Photochemistry of Atmospheric Dust: Ozone Decomposition on

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Environ. Sci. Technol. 2009, 43, 7437–7442

Photochemistry of Atmospheric Dust: Ozone Decomposition on Illuminated Titanium Dioxide ´ L A N I E N I C O L A S , † M A R I E M E N D O U R , †,‡ ME OUMAR KA,‡ BARBARA D’ANNA,† AND C H R I S T I A N G E O R G E * ,† Institut de recherches sur la catalyse et l’environnement de Lyon, Universite´ de Lyon 1, CNRS, UMR5256, IRCELYON, Villeurbanne, F-69626, France, and Universite´ Cheikh Anta Diop, Faculte´ des Sciences et Techniques, Dakar, Senegal

Received May 29, 2009. Revised manuscript received July 27, 2009. Accepted July 29, 2009.

The ozone decomposition onto mineral surfaces prepared with traces of solid TiO2 in a matrix of SiO2 in order to mimic mineral dust particles has been investigated using a coatedwall flow-tube system at room temperature and atmospheric pressure. The ozone uptake coefficients were measured both under dark conditions and irradiation using near UV-light. While uptake in the dark was negligible, a large photoenhanced ozone uptake was observed. For TiO2/SiO2 mixtures under irradiation, the uptake coefficients increased with increasing TiO2 mass fraction (from 1 to 3 wt %), and the corresponding uptake coefficient based on the geometric surfaces ranged from 3 × 10-6 to 3 × 10-5. The uptake kinetics was also observed to increase with decreasing ozone concentration between 290 and 50 ppbv. Relative humidity influenced the ozone uptake on the film, and a reduced ozone loss was observed for relative humidity above 30%. The experimental results suggest that under atmospherically relevant conditions the photochemistry of dust can represent an important sink of ozone inside the dust plume.

1. Introduction Following erosion from the ground, between 1400 and 2000 Tg of dust particles are annually uplifted by strong winds and are introduced into the atmosphere (1, 2). Mineral aerosol can provide a large surface onto which adsorption and reaction of trace gases occur. Gas uptake can therefore represent an important removal process for gaseous traces and can alter the chemical composition and the physical properties of mineral dust aerosol (3-5). Mineral aerosol chemical composition mainly reflects the crustal material from which it originated (6). The major components are silicon oxide (SiO2) and aluminum oxide (Al2O3), which account for about 60 and 10-15 wt %, respectively. Beside these main compounds, other oxides, e.g., iron oxide (Fe2O3), magnesium oxide (MgO), calcium oxide (CaO), and titanium oxide (TiO2), are also present at varying percentages depending on source location (7). Among these compounds both titanium and iron oxides are known semiconductors used as photochemical source of radicals. Fe2O3 is used to induce the so-called Fenton or Photo-Fenton * Corresponding author ircelyon.univ-lyon1.fr. † Universite´ de Lyon 1. ‡ Universite´ Cheikh Anta Diop. 10.1021/es901569d CCC: $40.75

Published on Web 09/02/2009

e-mail:

christian.george@

 2009 American Chemical Society

reactions (8). Pure TiO2 is used in a variety of remediation processes due to its photocatalytic properties. The resulting free-radicals are very efficient oxidizers of organic and inorganic matter (9). Recently, Ndour et al. (10, 11) have shown that near UV radiation is able to activate nitrogen dioxide conversion on SiO2/TiO2 films, Saharan dust, and Arizona test dust. In the present paper, we investigated the effect of near UV irradiation on the heterogeneous uptake of ozone on mineral surfaces of SiO2 doped with traces of TiO2 to mimic mineral dust composition. The destruction of ozone by mineral surfaces has been previously suggested by various field studies and models (3, 4). This process is of particular interest because of the importance of ozone as tropospheric photochemical pollutant, greenhouse gas, photolytic precursor of the OH radical, and selective oxidant. Several laboratory studies demonstrated that under dark conditions there is a direct reactive uptake of ozone onto mineral aerosol and various mineral oxides (12-22). The present paper extends the earlier work on mineral oxides, assessing quantitatively the effect of the light on the atmospheric heterogeneous chemistry of oxides containing small traces of photocatalytic compounds. The kinetic values have been determined as a function of ozone mixing ratio, film mass, humidity, and light irradiance using a coated flow tube apparatus at atmospheric pressure and room temperature.

