Laboratory study of cluster ions formation in H2SO4–H2O system: Implications for threshold concentration of gaseous H2SO4 and ion-induced nucleation kinetics

Ion-induced binary H₂SO₄–H₂O nucleation is an important mechanism of aerosol formation in the atmosphere. Ions are created in the atmosphere mainly by galactic cosmic rays. The importance of ion-induced nucleation is recognized in some of the observed nucleation events in the background atmosphere. However, the predictions of current ion–aerosol models are highly uncertain mostly due to the lack of detailed experimental information concerning the thermodynamics and kinetics of ion clustering reactions. Here we continue the report of results of our laboratory experiments on the formation and growth of positive and negative cluster ions in H₂SO₄–H₂O vapours in the flow reactor started in Wilhelm et al. [2004. Ion-induced aerosol formation: new insights from laboratory measurements of mixed cluster ions HSO₄⁻(H₂SO₄)_a(H₂O)_w and H⁺ (H₂SO₄)_a(H₂O)_w. Atmospheric Environment 38, 1735–1744] and Sorokin et al. [2006. Formation and growth of sulphuric acid–water cluster ions: experiments, modelling, and implications for ion-induced aerosol formation. Atmospheric Environment 40, 2030–2045]. The main attention is given to the definition of the concentration of gaseous sulphuric acid in experiment and also to some aspects of the kinetics of small cluster ions formation. The performed analysis has indicated a threshold concentration of gaseous sulphuric acid for binary homogeneous nucleation of at least about 10¹⁰ cm⁻³ at room temperature and low relative humidity.     

Deriving sorption indices for the prediction of potential phosphorus loss from calcareous soils

The aim of this study was to develop techniques to evaluate soil phosphorus (P) sorption capacity (PSC) and determine critical soil P levels to predict P loss potential for calcareous soils. Seventy-five soils mostly from Northern China were analyzed for soil P using four extraction methods (water, P_w; carbonate, P_Ols; ammonium oxalate, P_ox; and Mehlich 3, P_M3) as well as PSC derived from single-point (PSC₁₅₀) and multipoint sorption (S _t) isotherms. Strong correlation was found between PSC₁₅₀ and S _t (r ²=0.89, p<0.001). The sum of αCa_M3 and βMg_M3 as an index of PSC (PSC_{(CaM3 + MgM3)}) was most closely related to the maximum amount of P sorbed (S _max) as given by the sum of S _t and soil initial P setting α=0.039 and β=0.462 (r ²=0.80, p<0.001). The degree of P saturation (DPS) was thereafter calculated from PSC_{(CaM3 + MgM3)} (DPS_{(CaM3 + MgM3)}), to which Olsen P (P_Ols) was significantly correlated (r ²=0.82, p<0.001). In a split-line regression from P_w against DPS_{(CaM3 + MgM3)} (r ²=0.87, p<0.05), a change point was identified at 28.1% DPS_{(CaM3 + MgM3)}, which was equivalent to 49.2 mg kg^?1 P_Ols and corresponded to a P_w concentration of 8.8 mg kg^?1. After the change point, a sharp increase in P_w was observed. Our results reveal a new approach to approximating DPS from Ca_M3 and Mg_M3 for calcareous soils without the need to generate a S _max. We conclude that in the absence of an environmental soil test criteria for P, the DPS_{(CaM3 + MgM3)} and P_Ols could be used to predict P loss potential from calcareous soils.     

