Aspects of Atmospheric Aqueous Chemistry

The oxidation of aqueous sulphur dioxide has been studied for many decades as the importance of its extensive droplet chemistry was realised very early. Although chemical engineering and flash photolysis studies suggested radical chain reactions contributed to the autoxidation of sulphites the details are often neglected and the reaction were treated in a simpler overall manner. At typical atmospheric pH values, 2-6 most of the S(IV) is present as bisulphite anions.

0.5O₂ + HSO₃^- ® H⁺ + SO₄^2-

The oxidation of sulphur(IV) by molecular oxygen is very slow, although in polluted environments there is the potential for the reaction to become catalysed by a range of transition metals, such as iron and manganese. The mechanisms are seen as occurring through an electron transfer such that the metal is reduced in solution, so we might represent it:

M(III)(OH)_n + HSO₃^- ® M(II)(OH)_n-1 + SO₃^- + H₂O

Essentially initiating a radical chain:

SO₃^- + O₂ ® SO₅^-

SO₅^- + SO₃^2- ® SO₄^- + SO₄^2-

SO₄^- + SO₃^2- ® SO₃^- + SO₄^2-

This was a mechanism originally proposed by Bäckström in the 1930’s, but has found much support through mechanistic studies. The metal catalyzed pathways are likely to be more important at relatively modest acidities because the reactions slow down as pH decreases and the reaction generates acidity overall with the conversion of sulphurous to sulphuric acid. Organic materials such as terpenes may decrease the chain length and thus reduce the overall oxidation rate showed that other oxidants, most particularly hydrogen peroxide and ozone are capable of dissolving and oxidising bisulphite to sulphate. The hydrogen peroxide route is a particularly significant one as the reaction is faster in acid solution. This means that it would not slow down as the system became more acidic with the production of sulphuric acid. The oxidation by ozone can readily be represented:

HSO₃^- + O₃ ® H⁺ + SO₄^2- + O₂

Although the exact rearrangement during in this oxidation can be complex. Hydrogen peroxide and organo-peroxides will react:

ROOH + HSO₃^- = ROOSO₂^- + H₂O

ROOSO₂^- ® ROSO₃^-

ROSO₃^- + H₂O ® ROH + SO₄^2-

Alternatively one can imagine OH in atmospheric droplets initiating an opening step and subsequent reaction chain:

OH + HSO₃^- ® SO₃^- + H₂O

This brings us on to the general question of the overall fraction of oxidation taking place through various mechanisms. Warneck has investigated the efficiency of various reactions contributing to the oxidation of sulfur dioxide and nitrogen dioxide in cloud water. Ozone and hydrogen peroxide are the most important oxidants, in the aqueous phase, but the reaction of peroxynitric acid with the bisulphite anion can be make a significant contribution.

HOONO₂ + HSO₃^{- ®} 2H⁺ + NO₃^- + SO₄^2-

In more polluted situations hydrogen peroxide could be overwhelmed by much larger amounts of sulphur dioxide in the air. Under these types of situations oxidation by hydrogen peroxide and ozone will be typically be less important than oxidation by OH, Br₂^-and Cl₂^- .

The nitrogen chemistry of droplets has been less extensively studied than sulphur chemistry. Peroxynitric acid was mentioned in the section above. It is a very soluble product of atmospheric oxidation. It is a moderately strong acid (pKa » 5) and if the pH is low enough so that most of the acid is in the molecular form it is relatively stable and so available to react with S(IV) As seen in the equation above this also represents a source of nitric acid in droplets. However, most nitrate is formed from the oxidation of nitrogen dioxide to nitric acid in the gas phase by reaction with OH radicals. In addition to oxidizing S(IV), peroxynitric acid also contributes appreciably, to nitrite

HOONO₂ = H⁺ + NO₄^-

NO₄^{- ®} NO₂^- + O₂

Although we have not mentioned reactions in anything but the liquid phase it is worth noting observations that nitrite oxidation seems to proceed very quickly in freezing particles.

Cloud and fog water processes are potentially important contributors to secondary organic aerosol formation . Organic vapours dissolve within suspended droplets and participate in aqueous-phase reactions. Typically aldehydes, ketones, alcohols, monocarboxylic acids, and organic peroxides can lead to carboxylic acids (especially poly-functional ones), such as polyols, glyoxal and esters. The organic compounds that dissolve in atmospheric water most likely react via photochemically induced oxidation reactions, which are covered in the section below.