2. Experimental Section 2.1. Coated-Wall Flow-Tube System. The heterogeneous interactions between ozone and mineral surfaces were studied by exposing films of SiO2/TiO2 mixtures to known ozone concentrations. All experiments were performed under atmospheric pressure at 298 K using a thermostated horizontal coated-wall reactor detailed elsewhere (23-26). The characteristics of the system are presented in the Supporting Information (SI). 2.2. Uptake Coefficient Determination. The uptake of ozone on the mineral surfaces was determined by monitoring its loss in the gas phase as a function of the position of the movable injector, i.e., as a function of the exposure time of the oxide surface to the gas, as detailed in the SI. The derived pseudofirst order coefficient is related to the uptake coefficient (γgeom) through eq I: k)

γgeom · 〈c〉 2rtube

(I)

where γgeom is the uptake coefficient based on the geometric surface. The measured kinetics were corrected for gas phase diffusion limitations by using the Cooney-Kim-Davis (CKD) method (27-29), which takes into account axial and lateral diffusion combined with a first order loss at the inner surface of a cylindrical tube under laminar flow conditions. The time resolution of this experimental layout is such that measuring the initial uptake coefficient is not possible. Therefore, the kinetic parameters were determined after 10-15 min of exposure to the film when the ozone concentration reached a plateau allowing the determination of the steady-state uptake coefficients (γss). An uncertainty analysis led to an estimated error of about 30% (see SI). The experiments were performed with an ozone mixing ratio from 50 to 290 ppbv, a TiO2 content from 1 to 5 wt %, a film mass that ranged between 10 and 90 mg, and a relative humidity between 3 and 60%. 2.3. Sample Preparation. SiO2 (DEGUSSA Aerosil 130) and TiO2 (DEGUSSA, P25 80% anatase, 20% rutile; P25), were VOL. 43, NO. 19, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Ozone outlet concentration at T ) 298 K and 24% of relative humidity, during exposure (15-25 min in the time scale) onto SiO2 (diamonds) and SiO2/TiO2 1 wt % (squares) films. Filled symbols: ozone profile for dark experiments; empty symbols: experiments under irradiation (2.7 × 1014 photons cm-2 s-1). used without further cleaning. A given mass of the mineral oxides powder was dissolved in 30 mL of water, dripped uniformly into the Pyrex insert and dried overnight in an oven at 373 K. The resulting film covered the entire inner surface of the tube and, to the eye, was fairly uniform in thickness. Each tube was weighted, ensuring that kinetic measurements were performed on films with reproducible mass (i.e., with mass fluctuations less than 1 mg). The measured uptake coefficients measured on different films were comparable within our experimental error (see above). The specific surface area of a film prepared with 1 wt % of TiO2 was measured by means of the Brunauer-EmmettTeller (BET) technique using nitrogen and found to be 1.48 × 103 cm2 mg-1.

3. Results and Discussion 3.1. Ozone Uptake Coefficient. Figure 1 shows the raw data for the ozone concentration profile when exposed to SiO2 and SiO2/TiO2 (1 wt %) coatings under dark and light conditions (the ozone inlet concentration was 135 ppbv). For a pure SiO2 film (full and empty diamonds) the ozone mixing ratio reduction accounts for only 4-5%, for both dark and near UV light experiments (light irradiance of 2.7 × 1014 photons cm-2 s-1 in the range 340-420 nm). When the SiO2 film is doped with 1 wt % TiO2 a larger ozone uptake is observed under dark conditions (9). The ozone loss, however, rapidly decreases to an almost constant nonzero value under steady-state conditions (after 10-15 min of exposure to the oxidant), which accounts for an ozone loss of approximately 20%. Consequently, environmentally relevant measurements should be reported in terms of steadystate uptakes and not initial uptakes, which tend to overestimate the ozone loss because of the initial gas adsorption onto the surface. When the same film is exposed to near UV irradiation, about 60% of ozone reduction is detected under steady-state conditions, suggesting the importance of the photocatalytic ozone removal at the surface of the film containing traces of TiO2. The photoenhanced ozone loss shows another important feature. Indeed it does not exhibit any temporal profile, suggesting that saturation of the surface does not occur. For a film of SiO2/TiO2 (1 wt %) and 135 ppbv ozone concentration the geometric steady-state uptake coefficient (γgeom) is (1.4 ( 0.2) × 10-6 and (7.4 ( 1.1) × 10-6 in dark conditions and under irradiation, respectively. To our knowledge there are no data available for the ozone uptake coefficient on irradiated mineral dust films or their proxy based on TiO2 traces dispersed in a SiO2 matrix. 7438