Nitrous oxide emission from an agricultural field: Comparison between measurements by flux chamber and micrometerological techniques

The soil in a drained fjord area, reclaimed for arable farming, produced N₂O mainly at 75–105 cm depth, just above the ground water level. Surface emissions of N₂O were measured from discrete small areas by closed and open-flow chamber methods, using gas chromatographic analysis and over larger areas by integrative methods: flux gradient (analysis by FTIR), conditional sampling (analysis by TDLAS), and eddy covariance (analysis by TDLAS). The mean emission of N₂O as determined by chamber procedures during a 9-day campaign was 162–202 μg N₂ONm⁻²h⁻¹ from a wheat stubble and 328–467 μg N₂ONm⁻² h⁻¹ from a carrot field. The integrative approaches gave N₂O emissions of 149–495 μg N₂ONm⁻² h⁻¹, i.e. a range similar to those determined with the chamber methods. Wind direction affected the comparison of chamber and integrative methods because of patchiness of the N₂O emission over the area. When a uniform area with a single type of vegetation had a dominant effect on the N₂O gradient at the sampling mast, the temporal variation in N₂O emission determined by the flux gradient/FTIR method and chamber methods was very similar, with differences of only 18% or less in mean N₂O emission, well below the variation encountered with the chamber methods themselves. A detailed comparison of FTIR gradient and chamber data taking into account the precise emission footprint showed good agreement. It is concluded that there was no bias between the different approaches used to measure the N₂O emission and that the precision of the measurements was determined by the spatial variability of the N₂O emission at the site and the variability inherent in the individual techniques. These results confirm that measurements of N₂O emissions from different ecosystems obtained by the different methods can be meaningfully compared.     

Comparison of photochemical behavior of various humic substances in water: III. Spectroscopic properties of humic substances

Spectral absorption coefficients and fluorescence quantum efficiencies were determined for humic substances from a variety of sources. Specific absorption coefficients k_h, for humic substances at wavelengths λ from 300 to 500 nm can be closely described by the relation Ae^B(450-λ), where A and B are constants. When the k_h values are in units of liter (mg organic carbon)^?1meter^?1 and wavelength λ is in nanometers, mean values of A and B for aquatic humus in the 12 water bodies studied were 0.6±0.3 and 0.014±0.001, respectively. Spectral absorption coefficients of dissolved organic matter in blackwater rivers, of the “yellow substance” in the sea, and of fulvic acids extracted from soils are very similar. Fluorescence quantum yields of humic substances were low and more variable than the absorption coefficients, ranging from 0.0005 to 0.012 with excitation at 350 nm (average of 0.0045±0.0038 for 6 waters). Fluorescence spectra for the humic substances were remarkably similar with maximum emission at 430 to 470 nm. Results of this study can be used to compute photolysis rates of pollutants as a function of depth in natural water bodies.     

Do lagoons near concentrated animal feeding operations promote nitrous oxide supersaturation?

Animal wastewater lagoons nearby concentrated animal feeding operations (CAFOs) represent the latest tendency in global animal farming, severely impacting the magnitude of greenhouse gas emissions, including nitrous oxide (N₂O). We hypothesized that lagoon wastewater could be supersaturated with N₂O as part of incomplete microbial nitrification/denitrification processes, thereby regulating the N₂O partitioning in the gaseous phase. The objectives of this study were: (i) to investigate the magnitude of dissolved N₂O concentrations in the lagoon; and (ii) to determine the extent to which supersaturation of N₂O occurs in wastewater lagoons. Dissolved N₂O concentrations in the wastewater samples were high, ranging from 0.4 to 40.5 μg N₂O mL⁻¹. Calculated dissolved N₂O concentrations from the experimentally measured partition coefficients were much greater than those typically expected in aquatic systems (<∼0.6 μg N₂O mL⁻¹). Knowledge of the factors controlling the magnitude of N₂O supersaturation could potentially bridge mass balance differences between in situ measurements and global N₂O models.     

