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The dependences on ionic strength of the hydrolysis constants of Fe3+ and of the first dissociation constant of sulfurous acid are briefly reviewed. The data are needed to derive from apparent stability constants reported in the literature the stability constants for the three iron–sulfito complexes defined by the equilibria (c1) FeOH2+ + HSO3− = FeSO3+ + H2O, (c2) FeSO3+ + HSO3− = Fe(SO3)2− + H+, (c3a) Fe(SO3)2− + HSO3− = Fe(SO3)3H2−, where Kc1 = 1982 ± 518 dm3 mol−1, Kc2 = 0.72 ± 0.08, Kc3a = 189 ± 9 dm3 mol−1 (ionic strength μ = 0.1 mol dm−3). The rapid formation of these complexes is followed by a slower decomposition leading to the formation of SO3− radicals; the associated rate coefficients are k1 = 0.19 s−1, k1a ≈ 0.04 s−1, and k1b ≈ 0.08 s−1, respectively. The subsequent reaction leads to dithionate and sulfate as products. Overall rates and product yields from a variety of studies of the slow reaction are found to be consistent with a mechanism, in which the production of dithionate occurs mainly by the reaction of SO3− with FeSO3+ and that of sulfate by the reaction of SO3− with FeOH2+ and/or Fe3+. The role of copper as a catalyst is also analyzed. Rate coefficients for individual reactions are estimated from the data at low pH (μ = 1.0 mol dm−3) under conditions where the 1 : 1-complex is prevalent. They are extrapolated to lower ionic strengths for an analysis of the data obtained at higher pH to explore conditions when reactions of the higher complexes become important. The overall rate and the product yields of the reaction depend critically on the pH, the initial ratio of S(IV) to Fe(III) and the ionic strength of the solution.

The experimental data available for the photodissociation quantum yields of acetaldehyde are evaluated to derive formal expressions for their dependence on wavelength and pressure. The formulae are used to calculate the photodissociation coefficient of acetaldehyde as a function of altitude in the troposphere. The processes active in the photodecomposition of acetaldehyde are comparable to those in acetone, but the photodissociation coefficients of acetaldehyde (at room temperature) are six times greater. The dependence of the quantum yields on wavelength and pressure supports recent evidence of participation of both singlet and triplet states in the photodissociation process.

The quantum yields for CO and CH(4), formed in the photolysis of about 200 μmol mol(-1) of acetaldehyde in air at atmospheric pressure, were determined at 15 wavelengths in the range: 250-330 nm. The quantum yields for CO(2) were determined at nine wavelengths in the range: 250-315 nm. The products are mainly assigned to three primary processes: I) CH(3)CHO*→CH(3)+HCO, II) CH(3)CHO*→CH(4)+CO, III) CH(3)CHO*→CH(3)CO+H, with dissociation occurring from the initially populated S(1) singlet state of acetaldehyde. The pressure dependence of the quantum yields at wavelengths of 270, 304.4, and 313 nm exhibits a Stern-Volmer behavior in all cases. Collisional quenching is shown to deactivate the singlet state S(1) by pressure-induced intersystem crossing to the neighboring triplet state, which is subsequently rapidly quenched by oxygen. Quenching coefficients as a function of excitation energy are obtained by comparison with literature data; the wavelength dependence of individual primary quantum yields is also derived. CO(2) quantum yields at 304.4 and 313 nm were found to increase with rising CH(3)CHO concentrations due to secondary reactions. The reaction mechanism responsible for this effect was explored by means of computer simulations.

Measurements reported in the literature of gas–liquid partition coefficients for methanol and ethanol dissolved in water are compiled and critically evaluated to establish the temperature dependence. The data are linearly correlated in the ln( K H [mol dm −3 atm −1 ]) versus reciprocal absolute temperature coordinate frame and, when treated by a linear regression analysis, yield: ln( K H )=−(12.46±0.25)+(5312.4±76.0)/ T in the case of methanol (0–80 °C) and ln( K H )=−(15.87±0.82)+(6274.0±241.6)/ T in the case of ethanol (0–60 °C). The measurements at 25 °C average to K H (298.15)=(2.16±0.14)×10 2 mol dm −3 atm −1 ( n =8) and K H (298.15)=(1.94±0.13)×10 2 mol dm −3 atm −1 ( n =8), respectively. Enthalpies of solution derived from the temperature dependence are 44.17±0.63 kJ mol −1 for methanol and 52.16±2.01 kJ mol −1 for ethanol.