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FIGURE 2. Dependence of the ozone geometric (a) and the BET (b) uptake coefficients as a function of the film mass, at T ) 298 K and 24% of relative humidity for films prepared with SiO2/ TiO2 (1 wt %) mixture. Filled symbols: values for dark experiments; empty symbols: values for experiments under irradiation (2.7 × 1014 photons cm-2 s-1). 3.2. Dependence on the Mass of the Film. In our experiments, ozone removal can take place at the film surface, in that case the geometric surface should be used or it may occur on the internal layers of the film either completely or just a fraction of it. In such a case, the uptake coefficient based on the geometric surface displays a mass-dependence as shown in Figure 2(a) under irradiation and in the dark, at 24% RH and 130 ppbv of ozone. For sample masses varying from 10 to 75 mg, the uptake coefficient based on the geometric surface for experiments under irradiation shows an almost linear dependence for values between 2 × 10-6 and 3 × 10-5. A plateau seems to occur above 75 mg, a value that probably corresponds to the maximum of the reactive thickness of ozone into the mineral film. Other experimental determinations observed similar mass-dependence profiles on mineral surfaces (15-17, 22). This phenomenon is frequently observed for interaction of gaseous species with porous substrates and it indicates that diffusion and reactivity of the gas species occur in the interstitial space. The linear domain in Figure 2(a) reflects the mass region where the complete internal surface is accessible to the gaseous ozone, therefore, a mass correction should be applied in order to estimate a more realistic kinetic uptake; in this case the BET surface area has been used when linear dependence is observed. Hence, the mass-independent uptake, γBET, for deposits of less than 75 mg was calculated from the γgeom value according to eq II:

(

γBET ) γgeom

Sgeometric SBET × msample

)

(II)

where Sgeometric is the geometric area of the sample holder (in cm2), msample the sample mass (in mg) and SBET the specific BET surface area (in cm2 mg-1). This type of correction allows taking into consideration the diffusion of the gas into the solid layers and estimating the whole internal reactive surface of the mineral (30). The results of this correction are shown in Figure (2b) and no significant differences are observed in γBET values for a sample mass between 10 and 75 mg. Consequently, the uptake coefficients of all experiments with a film mass below 75 mg were calculated taking into account the BET surface area at 298 K. The interactions of ozone with dust (or proxies) have recently been reviewed by Usher et al. (7), and more recently Mogili et al. (19) and Karagulian and Rossi (16) also reported uptake kinetics of ozone on different minerals. It appears that all the reported uptake kinetics have only been reported under dark conditions. Also, most of these “dark” studies have been performed using a Knudsen cell or other experimental systems under dry conditions and low pressure, and often expressed in term of initial uptake coefficient. The latter characterizes the initial molecular interactions of ozone with a clean surface and is therefore directly related to adsorption. As a consequence, the initial uptake coefficients in these studies are generally very high (10-4-10-2). However, measuring the initial uptake coefficient is beyond the time resolution of our experimental layout from which we only derive steady-state uptake coefficients. Accordingly, we cannot compare our results to these data. Some studies, have also reported steady state uptake coefficient which are much smaller (in fact the uptake kinetics slow down as the surface is processed by ozone) in the order from 10-3-10-9 in the dark. The lowest values reported so far are those from Mogili et al., on R-Fe2O3 and R-Al2O3 measured in an environmental aerosol reaction chamber operating at atmospheric pressure and variable humidity (19), i.e., under conditions close to those used here and those encountered in the atmosphere. Our dark uptake coefficients are very small (1-3 × 10-9) values which can easily be explained by the fact that the SiO2 matrix is slightly less reactive toward ozone than R-Fe2O3 and R-Al2O3 (13). 3.3. Irradiance. As seen in Figures 1 and 2, the ozone removal is highly photoenhanced in the presence of the very weak near UV irradiation (irradiance of 2.7 × 1014 photons cm-2 s-1 in the 340-420 nm range) used during this study. Since such irradiance is approximately 50 times inferior to the solar irradiance between 340 and 420 nm reaching the Earth surface (26, 31), we investigated the dependence of the BET steady-state uptake coefficient on the irradiance at 298 K, 24% RH and 109 ppbv of ozone concentration. As shown in SI Figure S2, the BET uptake coefficient is linearly dependent on the irradiance, which varied from 0.12 to 1.15 × 1015 photons cm-2 s-1 confirming the photochemical nature of the ozone loss. Photocatalysis has been shown to depend almost linearly on the irradiance up to a level of a few mW cm-2 (32). Therefore, if the linearity observed in SI Figure S2, is extrapolated to the irradiance reaching the Earth ground in the same wavelength range at fixed ozone mixing ratio and humidity, it is then possible to estimate an atmospherically relevant uptake coefficient. Since the variation of parameters such as humidity or ozone concentration leads to different slopes a rougher approximation is used. The BET uptake coefficients are normalized according to Γphot )