Separate chemical characterizations of fog water,aerosol, and gas before,during, and after fog events near an industrialized area in Japan

Fog water, aerosol, and gas were separately collected at Mt. Rokko (altitude 931 m) in Kobe, Japan, using a new sampling method at a mountainous site near a highly industrialized area. The fog water was collected by an active string-fog collector and the aerosol and gas by using the filter pack method. Using plural filter packs and controlling or switching the airflow before, during, and after a fog event made it possible to collect the fog water, aerosol, and gas separately. Nitrate species such as NO₃⁻(p) and HNO₃(g) were effectively scavenged by fog water, while sulfur species such as SO₄²⁻(p) and SO₂(g) could not be easily and effectively scavenged because of the poor solubility of SO₂(g). This difficulty was experimentally examined through an in situ investigation. Ion species (especially Na⁺(p) and Ca²⁺(p)) which form coarse particles were easily and effectively scavenged by fog water. On the other hand, the difficulty of scavenging Mg²⁺(p) could not be explained by particle size.     

Atmospheric fate of acrylic acid and acrylonitrile: Rate constants with Cl atoms and OH radicals in the gas phase

Rate coefficients for the reactions of hydroxyl radicals and chlorine atoms with acrylic acid and acrylonitrile have been determined at 298 K and atmospheric pressure. The decay of the organics was followed using a gas chromatograph with a flame ionization detector (GC-FID) and the rate constants were determined using a relative rate method with different reference compounds. Room temperature rate constants are found to be (in cm³ molecule⁻¹ s⁻¹): k₁(OH+CH₂CHC(O)OH)=(1.75±0.47)×10⁻¹¹, k₂(Cl+CH₂CHC(O)OH)=(3.99±0.84)×10⁻¹⁰, k₃(OH+CH₂CHCN)=(1.11±0.33)×10⁻¹¹ and k₄(Cl+CH₂CHCN)=(1.11±0.23)×10⁻¹⁰ with uncertainties representing ±2σ. This is the first kinetic study for these reactions under atmospheric pressure. The rate coefficients are compared with previous determinations taking into account the effect of pressure on the rate constants. The effect of substituent atoms or groups on the overall rate constants is analyzed in comparison with other unsaturated compounds in the literature. In addition, atmospheric lifetimes based on the homogeneous sinks of acrylic acid and acrylonitrile are estimated and compared with other tropospheric sinks for these compounds.     

Temperature-dependent rate coefficients for the reactions of Cl atoms with methyl methacrylate,methyl acrylate and butyl methacrylate at atmospheric pressure

Rate coefficients for the gas-phase reactions of Cl atoms with a series of unsaturated esters CH₂C(CH₃)C(O)OCH₃ (MMA), CH₂CHC(O)OCH₃ (MAC) and CH₂C(CH₃)C(O)O(CH₂)₃CH₃ (BMA) have been measured as a function of temperature by the relative technique in an environmental chamber with in situ FTIR detection of reactants. The rate coefficients obtained at 298 K in one atmosphere of nitrogen or synthetic air using propene, isobutene and 1,3-butadiene as reference hydrocarbons were (in units of 10^?10 cm³ molecule^?1 s^?1) as follows: k_(Cl+MMA) = 2.82 ± 0.93, k_(Cl+MAC) = 2.04 ± 0.54 and k_(Cl+BMA) = 3.60 ± 0.87. The kinetic data obtained over the temperature range 287–313 K were used to derive the following Arrhenius expressions (in units of cm³ molecule^?1 s^?1): k_(Cl+MMA) = (13.9 ± 7.8) × 10^?15 exp[(2904 ± 420)/T], k_(Cl+MAC) = (0.4 ± 0.2) × 10^?15 exp[(3884 ± 879)/T], k_(Cl+BMA) = (0.98 ± 0.42) × 10^?15 exp[(3779 ± 850)/T]. All the rate coefficients display a slight negative temperature dependence which points to the importance of the reversibility of the addition mechanism for these reactions. This work constitutes the first kinetic and temperature dependence study of the reactions cited above.An analysis of the available rates of addition of Cl atoms and OH radicals to the double bond of alkenes and unsaturated and oxygenated volatile organic compounds (VOCs) at 298 K has shown that they can be related by the expression: log k_OH = 1.09 log k_Cl ? 0.10. In addition, a correlation between the reactivity of unsaturated VOCs toward OH radicals and Cl atoms and the HOMO of the unsaturated VOC is presented. Tropospheric implications of the results are also discussed.     