Hydroxyl radicals, generated by photolysis of H2O2, were reacted with n-pentane and isopentane in air in the absence of nitrogen oxides. The observed product distributions were compared with similar data derived by computer simulations, based on the known reaction mechanisms, to determine relative probabilities for hydrogen abstraction at different sites of the parent compounds and to estimate branching ratios and relative rate coefficients for cross-combination reactions between different peroxy radicals. For n-pentane, the distribution of the pentanols indicates probabilities for hydrogen abstraction, in percent, of q1 = 9.1 ± 0.7, q2 = 56.1 ± 1.8, and q3 = 34.8 ± 1.3, which agree with predictions based on the algorithm proposed by Atkinson. Branching ratios needed to harmonize calculated and observed product distributions are somewhat larger than, although still within the error ranges of, the values found by us previously. Comparison between experimental and calculated data confirms the isomerization and decomposition constants recently established for the three pentoxyl radical isomers. The product distribution for isopentane, which is dominated by acetone, acetaldehyde, 2-methyl-butan-2-ol, and 2-methyl-butan-2-hydroperoxide, is in harmony with the predicted oxidation mechanism. Probabilities for hydrogen abstraction from isopentane were estimated to occur to 12% at the primary, 28% at the secondary, and 60% at the tertiary sites, again in agreement with predictions based on the algorithm of Atkinson. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 677–688, 2006

A box model is used to explore the detailed chemistry of C2 and C3 organic compounds in the marine troposphere by tracing the individual reaction paths resulting from the oxidation of ethane, ethene, acetylene, propane, propene and acetic acid. The mechanisms include chemical reactions in the gas phase and in the aqueous phase of clouds and aerosol particles at cloud level under conditions resembling those in the northern hemisphere. Organic hydroperoxides are found to be important intermediate products, with subsequent reactions leading partly to the formation of mixed hydroxy or carbonyl hydroperoxides that are readily absorbed into cloud water, where they contribute significantly to the formation of multifunctional organic compounds and organic acids. Organic hydroperoxides add little to the oxidation of sulfur dioxide dissolved in the aqueous phase, which is dominated by H2O2. Next to acetaldehyde and acetone, glycol aldehyde, glyoxal, methyl glyoxal and hydroxy propanone are prominent oxidation products in the gas and the aqueous phase. Acetaldehyde is not efficiently converted to acetic acid in clouds; the major local sources of acetic acid are gas-phase reactions. Other acids produced include hydroperoxy acetic, glycolic, glyoxylic, oxalic, pyruvic, and lactic acid. The mechanism of Schuchmann et al. (1985), which derives glycolic and glyoxylic acid from the oxidation of acetate, is found unimportant in the marine atmosphere. The principal precursors of glyoxylic acid are glyoxal and glycolic acid. The former derives mainly from acetylene and ethene, the latter from glycolaldehyde, also an oxidation product of ethene. The oxidation of glyoxylic acid leads to oxalic acid, which accumulates and is predicted to reach steady state concentrations in the range 30–90 ng m−3. This is greater, yet of the same magnitude, than the concentrations observed over the remote Pacific Ocean.

Mixtures in air of chlorine, acetone, nitrogen dioxide, and nitric oxide (partly) were photolysed at 330 nm wavelength to produce acetonyl peroxy radicals and to determine the fraction of acetonoxy radicals formed in one of the two branches of the self-reaction of acetonyl peroxy, 2CH3COCH2OO· → CH3COCHO + CH3COCH2OH + O2 (3a) and 2CH3COCH2OO· → 2CH3COCH3O· + O2 (3b). In these experiments the decomposition of acetonoxy gives rise to acetyl peroxy radicals, which react with NO2 to form peroxy acetyl nitrate (PAN). The quantum yield of PAN was measured as a function of time. Computer simulations were used to explore the effect of acetonyl peroxy nitrate as an unstable intermediate formed in the reaction CH3COCH2OO· +NO2 ⇄ CH3COCH2OONO2 (9). The experimental data were evaluated to derive for the rate coefficients associated with reaction (3) the branching ratio k3b/(k3a + k3b) = 0.50 ± 0.05 and for the reverse path of reaction (9) the rate coefficient k−9 = 10.0 ± 3 s−1.