γBET I

(III)

where I is the irradiance reaching the film during the experimental determination. The obtained normalized coefficient, Γphot, is in units of (mW cm-2)-1. Γphot can then be multiplied by the irradiance reaching the Earth ground or

FIGURE 3. Dependence on the ozone mixing ratio of the photochemical and dark BET ozone uptake onto SiO2/TiO2 1 wt % films. The experiments were carried out with 50-290 ppbv of ozone, at T ) 298 K and 24% of relative humidity. Filled symbols: γBET for dark experiments; empty symbols: Γphot for experiments under irradiation (2.7 × 1014 photons cm-2 s-1). The insert shows the same data but on the same scale. the troposphere, making possible a rough assessment of the impact of light on the heterogeneous ozone chemistry on SiO2-TiO2 oxides. This procedure was necessary because of the high reactivity of the surface containing traces of TiO2 and the distinct risk of diffusion issues. Therefore the irradiance used for the majority of the experiments was only 0.147 mW cm-2, i.e., lower by a factor 55 than the irradiance reaching the Earth ground (see section 4). 3.4. Ozone Concentration. Another important atmospheric parameter is the ozone mixing ratio. The dependence of Γphot on the initial ozone concentration at 298 K and 24% relative humidity is illustrated in Figure 3. The average mass of the different films was 30 mg and the ozone mixing ratio ranged from 50 to 290 ppbv, i.e., the values observed in urban polluted areas (33). The photochemical BET steady-state uptake coefficient decreases from (3.2 ( 0.5) × 10-7 to (3.0 ( 0.4) × 10-8 (mWcm-2)-1 when the concentration increases from 50 to 290 ppbv of ozone. Many authors report a similar trend for the uptake of ozone onto various mineral oxides and mineral dust samples (14, 15, 19, 34). However, it is not possible to make direct quantitative comparisons with the literature studies on mineral oxides and mineral dust samples performed in Knudsen cell apparatus or in other experimental systems using dry conditions and low pressure, as the uptake coefficient is therein generally expressed in terms of its initial value. Nevertheless, our data clearly show that there is a saturation of the rate coefficient as the rate is inversely proportional to the ozone concentration, as previously observed in many heterogeneous studies onto different solid and liquid surfaces, both organic and inorganic (14, 26, 35-38). Another intriguing feature is that, as it can be seen from Table 1, the ratio between uptake coefficients under illuminated and dark conditions increases by more than 1 order of magnitude with decreasing ozone concentration. At low ozone mixing ratios, the surface reaction probably dictates the overall kinetics. 3.5. Percentage of TiO2. The previous experiments present results for a mixing ratio of 1 wt % of TiO2 in a matrix of SiO2; however, dust composition has a variable TiO2 content. Hanish and Crowley (15) determined the composition of Saharan dust samples from the Cape Verde Islands and found it to be approximately 4.5 wt % in TiO2. We therefore tested mixing ratios between 1 and 3 wt % of TiO2 in SiO2 matrix and the experimental results are presented in Figure 4 for both dark conditions and under irradiation. For 1, 2, and 3 wt % of TiO2 and a film mass of 50 mg, the measured VOL. 43, NO. 19, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Determination of Atmospheric Ozone Uptake Coefficient, γatmo experimental conditions SiO2/TiO2

