, on ground level surfaces will be maximized. Of course, other removal processes including atmospheric reactions leading to the formation of organic nitrogen compounds such as the peroxyacyl nitrates (Stephens, 1969)and nitrate aerosols (Hidy and Burton, 1975)can also be important. In remote areas where oxidant levels are generally lower, a higher fraction of the NO + NO2 would be expected to be in the form of NO. The results obtained in this study suggest that the fractional removal of atmospheric NO + NO, by ground level deposition in remote areas would be lower than the fractional removal in urban areas, due to the lower NO, to NO ratios and lower deposition velocities and capacities observed for NO relative to NOz. In both urban and remote areas, the actual removal at surfaces would be dependent upon the degree of saturation of those surfaces with regard to NO and NO2 removal, or the degree of surface regeneration through interaction with atmospheric ammonia or precipitation washing away soluble surface reaction products. In areas such as the Los Angeles basin where midsummer precipitation is virtually non-existent, saturation effects could be quite important. Acknowledgements-The authors extend their appreciation to Thomas B. Stewart for conducting the analyses of surface

reaction products and to James E. Foster for the conduct of the deposition measurements. REFERENCES

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Payrissat M. and Beilke S. (1975) Laboratory rn~ur~ents of the uptake of sulphur dioxide by different European soils. Atmospheric Enoironment 9, 21 l-217. Schuck E. A. and Stephens E. R. (1969) Oxides of nitrogen. Adv. Enuir. Sci. 1, 73-118. Stephens E. R. (1969) The formation, reactions, and properties of peroxyacyl nitrates (PANS) in photochemical air pollution. Adu. En&. Sci. 1, 119-146.