Product distributions from the 254-nm photooxidation of the three iodopentane isomers were explored as a technique for studying the self-reactions of individual pentyl peroxyl radicals (in air at ambient temperature and pressure). Pentanols and the associated carbonyl compounds (pentanal or pentanones) were major products as expected. Other major products resulted from the isomerization of pentan-1-oxyl and pentan-2-oxyl radicals, but their nature could not be identified. Minor products were alcohols and carbonyl compounds arising from the decomposition of pentoxyl radicals. Diols and mixed hydroxycarbonyl compounds from cross-combination reactions were essentially absent, in contrast to expectation. The observed product distributions were evaluated to derive branching ratios for the radical-preserving pathways of the self-reactions, 0.42 ±0.17, 0.46 ± 0.10, 0.39 ± 0.08, for pentan-1-yl peroxyl, pentan-2-yl peroxyl, and pentan-3-yl peroxyl, respectively. Rate coefficients derived for the decomposition of the corresponding pentoxyl radicals, relative to their reaction with oxygen, are (5.1 ± 0.5) × 1018, (1.0 ± 0.2) × 1018, and (3.2 ± 0.3) × 1018 molecule cm−3, respectively. Rate constants for the isomerization of pentan-1-oxyl and pentan-2-oxyl were estimated from the contributions of isomerization products to the total amounts of products as (4.0 ± 1.1) × 105 s−1 and (1.0 ± 2.0) × 105 s−1, respectively. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 126–138, 2002

Product distributions resulting from the OH-induced oxidation of but-1-ene, 2-methylbut-1-ene, 3-methylbut-1-ene and isoprene in air were measured in the absence of nitrogen oxides and compared with predictions based on currently accepted oxidation mechanisms. In the case of butenes, the observed distributions of carbonyl compounds, hydroxyketones, hydroxyalkanals and diols were evaluated to obtain probabilities for the initial attack of OH radical on the outer position of the double bond (γ = 0.90 ± 0.03 for 2-Me-but-1-ene and γ = 0.76 ± 0.05 for both but-1-ene and 3-Me-but-1-ene), for the probability of formation of stable products in the self-reaction of secondary β-hydroxyperoxyl radicals (kssb/kss = 0.29 ± 0.07 for but-1-ene and kssb/kss = 0.19 ± 0.06 for 3-Me-but-1-ene), and for the ratio of the reaction with oxygen s. decomposition of β-hydroxyalkoxyl radicals, k3[O2]/(k4 + k3[O2]) = 0.25 ± 0.04 for but-1-ene and = 0.38 ± 0.04 for 3-Me-but-1-ene. The last two values disagree with other published data, which suggest a smaller effect of oxygen. The oxidation of isoprene produced methacrolein and methyl vinyl ketone with a ratio 0.93 ± 0.10, the ratio of methyl vinyl ketone and 3-methylfuran was 7.3 ± 1.0. Other products were 1-hydroxy-3-methylbut-3-en-2-one (identified by mass spectrometry) and 3-methyl-3-oxo-butane (tentatively identified). The overall product distribution was complex and could not be fully elucidated. Computer simulations based on several mechanisms applied the relative probabilities for OH addition found for the but-1-enes. Comparison with the experimental data suggests probabilities for OH addition to the methylated double bond of 0.504 ± 0.027 (outer position) and 0.056 ± 0.003 (inner position), and to the non-methylated double bond of 0.335 ± 0.023 (outer position) and 0.105 ± 0.008 (inner position).