Estimating the vapor pressures of multi-functional oxygen-containing organic compounds using group contribution methods

A UNIFAC-based method for estimating the vapor pressure (p_L^o) values of oxygen-containing compounds of intermediate-to-low volatility has been developed as an aid in modeling the formation and behavior of organic aerosols. This UNIFAC-p_L^o method was constructed using a set of 76 compounds with experimentally determined p_L^o values. The compounds chosen are of intermediate-to-low volatility and contain multiple oxygen-containing functionalities. For test and development purposes, the 76 compounds were divided into a basis set of 43 compounds used to generate the coefficients required in the UNIFAC-p_L^o method and a second set of 33 compounds that was used to test the coefficients generated using the basis set. Both the basis and test sets contained compounds that possessed similar structures and functionalities. For the 33 compounds in the test set, on average UNIFAC-p_L^o predicted the p_L^o values to within a factor of 2 over the temperature range 290–320 K. Furthermore, the UNIFAC-p_L^o method did not show any correlation in prediction error with p_L^o so that it was equally likely to underpredict as overpredict p_L^o regardless of volatility. For comparison, three other vapor pressure estimation methods were applied to the test set of compounds. On average, these other methods all predicted the test set p_L^o values to within a factor of 3 over the temperature range 290–320 K. In contrast to the UNIFAC-p_L^o method, the prediction errors from the methods were found to be correlated with p_L^o so that the other methods overpredicted p_L^o as volatility decreased.     

Impact of initial and boundary conditions on preferential flow

Preferential flow in soil is approached by a water-content wave, WCW, that proceeds downward from the ground surface. WCWs were obtained from sprinkler experiments with infiltration rates varying from 5 to 40 mm h^− 1. TDR-probes and tensiometers measured volumetric water contents θ(z,t) at seven depths, and capillary heads, h(z,t) at six depths in a column of an undisturbed soil. The wave is characterized by the velocity of the wetting front, c_W, the amplitude, w_S, and the final water content, θ. We tested with uni-variate and bi-variate linear regressions the impacts of initial volumetric water contents, θ_ini, and input rates, q_S, on c_W, w_S and θ.The test showed that θ_ini influenced θ and w_S and q_S effected c_W. The expected proportionality of w_S ≈ qs^1/3 was weak and c_W ≈ qs^2/3 was strong.     

An Analysis of Extinction Coefficients of Particles and Water Moisture in the Stack after Flue Gas Desulfurization at a Coal-Fired Power Plant

ABSTRACT

Two important factors that affect in-stack opacity—light extinction by emitted particles and that by water moisture after a flue gas desulfurization (FGD) unit—are investigated. The mass light extinction coefficients for particles and water moisture, k _p and k _w, respectively, were determined using the Lambert-Beer law of opacity with a nonlinear least-squares regression method. The estimated k _p and k _w values vary from 0.199 to 0.316 m²/g and 0.000345 to 0.000426 m²/g, respectively, and the overall mean estimated values are 0.229 and 0.000397 m²/g, respectively. Although k _w is 3 orders of magnitude smaller than k _p, experimental results show that the effect on light extinction by water moisture was comparable to that by particles because of the existence of a considerable mass of water moisture after a FGD unit. The mass light extinction coefficient was also estimated using Mie theory with measured particle size distributions and a complex refractive index of 1.5-ni for fly ash particles. The k _p obtained using Mie theory ranges from 0.282 to 0.286 m²/g and is slightly greater than the averaged estimated k _p of 0.229 m²/g from measured opacity. The discrepancy may be partly due to a difference in the microstructure of the fly ash from the assumption of solid spheres because the fly ash may have been formed as spheres attached with smaller particles or as hollow spheres that contained solid spheres. Previously reported values of measured k _p obtained without considering the effects of water moisture are greater than that obtained in this study, which is reasonable because it reflects the effect of extinction by water moisture in the flue gas. Additionally, the moisture absorbed by particulate matter, corresponding to the effect of water moisture on the particulates, was clarified and found to be negligible.