1 wt %

1 wt %

1 wt %

3 wt %

ozone concentration 51 ( 2 (ppb) film mass (mg) 31 ( 2

195 ( 5

129 ( 3

130 ( 3

38 ( 2

32 ( 2

51 ( 3

Uptake Coefficients γBET/dark/10-9 2.8 ( 0.4 1.2 ( 0.2 γBET/light/10-8 4.7 ( 0.7 0.4 ( 0.1 Γphot ((mW cm-2)-1)/ 3.2 ( 0.5 0.3 ( 0.0 10-7 γatmo/10-6 2.6 ( 0.4 0.2 ( 0.0

1.9 ( 0.3 1.1 ( 0.2 1.0 ( 0.1 5.8 ( 0.9 0.7 ( 0.1 3.9 ( 0.6 0.6 ( 0.1 3.2 ( 0.5

photochemical uptake coefficient (Γphot) is (1.5 ( 0.2) × 10-7, (3.1 ( 0.5) × 10-7 and (3.9 ( 0.6) × 10-7 (mW cm-2)-1, respectively. This shows again a linear dependence of the normalized photochemical uptake with respect to the wt% of TiO2. Above 3 wt % in TiO2, diffusion limitations became very important and a correct determination of the ozone uptake was not possible with our experimental setup. However the linear dependence can be used to extrapolate to larger wt% of TiO2. 3.6. Relative Humidity. Water partial pressures is another important parameter in the natural environment, and SI Figure S3 shows the photochemical steady-state uptake on a 25 mg SiO2/1 wt % TiO2 film at a relative humidity ranging from 3 to 63%. The film was exposed to 38 ppbv of ozone at 298 K and to an irradiance of 2.7 × 1014 photons cm-2 s-1. Initially, increasing humidity leads to a faster photochemical uptake of ozone. Indeed, Γphot increases from (2.9 ( 0.4) × 10-7 for 0% RH to (4.3 ( 0.6) × 10-7 (mW cm-2)-1 for 20% RH. Above 35% RH, the reactive uptake starts decreasing and finally reaches a plateau at (0.4 ( 0.1) × 10-7 (mW cm-2)-1. This behavior has been often observed on photocatalytic surfaces and suggests a competitive dual adsorption of water and ozone, first facilitating the OH radical formation at the mineral surface and hence the ozone conversion, followed by a regime where dual adsorption will inhibit the reaction (see section 3.7) (39). Mogili et al. (19) also show an inverse dependence between the ozone uptake and the humidity under dark condition in their environmental aerosol chamber. A different behavior has been observed by Chang et al.

(14) who investigated the initial ozone uptake on authentic Saharan dust at various relative humidity. Their results show similar initial uptake coefficients at 0, 50, and 75% of relative humidity. Sullivan et al. (34) have obtained a similar behavior on alumina films. These authors concluded that water vapor does not affect the initial uptake coefficient of ozone on Saharan dust, in opposition to our observation. However, all the literature findings are relative to a “dark chemistry” and are not comparable with our observations where light promotes a different mechanism due to the photocatalytic reaction of TiO2 and justifying the different humidity dependence. 3.7. Ozone Uptake Mechanism. Previous studies of ozone decomposition onto mineral dust under dark conditions report oxygen formation with a yield of 100% (15). In the present study, oxygen formation has not been monitored. As already mentioned, irradiating the surface may imply a completely different reaction mechanism with respect to the dark. Indeed, the formation of holes and electrons on irradiated TiO2 does activate catalytic chain reactions (32) with possibly nonlinear effects on the uptake kinetics: TiO2 + hν f TiO2(e- + h+)

(1)

O3 + e- f O•3

(2)

+ • O•3 + H f HO3

(3)

HO•3 f O2 + HO•

(4)

HO• + O3 f O2 + HO•2

(5)

O2 + e- f O•2

(6)