A simple box model of a sunlit small cumulus cloud has been used to explore the efficiency of various chemical reactions contributing to the oxidation of sulfur dioxide and nitrogen dioxide in clouds. The principal aqueous-phase processes of sulfur(IV) oxidation are reactions with ozone, with hydroperoxides, with OH radicals, and catalytic reactions involving transition metals. The last two oxidants initiate chain oxidation processes, which were analyzed in detail. The results indicate that chain reactions are not very effective, partly because the chain carriers are scavenged, and partly because chain termination overrides chain propagation. Hydrogen peroxide is the most effective oxidant in S(IV) oxidation, contributing about 80% to the total rate. Peroxynitric acid also contributes appreciably, in addition to ozone. The oxidation of nitrogen dioxide to nitric acid occurs to 60% in the gas phase by reaction with OH radicals. In the aqueous phase, the reaction of peroxynitric acid with hydrogensulfite is most important, contributing 20–30% to the total rate.

A zinc arc lamp and a mercury lamp, respectively, were used to study the photodecomposition of HSO3- and SO32- in aqueous solutions saturated with either argon or nitrous oxide. The main products i...

Quantum yields of phenol and nitrate, produced by photodecomposition in aqueous solutions of NO2- and HNO2 in the presence of benzene as scavenger for OH radicals, have been determined as a function of wavelength between 280 and 390 nm. The production of phenol was used to calculate primary OH quantum yields. For NO2- photolysis at pH 6 Φ1(OH) was found to decrease with increasing wavelength from 0.069 ± 0.008 at 280 nm to 0.022 ± 0.004 at 390 nm, in agreement with previous data. The OH quantum yield Φ2(OH) for the photolysis of HNO2 at pH 2 was essentially constant over the entire wavelength range with Φ2 = 0.35 ± 0.02 (2σ). Quantum yields for NO3- are comparable in magnitude to those of phenol, indicating that NO as primary product is largely oxidized to nitrate. The most likely conversion processes are reactions of NO with O2- (pH 6), the latter resulting from the oxidation of benzene, to form peroxynitrous acid, which undergoes thermal decomposition, and of NO2 with HO2 (pH 2) to form peroxynitric aci...

Nitric acid and particulate nitrate in addition to other trace species were measured in the air at Deuselbach, a rural site in western Germany, in June and July 1985 under background atmospheric conditions. High-volume open face triple filter packs and cascade impactors were used together with ion-chromatographic analyses. Laboratory tests showed good correspondence between gaseous nitric acid and nitrate deposited on nylon back-up filters for low ambient aerosol concentrations as observed in the field. High aerosol loadings typically found in Mainz caused part of nitric acid to be retained together with particulate nitrate on the teflon front filter. The concentration of nitric acid observed in the field went through a maximum during the day and a minimum at night with a clear anti-correlation with relative humidity. For r.h. ⩽ 60% the average fraction of gaseous to total nitrate was 39 ± 8%. The average fraction from all data was 22%. The molar fraction of total nitrate to nitrogen dioxide was 24%. It is shown that the diurnal variation of HNO3 is partly due to absorption by liquid water associated with the aerosol, which increases with rising relative humidity (at night). The absorption is significant only because solution pH is buffered by the presence of sulfate and the formation of bisulfate. Most of the field data showed particulate nitrate to occur primarily in the coarse size range ( ⩾ 2 μm diameter) with sodium providing the main cation. Sea salt was identified as the principal source of sodium. Ammonium nitrate occurred only sporadically in the fine particle mode ( ⩽ 2 pm diameter). Ammonium nitrate was largely absent because the product of the concentrations of nitric acid (observed) and ammonia (inferred) was below the minimum required for equilibrium concentrations of particulate NH4NO3 to form. In addition, there often was insufficient ammonium (and other measurable cations) present in fine particles to balance the amount of sulfate.