Dimerization might well occur with dissolved compounds such as methacrylic acid that forms from the oxidation of isoprene. However, the low concentrations of organic compounds expected in most atmospheric droplets limits the potential for polymerisation reactions.

In systems with a rich biology there is an opportunity for biochemical reactions. Thus urea might be expected to degrade biologically:

NH₂CO NH₂ + H₂O _® 2NH₃ + CO₂

In leaf wetness enzymes such as the peroxidases are present. These could also catalyze the oxidative cross-linking and polymerization of organic compounds utilising hydrogen peroxide and other organic peroxides that are present in guttation. Legrand showed that at coastal Antarctic sites, bacterial decomposition of uric acid, is a source of ammonium, oxalate, and cations (such as potassium and calcium) in aerosols, in addition to a subsequent large ammonia loss from ornithogenic soils to the atmosphere.

Radical and photochemical reactions

In the last few decades the photochemistry and radical chemistry of atmospheric water droplets has been seen as increasingly important. Photolysis frequencies in the aqueous phase are more rapid than might be expected as the actinic flux inside cloud droplets is on the average more than twice as large as compared to the interstitial air. In addition we can expect different types of photochemistry in aqueous systems compared with that which has become familiar in the gas phase. Spectral shifts can stabilize some molecules, so for example, HCHO is photochemically degraded in the gas phase, but when hydrolysed to CH₂(OH)₂ in droplets it is not sensitive to photolysis. There is also the potential for some reactions to be photosensitized perhaps through the presence of the ferric ion. Despite the fact that the importance of these processes in the liquid phase was pointed out more than twenty years ago the development of detailed mechanisms was at first rather slow and study of these reactions in situ is not easy. The understanding of the balance of reactions in the liquid phase remains limited compared with the gas phase.

The production and loss of hydrogen peroxide, and the related processes with the HO2 and OH radical, lie at the heart of liquid this phase chemistry. Hydrogen peroxide partitions very effectively into the liquid phase and is an important oxidant. Although hydrogen peroxide can be photodissociated in solution to give OH this is less effective than transfer into aqueous solution.

Just as trace gases dissolve in droplets, radical species can also be absorbed into solution. Some of the most notable for droplet chemistry are: OH, HO₂, NO₃ and CH₃O₂. This dissolution process represents the most important source of aqueous HO₂. The dissolved hydroperoxide ion is a moderately strong acid (pKa 4.88) and gives the O₂-.

HO₂ = H⁺ + O₂^-

The hydroperoxide radical can also be produced in solution. This can be by reaction with OH:

H₂O₂ + OH ® HO₂ + H₂O

In addition to transfer into solution from the gas phase, hydrogen peroxide can be produced in solution via a Fe(II)_(aq) mediated photo-production. This has been observed in simulated cloudwater experiments. Potential electron donors for these types of processes, such as oxalate, formate, or acetate commonly found in cloud-water.

When H₂O₂ is abundant in solution nearly all of the OH produced comes from an iron(II)-HOOH photo-Fenton reaction mechanism:

Fe(II) + HOOH ® Fe(III) + OH + OH^-

initiated by photoreduction of Fe(III) to Fe(II) in the presence of HOOH.

The O₂^- from the peroxide system can react rapidly with dissolved ozone to give O₃^-

O₂^- + O₃ ® O₂ + O₃^-

O₃^- ® O₂ + O^-

O^- + H^{+ ®} OH

methyleneglycol (CH₂(OH)₂, hydrated formaldehyde) is an important sink of OH:

CH₂(OH)₂ + OH ® CH(OH)₂ + H₂O

CH(OH)₂ + O₂ ® HCOOH + HO₂

The formic acid produced can react further with OH to oxidize to carbon dioxide. Bicarbonate ions, although ubiquitous in atmospheric solutions react only slowly with OH.

The alkylperoxy radical CH₃O₂ can also dissolve effectively, but it is probably converted to the peroxide

CH₃O₂ + HO₂ ® O₂ + CH₃OOH

and as CH₃OOH is not very soluble it can be readily lost from the droplet.

Dissolving NO₃ can react with chloride or the bisulfite anion which means that the presence of Cl tends to moderate the chemistry of NO₃ through the reversible reactions

NO₃ + Cl^{- ®} NO₃^- + Cl

The concentration may be much modified by the presence of aldehydes which have a high rate of reaction with the nitrate radical, converting them via hydrogen abstraction to the acids. There is the possibility of nitration of disolved phenols by NO₃ or NO₂⁺ to form potentially carcinogenic and soluble nitrophenols.