IMPLICATIONS In-stack opacity is used as a surrogate for particle concentration and can be measured using light transmission meters as part of a continuous emission monitoring system. Because emission standards have become increasingly strict, FGD with wet scrubbing is generally used for coal-fired power plants. However, after a FGD unit with wet scrubbing is set up, the concentration of water moisture increases, affecting the measured opacity. This study evaluates the contributions of particles and water moisture to opacity. The results should provide useful information and can be utilized for modifying measurements for monitoring particulate emissions using opacity.     

FTIR gas-phase kinetic study on the reactions of some acrylate esters with OH radicals and Cl atoms

Acrylate esters are α,β-unsaturated esters that contain vinyl groups directly attached to the carbonyl carbon. These compounds are widely used in the production of plastics and resins. Atmospheric degradation processes of these compounds are currently not well understood. The kinetics of the gas phase reactions of OH radicals with methyl 3-methylacrylate and methyl 3,3-dimethylacrylate were determined using the relative rate technique in a 50 L Pyrex photoreactor using in situ FTIR spectroscopy at room temperature (298?±?2 K) and atmospheric pressure (708?±?8 Torr) with air as the bath gas. Rate coefficients obtained were (in units cm³ molecule^?1 s^?1): (3.27?±?0.33)?×?10^?11 and (4.43?±?0.42)?×?10^?11, for CH₃CH═CHC(O)OCH₃ and (CH₃)₂CH═CHC(O)OCH_3, respectively. The same technique was used to study the gas phase reactions of hexyl acrylate and ethyl hexyl acrylate with OH radicals and Cl atoms. In the experiments with Cl, N₂ and air were used as the bath gases. The following rate coefficients were obtained (in cm³ molecule^?1 s^?1): k₃ (CH₂═CHC(O)O(CH₂)₅CH₃?+?Cl)?=?(3.31?±?0.31)?×?10^?10, k₄(CH₂═CHC(O)OCH₂CH(CH₂CH₃)(CH₂)₃CH₃?+?Cl)?=?(3.46?±?0.31)?×?10^?10, k₅(CH₂═CHC(O)O(CH₂)₅CH₃?+?OH)?=?(2.28?±?0.23)?×?10^?11, and k₆(CH₂═CHC(O)OCH₂CH(CH₂CH₃)(CH₂)₃CH₃?+?OH)?=?(2.74?±?0.26)?×?10^?11. The reactivity increased with the number of methyl substituents on the double bond and with the chain length of the alkyl group in –C(O)OR. Estimations of the atmospheric lifetimes clearly indicate that the dominant atmospheric loss process for these compounds is their daytime reaction with the hydroxyl radical. In coastal areas and in some polluted environments, Cl atom-initiated degradation of these compounds can be significant, if not dominant. Maximum Incremental Reactivity (MIR) index and global warming potential (GWP) were also calculated, and it was concluded that these compounds have significant MIR values, but they do not influence global warming.     

Hydrophobic organic compound partitioning from bulk water to the water/air interface

Partitioning of hydrophobic organic compounds to the interface between water and air may significantly affect the distribution and transfer of many xenobiotic chemicals between vapor and aqueous phases. The fluorescent probe, 1-methylperylene, was used to investigate the affinity of hydrophobic compounds for the water–air interface by varying the ratio of interfacial surface area to water volume in a fused-quartz cuvette. We found that the water–air/water interface partitioning coefficient [K_w−awi=1.2 mol cm^-2_awi/(mol ml^-1_w)] for this polycyclic aromatic hydrocarbon (PAH) was quantitatively consistent with partitioning to the same interface but from the airside, recently reported in the literature for less hydrophobic PAHs. Our results demonstrate significant partitioning from bulk water to the water/air interface for a hydrophobicity range relevant to many xenobiotic compounds. Anticipated implications of this process for the environmental chemistry of hydrophobic compounds include retarded gas-phase transport in unsaturated soils, bubble-mediated transport in water, droplet-mediated transport in the atmosphere, and photochemical reactions.     