Vapor phase concentrations of acetone, acetaldehyde and acetonitrile over their aqueous solutions were measured to determine Henry's law partition coefficients for these compounds in the temperature range 5–40 °C. The results are for acetone: ln(H 1/atm)=−(5286±100)T+(18.4±0.3); acetaldehyde: ln(H 1/atm)=−(5671±22)/T+(20.4±0.1); and acetonitrile: ln(H 1/atm)=−(4106±101)/T+(13.8±0.3). Artificial seawater of 3.5% salinity in place of deiionized water raisesH 1 by about 15%. A similar technique has been used to measure the equilibrium constants for the addition compounds of acetone and acetaldehyde with bisulfite in aqueous solution. The results are ln(K 1/M −1)=(4972±318)/T−(11.2±1.1) and ln(K 1/M −1)=(6240±427)/T−(8.1±1.3), respectively. The results are compared and partly combined with other data in the literature to provide an average representation.

The Fe(III)-catalyzed oxidation of bisulfite in aqueous solution was studied at pH3 in the presence of benzene by monitoring the rise of hydrogen ion activity and phenol product concentration as a function of time, keeping [HSO 3 - ]/[Fe tot ]>>1. After a brief initial period, during which product concentrations rose rapidly and Fe(III) was largely converted toward Fe(II), the reaction underwent a period of slower growth characterized by a quasistationary state and nearly constant [Fe(II)]/[Fe(III)] ratio until toward the end of the reaction, when most of the S(IV) was consumed, Fe(II) was reoxidized toward Fe(III). In two sets of experiments either the concentration of benzene or that of [Fe(III)] o was varied in ranges of 0.3-2.2 mmol dm -3 and 8.5-51 μmol dm -3 , respectively, while keeping the other parameters constant ([HSO 3 - ] o =2 mmol dm -3 ). The data were analyzed by means of a steady state expression developed for the time period of nearly constant [Fe(II)]/[Fe(III)] ratio on the basis of a radical-driven chain oxidation mechanism presented previously. The data display the behavior predicted by the steady state equation; this allowed the determination of ratios of rate coefficients for reaction pairs: SO 4 - reacting with HSO 3 and benzene, k 11 /k 12 =0.31±0.04; SO 5 - reacting with Fe 2+ and HSO 3 - , k 3 /k 10 =54.1±1.7; HSO 5 - reacting with Fe 2+ and HSO 3 , k 4 /k 16 =2.2±0.2; and the branching ratio for the reaction of SO 5 - with HSO 3 - leading to (a) SO 3 - and (b) SO 4 - radicals, k 10b /k 10 ≤0.04. The experimental results largely support the proposed reaction mechanism

Benzene and tertiary butanol have been used as radical scavengers to demonstrate the occurrence of sulphate radicals in the iron ion-catalysed autoxidation of bisulphite in aqueous solution. Ferric ions are rapidly reduced to ferrous ions by HSO 3 − and a steady state is established between both iron species for some time. A detailed reaction mechanism is presented. Computer simulations show that it can account for the essential features of the reaction.

The description of atmospheric gas-phase chemistry generally requires a consideration of large sets of elementary chemical reactions. The reactions are elementary in the sense that they cannot or need not be further broken up into sub-processes. Elementary reactions are classified according to the number of atoms or molecules participating in reactive molecular encounters: as bimolecular, when new products arise from the collision of two reactants, and as termolecular when the reaction requires three reactants. In particular, the association of two reactants requires a third collision partner to remove excess energy in order to stabilize the newly formed molecule. In the atmosphere the third partner is primarily nitrogen or oxygen.

Comments: Densities for super-cooled water in the temperature range 0 >T> −34°C are smoothed data from Hare and Sorensen (1987) who fitted their measurements by a sixth order polynomial: $$ \begin{array}{c}{r}_{\rm{ice}}\left[\rm{g}\rm{\rm{cm}}^{-3}\right]=0.99986+6.69\times {10}^{-5}T+8.486\times {10}^{-6}{T}^{2}+1.518\times {10}^{-7}{T}^{3}\\ -6.9484\times {10}^{-9}{T}^{4}-3.6449\times {10}^{-10}{T}^{5}-7.497\times {10}^{-12}{T}^{6}\left(T\rm{in °C}\right)\end{array}$$

