Degradation of Isoprene in the Presence of Sulphoxy Radical Anions

Autoxidation of S(IV) initiated by manganese sulphate or potassium peroxydisulphate in alkaline aqueous solutions was significantly slowed down by dissolved isoprene, which decayed in the process. The laboratory experiments were carried out in a batch, perfectly mixed reactor, which had no gas space. The concentration–time profiles of oxygen were measured with a Clark-type electrode. The profiles of sulphite species and of isoprene were evaluated from the UV spectra of solutions. The kinetic analysis indicated that isoprene reacted directly with sulphate radical anions produced during the S(IV) autoxidation. A relative second-order rate constant of (2.12 ± 0.37) × 10⁹ M^–1 s^–1 was determined for this reaction at 25 °C, pH (8.0–8.5) and ionic strength of (1.7–4.9) × 10^–3 M (the reference rate constant of the reaction of sulphate radical anions with sulphite ions equalled 3.4 × 10⁸ M^–1 s^–1). A tentative mechanism of isoprene oxidation during S(IV) autoxidation, which included formation of isoprene – SO^– ₄ adduct, was based on the analogy to the gas-phase reactions of isoprene and to the liquid-phase reactions of sulphate radical anions with other compounds. Atmospheric significance of the aqueous-phase reaction of isoprene with sulphate radicals was discussed. Approximate analysis showed the reaction is a potential sink for isoprene in the aqueous phase and in the gas–liquid systems of high liquid water content (LWC > 10^–5 m³ m^–3). The aqueous-phase oxidation of isoprene can produce secondary pollutants, and influence transformation and the long-range transport of SO₂ in the atmosphere.     

The Effect of Atmospheric Organic Compounds on the Fe-Catalyzed S(IV) Autoxidation in Aqueous Solution

Laboratory experiments were conducted with real atmospheric aerosol particles as well as with synthetic solutions under dark conditions, to simulate some of the chemical features of aerosols. In solutions obtained by the leaching of aerosols (size range >D _ae: 0.4–1.6 m) that contained sufficient amounts of transition metal ions (e.g. Fe) and organic species (e.g. oxalate), S(IV) oxidation rates were significantly lower than those expected from the Fe-catalyzed S(IV) autoxidation in Milli-Q water. The results suggest that oxalate is responsible for much of the observed inhibition. Acetate and formate also inhibit the reaction, but to a much lesser extent. Oxalate has a strong inhibiting effect on the Fe-catalyzed S(IV) autoxidation at all investigated pH values (2.8, 3.7 and 4.5). It was established that Fe(III)-oxalato complexes affect the redox cycling of Fe(II)/Fe(III) and that the observed decrease of the reaction rate is caused by the reduced amount of catalytically active Fe(III) due to the complexation with oxalate. For the system Fe-S(IV)-O₂-oxalate at initial pH 3.7 the reaction rate was calculated using exponential simplification to account for oxalate influence on the amount of free Fe(III) by the following equation:–r_S(IV) = k · [S(IV)] · [Fe(III))] · e ^-b·[Ox]     

Interactions Between Environmental Selenium and Sulphoxy Radicals

Chemical interactions between sulphoxy radical anions and elemental Se were investigated to show the influence of SO₂ pollution on the atmospheric fate of selenium. Laboratory experiments were performed with elemental Se plates or fine powder exposed to sulphoxy radicals formed in situ during the autoxidation of SO₂ dissolved in water. Impacts of the radicals were followed examining the topographic surface changes of Se (atomic force microscopy, AFM) and measuring the S(IV) autoxidation rate, essentially decreased in the presence of Se (conductometry). The experiments showed that remobilization of water insoluble selenium (elemental Se) may take place not only in atmosphere (cloud waters), but also in surface waters, soil and sediments contacting with SO₂ polluted air. By scavenging radicals, selenium causes the retardation of the S(IV) autoxidation to sulphuric acid (acid rain), thus contributing to long-distance transport of unreacted S(IV). The more general problem of sulphoxy radical-induced atmospheric corrosion has been brought forward into consideration, as the radicals appear capable to damage diverse solid surfaces.     

