Table 1 Photochemical reactions of naturally occurring substances

Carbon compounds

Plant material → CO₂

(Oxidative) photochemical decomposition (mineralization)

[124, 125, 166 (CO₂ implied), 167]

Plant material (litter and living foliage) → CO

Photochemical decomposition (mineralization)

[125, 168,169,170,171]

plant material (litter) → CH₄

(Reductive) photochemical decomposition (mineralization/methanification)

[172,173,174]

Plant material (foliage) → CH₄

(Reductive) photochemical mineralization

[171, 173,174,175,176]

Plant material → ethane, ethene, propene, butane, other hydrocarbons

(Reductive) photochemical decomposition

[171, 177]

Plant material → dissolved organic matter

Photochemical decomposition + dissolution

[115]

Plant material → biologically more labile compounds

Photochemical priming (encouraging subsequent biotic decomposition)

[136, 178, 179]

Solid organic matter → CO₂

(Oxidative) photochemical decomposition (mineralization)

Sand

[180]

Soil organic matter → CH₄

(Reductive) photochemical decomposition (mineralization/methanification)

[181]

Sorbed or particulate organic matter → dissolved organic matter

Photochemical dissolution

[115, 182, 183]

Dissolved and colloidal organic matter → amino acids

Photochemical decomposition (depolymerization)

[184]

(Nonspecific) decomposition of dissolved organic matter

Photochemical decomposition

No facilitator

Aqueous and solid iron(III) species

[70, 109, 185,186,187]

Dissolved organic matter → CO

(Oxidative) photochemical decomposition (mineralization)

[188,189,190, – 191]

Dissolved organic matter → CO₂

(Oxidative) photochemical decomposition (mineralization)

No facilitator

TiO₂

[190, 192,193,194]

Dissolved organic matter → CH₄

(Reductive) photochemical decomposition (mineralization/methanification)

[195]

Dissolved organic matter → biologically more labile compounds

Photochemical priming (encouraging subsequent biotic decomposition)

[134, 135, 196]

Humic substances → humic substances with increased carboxylic acid content

photochemical oxidation + acidification

[185]

Dissolved organic matter → organic matter with increased aliphatic content

Photochemical aliphatization

[63, 193]

Humic substances → small carboxylic acids; increased hydrophobicity of remaining organic matter

photochemical decomposition + acidification

[135, 186]

Humic substances → simple carbonyl compounds (e.g., formaldehyde, acetone, pyruvate)

Photochemical decomposition

[189, 197]

Dissolved organic matter → condensed aromatic structures (soluble and particulate)

Photochemical condensation

[193]

Carbohydrates and lipids → oxidized products

Photochemical oxidation

With and without ZnO

[198]

(Nonspecific) decomposition of cellulose

Photochemical decomposition

No facilitator

Organic dyes

Fe(III) compounds, ZnO, ZnS, TiO₂

[14, 50, 96, 97, 199]

Cellulose → less polymerized cellulose with increased carbonyl and carboxyl content

Photochemical depolymerization + oxidation

[96, 200]

(Nonspecific) decomposition of chitosan

Photochemical decomposition

[201]

(Nonspecific) decomposition of wool

Photochemical decomposition

[99]

(Nonspecific) decomposition of lignin

Photochemical decomposition

No facilitator

TiO₂

[98, 202, 203]

Lignin → CH₄, ethane

(Reductive) photochemical decomposition

[204]

Lignin → quinones

(Oxidative) photochemical decomposition

[99, 204, 205]

Lignin → aromatic and aliphatic aldehydes

(Oxidative) photochemical decomposition

[206]

Proteins → larger, aggregated proteins e.g., via intermolecular tyrosine dimerization

Photochemical crosslinking

[207]

Unconjugated unsaturated lipids → conjugated unsaturated lipids + insoluble material

Photochemical isomerization, condensation

Observed in seawater

[208]

Polyunsaturated lipids → humic substances (proposed reaction)

(Oxidative) photochemical crosslinking

[209]

Fatty acids →

CO₂, alkenes, aldehydes, ketones, fatty acid dimers

Photochemical oxidation, cleavage,

dimerization

No facilitator

TiO₂

[210, 211]

Hydrocarbons e.g., ethane, ethene, propane, butane, paraffin → CO₂

Photochemical oxidation

TiO₂

[211, 212]

Long-chain alkanes → ketones, alcohols, acids

Photochemical oxidation

Naphthol, xanthone, anthraquinone

[101]

Dienes + NO_x → carboxylic acids

Photochemical oxidation

[213]