The solid earth consists of the core, the mantel and the crust. Seismic data show that the (iron-rich) core is divided into a solid inner core, which is denser than iron and is probably an Fe-Ni alloy, and a molten outer core, which is slightly less dense than iron. The silicate mantle is also subdivided into a lower and an upper mantle by seismic discontinuities occurring at depths between 400 and 670 km. The crust is the uppermost layer of the earth. The continental crust (granitic) is enriched in incompatible elements and contains most of the earth alkaline elements as well as a large fraction of uranium and other radioactive elements. Most of the continental crust is 20–50 km thick, with an average thickness of 35–40 km. The oldest known rocks are nearly 4 × 109 years old. The (basaltic) oceanic crust is about 5–10 km thick. It is fairly young with an average age of 60 million years. The oceanic crust is less enriched with incompatible elements than the continental crust. The upper 100–200 km of the earth is made up of about 12 plates that float on the upper mantle. The thickness of oceanic plates is about 60 km, and that of continental plates 100–200 km. The mutual interaction of plates gives rise to divergence zones, such as the mid-ocean ridges where they spread apart, and convergence zones, where the higher density oceanic plates are subducted under the lower density continental plates and mountain ranges are formed. These zones are active regions of volcanism.

Chemical reactions in the atmosphere result largely from the absorption of solar radiation in the visible and ultraviolet spectral regions provided the incoming photons (light quanta) carry sufficient energy for chemical processes to be initiated. Photochemical reactions always occur in two steps: the first is the excitation of a molecule by the absorption of radiation; the second is a reaction of the excited molecule. In atmospheric chemistry, the most important process to be considered is dissociation of the excited molecule. Other modes of energy dissipation, such as fluorescence or collisional energy transfer, usually do not lead to chemical changes so that they are of lesser interest. Any quantitative assessment of the photochemical activity of an atmospheric constituent requires knowledge of (a) the photon flux of solar radiation, (b) the absorption cross section of the species under consideration, and (c) the primary quantum yield of the photo-dissociation process. These parameters depend on the wavelength of radiation; absorption cross sections and quantum yields may additionally depend on temperature and/or pressure.

Atmospheric pressure and density decrease quasi-exponentially with increasing altitude in accordance with the barometric law. Figure 3.1 presents an idealized vertical temperature profile of the atmosphere and the nomenclature adopted by international convention to identify different altitude regimes, which are distinguished by the prevailing sign of the temperature gradient.

Conditions in the outermost region of the atmosphere differ radically from those in the lower atmosphere. The principal characteristic features are the following

The term aerosol refers to a suspension of fine particles in air. The principal sources of the aerosol in the troposphere are: (a) surface agitation by wind force, which generates soil dust, sea salt particles, and biogenic material; (b) high temperature processes including volcanic eruptions, forest fires and anthropogenic combustion processes; (c) chemical reactions leading to compounds with low vapor pressures such as the oxidation of SO2 to sulfuric acid, NO2 to nitric acid, or glyoxal to oxalic acid, and the neutralization of the acids by ammonia or alkaline minerals to form water-soluble salts. Particles in the size range above ∼0.1 μm act as cloud condensation nuclei (CCN). While most clouds evaporate again so that the aerosol particles are recycled, precipitation transfers the material to the earth surface. The particles are incorporated in rain water during the formation of precipitating clouds (nucleation scavenging) and by attachment to rain drops (below-cloud scavenging). Dry deposition also contributes to the removal of particles from the atmospheric boundary layer (primarily in the size range >5 μm). Directly emitted particles usually retain their original chemical character in the vicinity of sources, but during transport in the troposphere all particles undergo chemical modification both by the coagulation of small particles and by the accumulation of material resulting from chemical reactions occurring in the gas phase and in clouds. In the stratosphere the principal source of aerosol particles is the chemical conversion of sulfur compounds to sulfuric acid.

Gas chromatography: This is a widely used technique for the separation and analysis of different species present in a gas sample, based on the principle of selective adsorption. An inert carrier gas, taken from a storage tank, flows through (a) a valve or port for the injection of an aliquot of the sample; (b) a column packed or coated with a suitable sorbent, with which the individual compounds present in the sample interact more or less strongly so that they are separated owing to different retention times; (c) a detector producing an electric signal when a compound elutes from the column and passes through the detector.