On the pH-dependent formation constants of iron(III)-sulfur(IV) transient complexes

An experimental study is described of Fe(III)-S(IV) formation constants measured as a function of pH (1–3), ionic strength (0.2–0.5 M) and [Fe(III)] _T (2.5–5.0×10^–4 M) using a continuous-flow spectrophotometric technique to make observations 160 ms after mixing. Preliminary experiments using pulse-accelerated-flow (PAF) spectrophotometry to measure rate constants on a microsecond timescale are also described. The conditional formation constant at 25 °C can be modeled with the following equation: {ie307-1} where {ie307-2}K ₇ andK ₈ can be interpreted as intrinsic constants for the coordination of HSO ₃ ^– by FeOH²⁺ and Fe³⁺, respectively, but until further evidence is obtained they should be regarded as fitting constants. PAF spectrophotometry showed that the initial reaction of Fe(III) with S(IV) (pH 2.0) is characterized by a second-order rate constant of 4×10⁶ M^–1 s^–1 which is comparable to rate of reaction of FeOH²⁺ with SO ₄ ^2– . However, the PAF results should be regarded as preliminary since unexpected features in the initial data indicate that the reaction may be more complex than expected.     

S(IV) Autoxidation in Atmospheric Liquid Water: The Role of Fe(II) and the Effect of Oxalate

The reactivity of dissolved iron compounds towards different pollutants and photooxidants in atmospheric liquid water depends upon the oxidation state and speciation of iron. Our measurements of the oxidation state of dissolved iron eluted from aerosol particles (Dae: 0.4–1.6 m) collected in the urban atmosphere of Ljubljana showed that a large fraction of the iron content is present as Fe(II). The concentration ratio [Fe(II)]/[Fe(III)] varied between 0.9 and 3.1. The kinetics of S(IV) autoxidation catalyzed by Fe(II) under the conditions representative for acidified atmospheric liquid water and the influence of oxalate on this reaction under dark conditions was investigated. The reaction rate is the same if Fe(II) or Fe(III) is used as a catalyst under the condition that Fe(II) can be oxidized in Fe(III), which is the catalytically active species. Oxalate has a strong inhibiting effect on the S(IV) autoxidation in the presence of Fe(II). The reaction is autocatalytic with an induction period, that increases with higher concentrations of oxalate. The inhibiting effect of oxalate differs according to whether iron is initially in the Fe(II) or Fe(III) state. However, in both cases the inhibition by oxalate is a result of the formation of complexes with the catalyst.     

Henry's law and the behavior of weak acids and bases in fog and cloud

Experimental data from two field experiments on ground based clouds were used to study the distribution of formic acid, acetic acid, ammonia and S(IV) species between liquid and gas phase. The ratio of the concentrations of these compounds between the phases during concurrent measurements was compared to ratios expected according to Henry's law (considering the pH influence). Large discrepancies of several orders of magnitude were seen. Three hypotheses have been investigated to explain the observed discrepancies: The existence of a microscale equilibrium which does not persist in a bulk sample, a thermodynamic shift of the equilibrium due to competing reactions, and nonequilibrium conditions due to mass transfer limitations. Approximate quantitative calculations show that none of these hypotheses is sufficient to explain all of the discrepancies, so a combination of different effects seems to be responsible for this observation. The same theoretical considerations also suggest that mass transfer limitation may be an important factor for highly soluble compounds. The data presented here indicates that it is not possible to simply extrapolate interstitial gas phase composition from measured bulk liquid phase concentrations of a fog or cloud.Notation [r _max] liquid phase molar uptake rate (mol l^–1 s^–1) - [A _g ] concentration ofA in gas phase (atm) - [A _l ] concentration ofA in liquid phase (mol l^–1) - [A _g , 0] concentration ofA in gas phase (atm) at time 0 - LWC liquid water content (g m^–3) - R universal gas constant (0.082 l atm mol^–1 K^–1 - D _g diffusivity (for all gases 0.1 cm² s^–1 was used) - K _H ^* effective Henry's law coefficient (mol l^–1 atm^–1) - t _f lifetime of fog droplet (s) - a droplet radius (cm) - accommodation coefficient - R factor of discrepancy - T temperature (K) - v mean molecular speed (cm s^–1) formic acid: 35 000 acetic acid: 31 000 ammonia: 58 000     