Plutonium partitioning in three-phase systems with water,granite grains,and different colloids

Low-solubility contaminants with high affinity for colloid surfaces may form colloid-associated species. The mobile characteristics of this species are, however, ignored by the traditional sorption/distribution experiments in which colloidal species contributed to the immobile fraction of the contaminants retained on the solids as a result of centrifugation or ultrafiltration procedures. The mobility of the contaminants in subsurface environments might be underestimated accordingly. Our results show that colloidal species of ²³⁹Pu in three-phase systems remained the highest percentages in comparison to both the dissolved species and the immobile species retained on the granite grains (solid phase), although the relative fraction of these three species depended on the colloid types. The real solid/liquid distribution coefficients (K _s/d) experimentally determined were generally smaller than the traditional K _s/d (i.e., the K _s+c/d in this study) by ~1,000 mL/g for the three-phase systems with the mineral colloids (granite particle, soil colloid, or kaolinite colloid). For the humic acid system, the traditional K _s/d was 140 mL/g, whereas the real K _s/d was approximately zero. The deviations from the real solid/liquid K _s/d were caused by the artificially increased immobile fraction of Pu. One has to be cautious in using K _s/d-based transport models to predict the fate and transport of Pu in the environment.     

Probing the sensitivity of gaseous Br2 production from the oxidation of aqueous bromide-containing aerosols and atmospheric implications

This paper presents a global sensitivity and uncertainty analysis of the bromine chemistry included in the Model of Aqueous, Gaseous and Interfacial Chemistry (MAGIC) in dark and photolytic conditions. Uncertainty ranges are established for input parameters (e.g. chemical rate constants, Henry's law constants, etc.) and are used in conjunction with Latin hypercube sampling and multiple linear regression to conduct a sensitivity analysis that determines the correlation between each input parameter and model output. The contribution of each input parameter to the uncertainty in the model output is calculated by combining results of the sensitivity analysis with input parameters' uncertainty ranges. Model runs are compared using the predicted concentrations of molecular bromine since Br_2(g) has been shown in previous studies to be generated via an interface reaction between O_3(g) and Br_(surface)^? during dark conditions [Hunt et al., 2004]. Formation of molecular bromine from the reaction of ozone with deliquesced NaBr aerosol: evidence for interface chemistry. Journal of Physical Chemistry A 108, 11559–11572]. This study also examines the influence of an interface reaction between OH_(g) and Br_(surface)^? in the production of Br_2(g) under photolytic conditions where OH_(g) is present in significant concentrations. Results indicate that the interface reaction between O_3(g) and Br_(surface)^? is significant and is most responsible for the uncertainty in MAGICs ability to calculate precisely Br_2(g) under dark conditions. However, under photolytic conditions the majority of Br_2(g) is produced from a complex mechanism involving gas-phase chemistry, aqueous-phase chemistry, and mass transport.     

Fumigation of pollutants in and above the entrainment zone into a growing convective boundary layer: a large-eddy simulation