Aromatic compounds + NO_x, NO₂ ⁻, or NO₃ ⁻ → nitrated aromatic compounds

Photochemical nitration

No facilitator

TiO₂, Fe₂O₃

[214,215,216,217,218]

(Nonspecific) decomposition of polycyclic aromatic hydrocarbons

Photochemical decomposition

No facilitator

Algae (live or dead)

TiO₂

[138,139,140, 219]

Polycyclic aromatic hydrocarbons → quinones

Photochemical oxidation

Al₂O₃

[78]

Condensed aromatic compounds (dissolved black carbon) → nonspecific products, CO₂

(Oxidative) photochemical decomposition

[63, 220, 221]

Soot → oxygen-containing species

Photochemical oxidation

[222]

Crude oil → CO₂

Photochemical oxidation (mineralization)

Sand containing magnetite and ilmenite

[223]

Amino acids → CO₂

Photochemical oxidation (mineralization)

Cu(II) (aq)

[224, 225]

Amino acids and peptides → smaller carboxylic acids, amines, and amides, NH₃, CO₂

(Oxidative) photochemical decomposition, mineralization

[226]

Lysine → pipecolinic acid

ornithine → proline

Photochemical cyclization

HgS, ZnS, CdS

[227, 228]

Phenolic ketones and aldehydes → brown carbon

Photochemical oxidation, oligomerization

[155]

Phenol → hydroquinone, catechol → further oxidation products, CO₂

Photochemical oxidation

Fe₂O₃, TiO₂

[211, 229, 230]

Decomposition of aqueous phenol, naphthol, methylphenols, methoxyphenols, anilines

Photochemical oxidation

Humic and fulvic acids, flavins

Algae (live or dead)

[219, 231, 232]

Phenols → phenol dimers

Photochemical coupling/dimerization

Fe(III) (aq)

[102]

Phenols → quinones, naphthols, aminonaphthols → naphthoquinones

Photochemical oxidation

No facilitator

NO₃ ⁻

[217, 233, 234]

Quinones → quinone dimers

Photochemical coupling/dimerization

[235, 236]

Quinones + benzocyclic olefins → addition products

Photochemical coupling

[237]

Ketones → carboxylic acids

Photochemical cleavage + acidification

[238,239, – 240]

Ketones → CH₄, ethane

photochemical reduction

[174, 240]

Aromatic ketones → condensed aromatic ring systems

Photochemical condensation

[241]

Vicinal diols → ketones, aldehydes, carboxylic acids

Photochemical cleavage + oxidation

Fe(III) porphyrins

[242]

Cinnamic acid → cinnamic acid dimer

Photochemical coupling/dimerization

[243]

Acetic acid → CH₄ + CO₂

Photochemical disproportionation/dismutation

TiO₂; α-Fe₂O₃; Fe₂O₃ on montmorillonite (in the absence of O₂); TiO₂, Fe₂O₃, SrTiO₃ plus an electron acceptor

[121, 122, 244]

Acetic acid → CO₂, CH₄, ethane; methanol, ethanol, propionic acid, other products

Various

α-Fe₂O₃; TiO₂, Fe₂O₃, SrTiO₃, WO₃ plus an electron acceptor

[122, 211, 244]

Acetate, terpenes + O₂ → organic (hydro)peroxides

Photochemical peroxidation

No facilitator

ZnO, organic sensitizers

[245,246,247]

Unsaturated lipids + O₂ → lipid hydroperoxides

Photochemical peroxidation

Chlorophyll

[248, 249]

Propionic acid → ethane + CO₂

Butyric acid → propane + CO₂

Salicylic acid → phenol + CO₂

Photochemical decarboxylation

Fe₂O₃ alone or on montmorillonite

Algae (live or dead)

[122, 250]

Lactic acid → pyruvic acid + H₂

Photochemical oxidation + dehydrogenation

ZnS

[251]

Lactic acid → acetaldehyde + CO₂

(Oxidative) photochemical decarboxylation

Aqueous Cu(II) and Fe(III)

[251, 252]

Glucose → CO₂

Photochemical oxidation

TiO₂

[211]

Oxalic acid → CO₂

Photochemical oxidation

TiO₂, sand, ash,

α-Fe₂O₃, γ-Fe₂O₃,

α-FeOOH, β-FeOOH,

γ-FeOOH, δ-FeOOH

[71, 211, 253, 254]

Tartaric, citric, oxalic, malonic acids → oxidized products

Photochemical oxidation

Ferritin

[255]