Saline droplets in the atmosphere, especially where they are concentrated, represent an important source of atomic bromine and chlorine that have a range of gas phase reactions. In addition to the processes with nitrogen and sulfur radicals noted in the section above there is the potential for reactions via OH

OH + Cl^{- ®} HOCl ^-

HOCl ^- + H^{+ ®} Cl + H₂O

A further important sequence is:

O₃^- + H⁺ + Br^{- ®} HOBr + O₂^-

HOBr + Cl^{- ®} BrCl + H₂O

Which represents an important source of BrCl to the gas phase. Alternate drivers for the same types of processes arise from the dissolution of the hypochlorous and hypobromous acids

HOCl_{(g) ®} HOCl_(aq)

HOCl_(aq) + Cl^- + H^{+ ®} Cl₂ + H₂O

Cl₂ + HO_{2 ®} Cl₂^- + H⁺ + O₂

Cl₂^- = Cl^- + Cl

The equilibrium constant is of the order of 1/10⁵, which means that in typical cloud water Cl dominates over Cl₂^-. Reactions with SO₄^- represent a further source:

SO₄^- + Cl^-® Cl + SO₄^2-

Chlorine atom can be lost with the production of OH :

Cl + H₂O ® Cl^- + H⁺ + OH

These radicals undergo interconversion , being equilibrated by chloride, sulfate and hydrogen ions such that no one radical acts as a sink. Thus at high Cl^- concentrations, for example, the radical chemistry of Cl₂. and Cl. will be important while at high sulfate that of SO4.^- will predominate..

There is also an active organic chemistry in sunlit droplets. The aqueous-phase photolysis of biacetyl is important source of organic acids and peroxides to aqueous aerosols, and fog and cloud drops. The half-life of aqueous-phase biacetyl with respect to photolysis is approximately 1.0- 1.6 h (solar zenith angle 36^o). Major products of aqueous biacetyl photolysis are acetic acid, peroxyacetic acid, and hydrogen peroxide, with pyruvic acid and methylhydroperoxide as minor photoproducts. Typical reductants in atmospheric waters are likely to formate, formaldehyde, glyoxal, phenolic compounds and carbohydrates.

Dew chemistry is likely to involve a novel range of organic and nitrogen compounds and typically has a higher pH than rainwater. The photolysis of aqueous-phase nitrous acid and nitrites could play a significant role in initiating oxidation reactions through the photo-production of the OH radical.

Continental aerosol chemistry likely to be a little different to that in marine clouds as several processes likely to consume more OH. Most notably these are:

1. Larger formic acid concentrations in the gas phase which can dissolve and consume OH
2. Transition metals can scavenge HO₂/O₂^- from the system
3. Higher concentrations of SO₂ to reduce H₂O₂

Liquid water in the atmosphere represents an important part of the process of removing trace substances from the atmosphere. The residence time of water in the atmosphere is a matter of days (4-10) and the lifetime of rain drops and dew drops considerably shorter. However the effectiveness of aqueous systems in removing gases from the atmosphere requires these gases to be very soluble in water or undergo rapid reactions.

Chemistry within the droplets may serve to reprocess materials. This is particularly evident where reactions within the droplet produce gases that have a low solubility and become lost from the liquid phase.

The influence of droplet phases on atmospheric chemistry can be more subtle than this. Sequestration can alter the chemistry of the gas phase, for example, through the high solubility of peroxyradicals, which has an impact of ozone concentrations. Lelieveld and Crutzen suggested that the removal of HO₂ from the gas phase into cloud water will prevent it from oxidizing NO to NO₂. This limits the gas phase production of O₃ from photolysis of NO₂ and thus effectively leads to a loss of ozone from the troposphere:

OH + O₃ ® HO₂ + O₂

HO₂ = O₂^- + H⁺

O₂^- + O₃ ® O₂ + O₃^-

O₃^- ® O₂ + O^-

O^- + H^{+ ®} OH

Which sums to give

2O₃ ® 3O₂

The chemistry of the liquid phase has been more difficult to resolve, because although gas phase chemistry is affected by the liquid phase it has been possible to treat the liquid phase simply as a sink as a first approximation. When considering the liquid phase the chemistry has often had to involve multiphase considerations from the outset. Nevertheless the recent decade has seen considerable advance to considering the radical and photochemistry of droplets in the atmosphere.