Fumigation of a passive plume located in or above the entrainment zone (EZ) into a growing convective boundary layer (CBL) has been simulated by large-eddy simulation (LES). Three non-dimensional parameters, α(=w_e0/w_*0), z₀/z_i0, and σ_z0/z_i0, are used to classify different cases, where w_*0 is the convective velocity scale, w_e0 the initial entrainment velocity, z_i0 the initial CBL height, z₀ the initial plume height, and σ_z0 is the initial plume half-depth. Forty cases have been run and analysed. The crosswind-integrated concentrations have been compared with existing laboratory data from a saline convection tank. The results show that LES is a promising tool to reproduce fumigation processes. With a relatively coarse grid mesh near the EZ, LES derives reliable results that are in a good agreement with the laboratory data. The first parameter, α, containing the effects due to inversion strength, plays an important role in determining C₀(T), the ground-level concentration (GLC) as a function of dimensionless time, T. For large α (say >0.03, corresponding to fast entrainment), variation of α gives significant change in C₀(T); whereas for a wide range of α between 0.01 and 0.02 (corresponding to slow entrainment), C₀(T) is almost independent of α. The starting time of fumigation does not vary significantly with the second parameter, z₀/z_i0 (relative height of plume), although C₀(T) is, in general, smaller for a higher plume. This confirms laboratory findings that the traditional notion of zero fumigation for a high plume (say above 1.10z_i) is not correct. The effect of the third parameter, σ_z0/z_i0, is on the magnitude of C₀(T); thinner initial plumes have higher GLCs.     

Sorption of polycyclic aromatic hydrocarbons (PAHs) on glass surfaces

Sorption of polycyclic aromatic hydrocarbons (PAHs) to glass commonly used in laboratories was studied. Sorption coefficients (K_d) of five selected PAHs to borosilicate glass surfaces were measured using column chromatography. A linear relationship between log K_d and the corresponding water solubility of the subcooled liquid (log S_w) of the investigated PAHs was observed. Based on the determined sorption coefficients our data revealed that mass loss caused by sorption on glass walls strongly depends on the ratio of solution volume to contacted surface area (V/S). The influence of solution chemistry such as ionic strength, solution pH, presence of cosolvent, and the influence of temperature on the sorption process were investigated. In the presence of ionic strength, sorption coefficients concurrently increased but less than a factor of 2 up to 0.005 M calcium chloride concentration. However, further increasing ionic strength had no influence on K_d. The cosolvent reduced sorption at a concentration of methanol in water above 0.5% (v/v); however, for benzo[a]pyrene even with 10% (v/v) methanol the mass loss would be still higher than 10% (with a V/S ratio less than 0.25). Significant effects of the solution pH and temperature were not observed. These results suggest that van der Waal’s forces dominate the sorption process. In the analysis of highly hydrophobic PAHs in aqueous samples, mass loss due to sorption on glass walls should be accounted for in the final result if untreated glass is used. The presented relationship between log K_d and log S_w may help to decide if such a correction is necessary. Furthermore, the frequently used silanization of glass surfaces may not be sufficient to suppress sorption for large PAHs.     

Cross-sections and quantum yields for the atmospheric photolysis of the potent greenhouse gas nitrogen trifluoride

Although NF₃, a trace gas of purely anthropogenic origin with a large global warming potential is accumulating in the Earth's atmosphere, little photochemical data exists from which to calculate its atmospheric removal rate. In this study, photodissociation quantum yields, Φ₁, were derived following 193.3 nm laser photolysis of NF₃, and quantitative conversion of the F-atom photoproducts to OH, which was detected by laser induced fluorescence. Values of Φ₁(P, T) = (1.03 ± 0.05) were determined at pressures between 28 and 100 mBar of He or N₂ and at either room temperature or 255 K. Absorption cross-sections, σ, obtained between 184 and 226 nm were combined with the values of Φ₁(P, T) to confirm a long (≈700 year) photolysis lifetime for NF₃. No evidence for reaction of OH with NF₃ was found, indicating that this process makes little or no contribution to NF₃ removal from the atmosphere. These results underpin recent calculations of an NF₃ atmospheric lifetime τ ≈ 550 years, largely controlled by photolysis in the stratosphere. In the course of this work the rate coefficient k₂(298 K) = (1.3 ± 0.2) × 10^?11 cm³ molecule^?1 s^?1 was obtained for the reaction F + H₂O.