A Simple Kinetic Model for Autoxidation of S(IV) Oxides Catalyzed by Iron and/or Manganese Ions

The reaction kinetics of S(IV) autoxidation catalyzed by single metal ions of Mn(II) and Fe(II) or Fe(III) and by a mixture of Mn(II) and Fe(II) under the conditions representative for acidified atmospheric liquid water was investigated. A simple power law kinetic model based on the stability constants for metal-sulfito complexes formed during the first step of a radical chain mechanism predicts well the kinetics for the reactions catalyzed by single metal ions. The calculated stability constants for iron (5.7×10³ dm³ mol^–1) and manganese (10×10⁴ dm³ mol^–1) sulfito complexes are close to those reported in the literature. The catalytic synergism between Mn(II) and Fe(II) was confirmed. For this system the following power law rate equation was suggested:r_tot = S_Fe · r_Fe + S_Mn · r_Mn ,where r_Fe and r_Mn are the reaction rates in the presence of Fe(II) and Mn(II), respectively. S_Fe and S_Mn are proportional factors, which account for the synergistic effect. The proposed power law rate equation predicts the reaction kinetics very well. The values of S_Fe (1.35) and S_Mn (15) indicate that the influence of Fe(II)/Fe(III) on Mn(II)/Mn(III) cycling is larger than, vice versa, agreeing with the reaction mechanism proposed for the S(IV) autoxidation catalyzed by mixed metal ions.     

A Generalized Structure-Activity Relationship for the Decomposition of (Substituted) Alkoxy Radicals

A novel and readily applicable Structure-Activity Relationship (SAR) for predicting the barrier height E_b to decomposition by C-C scission of (substituted) alkoxy radicals is presented. Alkoxy radicals are pivotal intermediates in the atmospheric oxidation of (biogenic) volatile organic compounds, and their fate is therefore of crucial importance to the understanding of atmospheric VOC degradation mechanisms. The SAR is based on available theoretical energy barriers and validated against barriers derived from experimental data. The SAR is expressed solely in terms of the number(s) N_i of alkyl-, hydroxy- and/or oxo-substituents on the - and -carbons of the breaking bond: E_b(kcal/mol) =17.5 – 2.1 × N(alk) – 3.1 ×N(alk) – 8.0 × N_,(OH) – 8.0 × N(O=) – 12 × N(O=). For barriers below 7 kcal/mol, an additional, second-order term accounts for the curvature. The SAR reproduces the available experimental and theoretical data within 0.5 to 1 kcal/mol. The SAR generally allows conclusive predictions as to the fate of alkoxy radicals; several examples concerning oxy radicals from prominent atmospheric VOC are presented. Specific limitations of the SAR are also discussed. Using the predicted barrier height E_b, the high-pressure rate coefficient for alkoxy decomposition k _diss (298 K) can be obtained from k _diss (298 K) = L ×1.8 × 10¹³ exp(–E_b/RT) s^–1, with L the reaction path degeneracy.     