The international system of units (systeme international d’unites, SI) has now largely replaced all earlier systems of units used to describe physical quantities. SI is built upon seven base quantities each having its own dimension: length, mass, time, thermodynamic temperature, amount of substance (chemical amount), electric current, and luminous intensity (Bureau International des Poids et Mesures 1991). All other quantities are derived quantities that acquire dimensions derived ­algebraically from the seven base quantities by multiplication and division. The possibility of combining SI units with prefixes designating decimal multiples or submultiples of units provides additional flexibility when dealing with quantities that range over many orders of magnitude, a feature which makes SI especially suitable for use in the atmospheric sciences. Accordingly, SI units are strongly recommended for use in atmospheric chemistry. However, some non-SI units are still required, and they will remain in use along with SI. Examples are time periods such as minute, hour, day and year, which can be expressed in terms of the second, but which defy decimalization. The tables that follow provide an overview on SI, on units that are not part of SI but are used along with it, and on a variety of non-SI units still in use together with the conversion factors to the corresponding SI units. For the correct treatment of symbols and units the reader should consult the manual Quantities, Units and Symbols in Physical Chemistry by Mills et al. (1993).

The product distribution resulting from the OH induced oxidation of 2,3-dimethylbutane in air was measured and compared with predictions based on a general reaction mechanism. Relative rates derived for the abstraction of primary and tertiary hydrogen atoms by OH radicals from the parent compound are 17% and 83%, respectively. The branching ratio for the alcohol versus alkoxyl radical producing pathways of the self-reaction of 2-propylperoxy radicals was determined to be (0.61 {plus_minus} 0.08):(0.39 {plus_minus} 0.08); the corresponding ratio for the self-reaction of primary 2,3-dimethylbutylperoxy radicals is (0.56 {plus_minus} 0.07):(0.44 {plus_minus} 0.07). Large amounts of 2,3-dimethyl-2-hydroperoxybutane and small amounts of 2,3-dimethyl-2-butanol were found, the latter as a product of the cross combination reactions of 2,3-dimethyl-2-butylperoxy with 2-propylperoxy and 2,3-dimethyl-1-butylperoxy radicals. Rate constants of 3.5 x 10{sup {minus}17} and 2 x 10{sup {minus}16} cm{sup 3}/(molecule s), respectively, were estimated for these reactions with the help of computer simulations. 25 refs., 6 figs., 4 tabs.

The product distribution resulting from the OH induced oxidation of 2,3-dimethylbutane in air was measured and compared with predictions based on a general reaction mechanism. Relative rates derived for the abstraction of primary and tertiary hydrogen atoms by OH radicals from the parent compound are 17% and 83%, respectively. The branching ratio for the alcohol versus alkoxyl radical producing pathways of the self-reaction of 2-propylperoxy radicals was determined to be (0.61 {plus_minus} 0.08):(0.39 {plus_minus} 0.08); the corresponding ratio for the self-reaction of primary 2,3-dimethylbutylperoxy radicals is (0.56 {plus_minus} 0.07):(0.44 {plus_minus} 0.07). Large amounts of 2,3-dimethyl-2-hydroperoxybutane and small amounts of 2,3-dimethyl-2-butanol were found, the latter as a product of the cross combination reactions of 2,3-dimethyl-2-butylperoxy with 2-propylperoxy and 2,3-dimethyl-1-butylperoxy radicals. Rate constants of 3.5 x 10{sup {minus}17} and 2 x 10{sup {minus}16} cm{sup 3}/(molecule s), respectively, were estimated for these reactions with the help of computer simulations. 25 refs., 6 figs., 4 tabs.

A survey is given of chemical reactions occurring in the aqueous phase of clouds and fogs. Expanding on a previous discussion of the subject, the following aspects are emphasised: nitrogen oxide chemistry, aqueous photochemistry, and the interaction of Fe(II) and Fe(III) with the ionic species following from the dissolution of sulphur dioxide in water.

Quantum yields for the products CH3CHO, CO2 and CH3COOH in the 350 nm photodecomposition of pyruvic acid are 0.48 ± 0.01, 1.27 ± 0.18 and 0.14, respectively, as measured in air at atmospheric pressure. In the presence of NO2 the quantum yield for CH3CHO was reduced to 0.30 ± 0.04 and peroxyacetyl nitrate was formed with a quantum yield of 0.15 ± 0.02. This is taken to indicate the formation of acetyl radicals in one of the primary processes.