•O•2 + O3 f O3 + O2

(7)

The ozone absorbed on the available surface sites is a stronger oxidant than oxygen and can be easily reduced by the photogenerated electron, producing an ozonide radical anion. This radical can evolve rapidly in the presence of small amounts of water to generate oxygen and hydroxyl radicals (reactions 2 and 3) (40). The presence of the latter, a stronger oxidant, leads to new oxygen formation from ozone through reactions 4 and 5 (41). Another pathway for ozone destruction involves the formation of superoxide radical anion (reaction 6), which can react with ozone (reaction 7) (42). This catalytic mechanism has been well characterized in aqueous phase but, to our knowledge, has not been identified so far at the gas/solid interface. Anyhow, it depicts that the surface chemistry occurring in these experiments is far from being elementary or simple and may therefore not strictly follow a Langmuir-Hinshelwood formalism. However, reactions 1-7 may explain partly the observed ozone loss. In fact, reactions 1-7 will only be effective under irradiation and will certainly mostly be important at low ozone surface coverage. This may lead to the observed increase of the uptake coefficients ratio (see Table 1).

4. Implications for Atmospheric Chemistry FIGURE 4. Dependence of the photochemical ozone uptake, γBET, on the percentage of TiO2 in the SiO2/TiO2 mixture. The experiments were carried out with 130 ppbv of ozone, at T ) 298 K and 24% of relative humidity and the average film mass was 55 mg. Filled columns: photochemical uptake coefficients for dark experiments; hatched columns: photochemical uptake coefficients for experiments under irradiation (2.7 × 1014 photons cm-2 s-1). 7440

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The main focus of the present study is still the evaluation of the light-induced ozone uptake on a proxy for mineral aerosol (SiO2/TiO2). The photochemical process is not only more important in magnitude (the uptakes increases from 10-9 in the dark to 10-7 under laboratory irradiation), but is time independent (there is no saturation of the surface), in opposition to the reaction in the dark where steady-state uptake coefficients are clearly lower than initial uptake coefficients. Owing to the high reactivity of the surface

containing traces of TiO2 and to avoid diffusion issues the irradiance used was only 0.147 mW cm-2 while at the Earth ground, in the same wavelength range, it reaches approximately 8.13 mW cm-2 at 48° solar zenith angle below 400 nm (as derived for clear sky conditions from the standard spectrum of the American Society for Testing and Materials (ASTM)) (31). A rough extrapolation of the uptakes with the irradiance reaching the Earth ground is presented in Table 1. The atmospheric ozone uptake coefficient, γatmo, reaches values of 10-6 for 51 ppbv of ozone. The γatmo corresponds to more realistic and meaningful uptake coefficients in term of environmental conditions. With respect to the dark ozone loss, the large photoinduced ozone uptake on dust may potentially change the impact of this sink on the atmospheric ozone budget. Mineral surfaces provide an important atmospheric surface for the adsorption and reaction of various trace gases, such as ozone. Indeed, several field campaigns have reported reduced ozone concentrations measured inside dust plumes (3, 43, 44). It is generally believed that the ozone reduction is driven by the much more rapid HNO3 uptake on dust leading to a reduced NO × concentration. The present data show a strongly enhanced reactivity of synthetic surfaces containing TiO2 and SiO2, taken as proxies for atmospheric dust particles when irradiated with weak near UV irradiation. This suggests that under atmospherically relevant conditions the photochemistry of dust could be an important sink for ozone in a dust plume. The global impact of such dust photochemistry is now facilitated by the report made here on the photoinduced uptake coefficients.

Acknowledgments We acknowledge the support of the French Ministry Environment (Primequal2 grant PhotoBat) and of the ANR (NeoRad grant). C.G. is grateful to J.-M. Herrmann, who initiated this study. M.N. is grateful to the Agence Universitaire de la Francophonie and the Re´gion Rhoˆne-Alpes for financial support.

Supporting Information Available Detailed information about the methodology used for measuring the uptake kinetics, Table S1 provides some basic information about the experimental method, and two Figures (S1 and S2) show the linear dependence of the uptake coefficient on the irradiance and the nonlinear behaviour of the relative humidity. This material is available free of charge via the Internet at http://pubs.acs.org.

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