FT-IR Study of the Kinetics and Products of the Reactions of Dimethylsulphide, Dimethylsulphoxide and Dimethylsulphone with Br and BrO

Kinetics and products of the gas-phase reactions of dimethylsulphide (DMS), dimethylsulphoxide (DMSO) and dimethylsulphone (DMSO₂) with Br atoms and BrO radicals in air have beeninvestigated using on-line Fourier Transform Infrared Spectroscopy (FT-IR) as analytical technique at 740 ± 5 Torr total pressure and at 296 ± 3 K in a480 L reaction chamber. Using a relative rate method for determining the rate constants; the following values (expressed in cm³molecule^–1 s^–1) were found: k_DMS+Br = (4.9 ±1.0) ×10^–14, k_DMSO + Br < 6 × 10^–14,k_{DMSO 2} + Br 1 × 10^–15,k_DMSO + BrO = (1.0 ± 0.3) × 10^–14 andk_{DMSO 2} + BrO 3 × 10^–15 (allvalues are given with one on the experimental data). DMSO, SO₂, COS, CH₃SBr andCH₃SO₂Br were identified as the main sulphur containing products of the oxidation of DMS by Br atoms. From the reaction between DMSO and Br atoms, DMSO₂and CH₃SO₂Br were the only sulphur containing products thatwere identified. DMSO, DMSO₂ and SO₂ were identified as themain sulphur containing products of the reaction between DMS and BrO.DMSO₂ was found to be the only product of the reaction between DMSO and BrO. For the reactions of DMSO₂ with Br and BrO no products were identified because the reactions were too slow.The implications of these results for atmospheric chemistry are discussed.     

Kinetics of ammonia and ammonium ion inhibition of the atmospheric oxidation of aqueous sulfur dioxide by oxygen

This is the first study, which shows both NH₃ and NH₄⁺ to inhibit the autoxidation of aqueous SO₂ in the pH range 7.0–8.5. The rate of the autoxidation, R _aut , in both buffered and unbuffered media at a fixed pH is in conformity with the rate law:

Influence of Transport over a Mountain Ridge on the Chemical Composition of Marine Aerosols during the ACE-2 Hillcloud Experiment

Aerosol chemical composition and trace gas measurements were made at twolocations on the northeastern peninsula of Tenerife during the ACE-2HILLCLOUD experiment, between 28 June and 23 July 1997. Measurementswere made of coarse (#gt;2.5 m aerodynamic diameter) and fine (#lt; 2.5m) aerosol Cl^–, NO₃ ^–,SO₄ ^2–, non-sea saltSO₄ ^2– (NSSS),CH₃SO₃ ^– (MSA) andNH₄ ⁺, and gas phase dimethylsulphide (DMS), HCl,HNO₃, SO₂, CH₃COOH, HCOOH andNH₃. Size distributions were measured using a cascadeimpactor. Results show that in marine air masses NSSS and MSA wereformed via DMS oxidation, with additional NSSS present in air massescontaining a continental component. Using a Eulerian box model approachfor aerosols transported between upwind and downwind sites, a mean NSSSproduction rate of 4.36 × 10^–4 gm^–3 s^–1 was calculated for daytimeclear sky periods (highest insolation), with values for cloudy periodsduring daytime and nighttime of 3.55 × 10^–4 and2.40 × 10^–4 g m^–3s^–1, respectively. The corresponding rates for MSA were6.23 × 10^–6, 8.49 × 10^–6and 6.95 × 10^–6 g m^–3s^–1, respectively. Molar concentration ratios forMSA/NSSS were 8.7% (1.8–18.2%) and 1.9%(1.3–3.5%) in clean and polluted air masses, respectively.Reactions occurring within clouds appeared to have a greater influenceon rates of MSA production, than of NSSS, while conversely daytime gasphase reactions were more important for NSSS. For MSA, nighttimein-cloud oxidation rates exceeded rates of daytime gas phase productionvia OH oxidation of DMS. NSSS, MSA and ammonium had trimodal sizedistributions, with modes at 0.3, 4.0 and >10.0 m (NSSS andNH₄ ⁺), and 0.3, 1.5 and 4.0 m (MSA). Nosignificant production of other aerosol species was observed, with theexception of ammonium, which was formed at variable rates dependent onneutralisation of the aerosol with ammonia released from spatiallynon-uniform surface sources. Seasalt components were mainly present incoarse particles, although sub-micrometre chloride was also measured.Losses by deposition exceeded calculated expectations for all species,and were highest for the seasalt fraction and nitrate.     