The photodecomposition of acetone in air with NO 2 admixed has been used to generate peroryacetyl nitrate (PAN) either by batch synthesis in an isolated flask or in a continuous-flow reactor. With NO 2 mixing ratios near 10 ppm, the method converts in the first case 88% of NO 2 to PAN and ∼6% to methyl nitrate within a time period of a few minutes. In the second case, the conversion is ∼90% and 10%, respectively. The reproducibility is better than 5%. The PAN mixing ratio produced by batch synthesis declines exponentially with time due to wall-catalyzed thermal decomposition

Aqueous solutions of sodium pyruvate/hydrochloric acid mixtures were studied with regard to changes in the intensity of optical absorption at 317 nm wavelength, caused by varying pH and temperature. The data were evaluated to determine the pyruvic acid apparent dissociation constant at infinite dilution. Its temperature dependence followed the relation ln Kod = −(0.55 ± 0.47) − (1538 ± 135)/T or, if previous results of Pedersen at two temperatures are included, In Kod = −(0.97 ± 0.35) − (1418 ± 102)T. The effective absorption coefficient for pyruvic acid and its anion differ because the former species is largely hydrated, whereas the latter is not. The known hydration constant and its temperature dependence were used to derive the dissociation constant K1b for unhydrated pyruvic acid at infinite dilution. Its pK is 1.88 at 25°C with In K1d = −(8.67 ± 0.35) + (1296 ± 102)/T. The absorption coefficients for unhydrated pyruvic acid and its anion are essentially the same.

In contrast to a fairly extensive knowledge of gas phase chemical reactions in the atmosphere, our current understanding of the chemistry in the aqueous phase of clouds is still inadequate. Particularly for continental clouds the difficulties arise primarily from uncertainties in reaction mechanisms for the oxidation of various sulfur(IV) species originating from the dissolution of sulfur dioxide in cloud water and the role of the ions of transition metals in this oxidation process. The importance of OH and other radicals in gas-phase reactions has led to models, in which radical reactions are held responsible for much of the chemical change also in the liquid phase. This viewpoint has gained support specifically from the identification of iron(III)-hydroxo complexes as a photochemical source of OH radicals in continental clouds. Their reactions with sulfur(IV) compounds in the aqueous phase initiate chain processes, which are currently being examined by laboratory studies.

A gas chromatographic procedure for the determination of acetonitrile (CH{sub 3}CN) in air is described, based on a thermionic nitrogen-selective detector. The lower limit of detection is 15 pg of CH{sub 3}CN for a blank value of 5.0 {plus minus} 3.2 pg. The method involves the collection of air in 2-dm{sup 3} vessels (glass or stainless steel) and preconcentration of acetonitrile by cryogenic trapping. Preconcentration by sorption on modified Chromosorb 102 was found unsuitable due to undesirable memory effects. Air samples stored in containers for periods longer than 3 days generally show a rise in CH{sub 3}CN concentration, so that air samples must be processed within a few days. precautions against contamination in the gas handling system or by intrusion of laboratory air also are important. Calibration tests based on gaseous and liquid dilution series lead to comparable accuracies of about 4% in each case but a better reproducibility for liquid calibration mixtures.

The unimolecular decay of pernitric acid (PNA), HO 2 NO 2 , in aqueous solutions is strongly pH dependent and yields predominantly nitrite above pH=5. This observation is of particular interest for atmospheric processes involving aerosol and cloud water chemistry. The detailed kinetic analysis of the underlying processes shows good agreement with the kinetics of a related phenomenon, the oxygen production from PNA solutions as observed in 1981 by Kenley, Trevor, and Lan. Both sets or data are analyzed on the basis of a common mechanism, and a number of important and hitherto unknown rate constants are evaluated. For reaction 1 HO 2 NO 2 ⇄H + +O 2 NO 2 - , the equilibrium constant is (1-2.5)×10 -5 M