The Influence of Oxalate on Fe-Catalyzed S(IV) Oxidation by Oxygen in Aqueous Solution

This study demonstrates that oxalate has a strong inhibiting effect onFe-catalyzed S(IV) oxidation by oxygen in aqueous solution. While thepseudo-first order rate constant of S(IV) oxidation was determined to be1.6 × 10³ M^-1 s^-1 in experimentswithout oxalate, the oxidation of S(IV) was totally inhibited at a molarconcentration ratio of iron:oxalate = 1:5 at an oxalate concentration of 4M. Under these conditions, the Fe(II)/Fe(III) ratio remained nearlyconstant during the observed reaction time. The determined rate constants wereindependent of the initial oxidation state of iron. However, with increasingconcentrations of oxalate, a longer induction period is observed forexperiments with iron initially in the Fe(II) oxidation state.     

A discharge flow mass-spectrometric study of the reaction between the NO3 radical and isoprene

The kinetics and mechanism of the reactionNO₃+CH₂=C(CH₃)–CH=CH₂productswere studied in two laboratories at 298 K in the pressure range 0.7–3 torr using the discharge-flow mass-spectrometric method. The rate constant obtained under pseudo-first-order conditions with excess of either NO₃ or isoprene was: k ₁=(7.8±0.6)×10^–13 cm³ molecule^–1 s^–1. The product analysis indicated that the primary addition of NO₃ occurred on both -bonds of the isprene molecule.     

Mixed-layer heat advection and entrainment during the sea breeze

A model is developed to simulate the potential temperature and the height of the mixed layer under advection conditions. It includes analytic expressions for the effects of mixed-layer conditions upwind of the interface between two different surfaces on the development of the mixed layer downwind from the interface. Model performance is evaluated against tethersonde data obtained on two summer days during sea breeze flow in Vancouver, Canada. It is found that the mixed-layer height and temperature over the ocean has a small but noticeable effect on the development of the mixed layer observed 10 km inland from the coast. For these two clear days, the subsidence velocity at the inversion base capping the mixed layer is estimated to be about 30 mm s^–1 from late morning to late afternoon. When the effects of subsidence are included in the model, the mixed-layer height is considerably underpredicted, while the prediction for the mean potential temperature in the mixed layer is considerably improved. Good predictions for both height and temperature can be obtained when values for the heat entrainment ratio,c, 0.44 and 0.68 for these two days respectively for the period from 1000 to 1300 LAT, were used. These values are estimated using an equation including the additional effects on heat entrainment due to the mechanical mixing caused by wind shear at the top of the mixed layer and surface friction. The contribution of wind shear to entrainment was equal to, or greater than, that from buoyant convection resulting from the surface heat flux. Strong wind shear occurred near the top of the mixed layer between the lower level inland flow and the return flow aloft in the sea breeze circulation.Symbols c entrainment parameter for sensible heat - c _p specific heat of air at constant pressure, 1010 J kg^–1 K^–1 - d ₁ the thickness of velocity shear at the mixed-layer top, m - Q _H surface sensible heat flux, W m^–2 - u _m mean mixed-layer wind speed, m s^–1 - u _* friction velocity at the surface, m s^–1 - w subsidence velocity, m s^–1 - W subsidence warming,^oC s^–1 - w _e entrainment velocity, m s^–1 - w _* convection velocity in the mixed layer, m s^–1 - x downwind horizontal distance from the water-land interface, m - y dummy variable forx, m - Z height above the surface, m - Z _i height of capping inversion, m - Z _m mixed-layer depth, i.e.,Z _i–Z_s, m - Z _s height of the surface layer, m - lapse rate of potential temperature aboveZ _i, K m^–1 - potential temperature step atZ _i, K - u _h velocity step change at the mixed-layer top - _m mean mixed-layer potential temperature, K     

Trace gas measurements at the monitoring station cape point,South Africa,between 1978 and 1988

Since 1978, a measuring station has been operated at Cape Point (34°21 S, 18°29 E). In this article, results of measurements of CO, CFCl₃, CCl₄, O₃, N₂O and CH₄ are presented as monthly means and analyzed with respect to long-term trends and seasonal variations. For CO and CH₄, very similar seasonal variations have been observed, indicating strong interrelations between these two gases. For CO and O₃, no significant changes of the mean annual concentrations can be established for the observation periods of 10 and 5 years, respectively. The measurements yield a growth rate of 9.1 pptv yr^-1 for CFCl₃ (1980–1987) and 0.6 ppbv yr^-1 for N₂O (1983–1987). The concentration increases of CH₄ (10.3 ppbv yr^-1 for 1983–1987) and of CCl₄ (2.1 pptv yr^-1 for 1980–1988) are analyzed for temporal changes during the last years.Presented at the Second Conference on Baseline Observations in Atmospheric Chemistry (SABOAC II) in Melbourne, Australia, November 1988.     

Autoxidation of N(III), S(IV), and other Species in Frozen Solution – A Possible Pathway for Enhanced Chemical Transformation in Freezing Systems

The chemistry of glycolaldehyde (hydroxyacetaldehyde) relevant to the troposphere has been investigated using UV absorption spectrometry and FTIR absorption spectrometry in an environmental chamber. Quantitative UV absorption spectra have been obtained for the first time. The UV spectrum peaks at 277 nm with a maximum cross section of (5.5± 0.7)×10^–20 cm² molecule^–1. Studies of the ultraviolet photolysis of glycolaldehyde ( = 285 ± 25 nm) indicated that the overall quantum yield is > 0.5 in one bar of air, with the major products being CH₂OH and HCO radicals. Rate coefficients for the reactions of Cl atoms and OH radicals with glycolaldehyde have been determined to be (7.6± 1.5)×10^–11 and (1.1± 0.3)×10^–11 cm³ molecule^–1 s^–1, respectively, in good agreement with the only previous study. The lifetime of glycolaldehyde in the atmosphere is about 1.0 day for reaction with OH, and > 2.5 days for photolysis, although both wet and dry deposition should also be considered in future modeling studies.     

Size-Resolved Characterisation of Soluble Ions in the Particles in the Tropospheric Plume of Masaya Volcano, Nicaragua: Origins and Plume Processing

We present the first application of a multi-stage impactor to study volcanic particle emissions to the troposphere from Masaya volcano, Nicaragua. Concentrations of soluble SO₄ ^2–,Cl^–, F^–, NO₃ ^–, K⁺, Na⁺,NH₄ ⁺, Ca²⁺ and Mg²⁺ were determined in 11 size bins from 0.07 m to >25.5 m. The near-source size distributions showed major modes at 0.5m (SO₄ ^2–, H⁺,NH₄ ⁺); 0.2 m and 5.0 m (Cl^–) and 2.0–5.0 m(F^–). K⁺ and Na⁺ mirrored the SO₄ ^2– size-resolvedconcentrations closely, suggesting that these were transported primarily asK₂SO₄ and Na₂SO₄ in acidic solution, while Mg²⁺ andCa²⁺ presented modes in both <1 m and >1 m particles. Changes in relative humidity were studied by comparing daytime (transparent plume) and night-time (condensed plume) results. Enhanced particle growth rates were observed in the night-time plume as well as preferential scavenging of soluble gases, such as HCl, by condensed water. Neutralisation of the acidic aerosol by background ammonia was observed at the crater rim and to a greater extent approximately 15 km downwind of the active crater. We report measurements of re-suspended near-source volcanic dust, which may form a component of the plume downwind. Elevated levels ofSO₄ ^2–, Cl^–, F^–,H⁺, Na⁺, K⁺ and Mg²⁺ were observed around the 10 m particle diameter in this dust. The volcanic SO₄ ^2– flux leaving the craterwas 0.07 kg s^–1.     

Vertical distribution of dimethylsulfide,sulfur dioxide,aerosol ions,and radon over the Northeast Pacific Ocean

Dimethylsulfide (DMS), sulfur dioxide (SO₂), methanesulfonate (MSA), nonsea-salt sulfate (nss-SO₄ ^2–), sodium (Na⁺), ammonium (NH₄ ⁺), and nitrate (NO₃ ^–) were determined in samples collected by aircraft over the open ocean in postfrontal maritime air masses off the northwest coast of the United States (3–12 May 1985). Measurements of radon daughter concentrations and isentropic trajectory calculations suggested that these air masses had been over the Pacific for 4–8 days since leaving the Asian continent. The DMS and MSA profiles showed very similar structures, with typical concentrations of 0.3–1.2 and 0.25–0.31 nmol m^–3 (STP) respectively in the mixed layer, decreasing to 0.01–0.12 and 0.03–0.13 nmol m^–3 (STP) at 3.6 km. These low atmospheric DMS concentrations are consistent with low levels of DMS measured in the surface waters of the northeastern Pacific during the study period.The atmospheric SO₂ concentrations always increased with altitude from <0.16–0.25 to 0.44–1.31 nmol m^–3 (STP). The nonsea-salt sulfate (ns-SO₄ ^2–) concentrations decreased with altitude in the boundary layer and increased again in the free troposphere. These data suggest that, at least under the conditions prevailing during our flights, the production of SO₂ and nss-SO₄ ^2– from DMS oxidation was significant only within the boundary layer and that transport from Asia dominated the sulfur cycle in the free troposphere. The existence of a sea-salt inversion layer was reflected in the profiles of those aerosol components, e.g., Na⁺ and NO₃ ^–, which were predominantly present as coarse particles. Our results show that long-range transport at mid-tropospheric levels plays an important role in determining the chemical composition of the atmosphere even in apparently remote northern hemispheric regions.     

Energy Fluxes of an Open Water Area in a mid-Latitude Prairie Wetland

Concurrent measurements of the surface energy balance components (net radiation, heat storage, and sensible and latent heat fluxes) were made in three communities (open water, Phragmites australis, Scirpus acutus) in a wetland in north-central Nebraska, U.S.A., during May-October, 1994. The Bowen ratio – energy balance method was used to calculate latent and sensible heat fluxes. This paper presents results from the open water area. The heat stored in water (G) was found to play a major role in the energy exchange over the water surface. During daytime, G consumed 45–60% of R _n , the net radiation (seasonally averaged daytime G was about 127 W m^–2). At night, G was a significant source of energy (seasonally averaged nighttime G was about -135 Wm^–). The diurnal pattern of latent heat flux ( E) did not follow that of R _n . On some days, E was near zero during midday periods with large R _n . The diurnal variability in E seemed to be significantly affected by temperature inversions formed over the cool water surface. The daily evaporation rate (E) ranged from 2 to 8 mm during the measurement period, and was generally between 70 and 135% of the equilibrium rate.     

A laboratory study of the mechanism of the oxygen airglow

Further laboratory studies of emission by O(¹ S) and by O₂ A ³ _u ⁺ ,A³ _u andc ¹ _u ^– in the oxygen afterglow lead to the conclusion that Barth's mechanism for the excitation of the auroral green line O ₂ ^* +O(³P=O₂+O(¹S)–(1) is correct and that levelsv=6 and 7 of O₂ A ³ _u ⁺ are Barth precursors. The value ofk ₁=7×10^–11 cm³ s^–1 deduced for these levels is shown to be in fair agreement with atmospheric measurements.