Pyruvic acid → pyruvic acid oligomers

Photochemical oligomerization

[256]

Salicylic acid → humic-like substances

Photochemical condensation

Accelerated in the presence of algae

[250]

Syringic acid and other methoxybenzoic acids → methanol

Photochemical decomposition

[257]

Syringic acid and related compounds + Cl⁻ → CH₃Cl

Photochemical decomposition + chlorination

[257]

Methanol → ethylene glycol + H₂

Ethanol → butane-2,3-diol + H₂

Photochemical coupling + dehydrogenation

ZnS in the absence of air

[258]

Isoprene → methylthreitol and methylerythritol (aerosols)

Photochemical oxidation

[259]

(Specific) plant compounds → compounds toxic to other organisms

Phototoxicity

[260, 261]

CO₂ → CO, HCOOH, HCHO, CH₃OH, CH₄

Photochemical reduction (one-carbon products)

Fe(III) oxides, FeCO₃, NiCO₃, CoCO₃, CuCO₃, Mn(II) (aq), ZnO, TiO₂, ZnS, CdS, ZrO₂, WO₃, CaFe₂O₄, BiVO₄, hydrous Cu₂O, transition metal ions and oxides in zeolites

[30, 31, 33, 262,263,264,265,266,267,268]

CO₂ + H₂ → CH₄

Photochemical reduction

α-Fe₂O₃ and Zn-Fe oxide in the presence of water, NiO

[269, 270]

CO₂ + H₂ → CO, HCOOH, CH₃OH

Photochemical reduction

α-Fe₂O₃ and Zn-Fe oxide in the presence of water

[269]

CO₂ → HCOOH

Photochemical reduction

Porphyrins, phthalocyanines

Elemental Cu on silicate rocks such as granite and shale

[271, 272]

CO₂ → ethanol

CO₂ → ethane, ethene, propane, propene

CO₂ → tartaric, glyoxylic, oxalic acids

Photochemical reduction (products with more than one carbon)

SiC, ZnS, BiVO₄, montmorillonite-modified TiO₂

[273,274,275,276,277]

CH₄ → HCOOH

CH₄ → CO, CO₂

Photochemical oxidation

TiO₂

[211, 278]

CH₄ → ethane + H₂

Photochemical coupling + dehydrogenation

SiO₂-Al₂O₃-TiO₂

[279]

Nitrogen compounds

Plant foliage → NO_x

[280]

Plant foliage → N₂O

[281]

Particulate organic N → dissolved organic N and NH₄ ⁺

Photochemical decomposition (dissolution + mineralization)

[115]

Dissolved organic N → biologically more labile N

Photochemical priming

[282]

Amino acids and other organic N (including biologically recalcitrant organic N) → NH₄ ⁺

Photochemical decomposition (mineralization/ammonification)

No facilitator

Organic matter,

Fe₂O₃, soil

[132, 184, 193, 194, 283,284,285,286]

Humic substances → NO₂ ⁻

(Oxidative) photochemical decomposition (mineralization)

[104, 287]

NH₃ → NO₂ ⁻

NH₃ → NO₃ ⁻

Photochemical oxidation (nitrification)

TiO₂, ZnO, Al₂O₃, SiO₂, MnO₂, soil

Observed in seawater

[288,289, – 290]

NH₃ → N₂O, N₂

Photochemical oxidation

TiO₂

[290, 291]

NH₄ ⁺ + NO₂ ⁻ → N₂

urea, protein → [NH₄NO₂] → N₂

Photochemical oxidation + reduction (denitrification)

TiO₂, ZnO, Fe₂O₃, soil

[292, 293]

NH₄NO₃ → N₂O

Photochemical oxidation + reduction (denitrification)

Al₂O₃

[294]

NO_x → NO₃ ⁻

Photochemical oxidation

TiO₂

[295, 296]

NO₂ → HONO, NO, N₂O

Photochemical reduction

TiO₂

[296]

NO₂ ⁻ → NO₃ ⁻

Photochemical oxidation

TiO₂, ZnO, Fe₂O₃, WO₃

[297]

NO₃ ⁻ → NH₃

Photochemical reduction

TiO₂ plus electron acceptor

[298]

NO₃ ⁻ or HNO₃ → N₂O, NO, HONO, NO₂

Photochemical reduction (denitrification/renoxification)

Al₂O₃, TiO₂, SiO₂,

α-Fe₂O₃, ZnO, CuCrO₂, Na zeolite, sand

Observed in snow

[299,300,301,302,303,304,305]

NO₃ ⁻ → NO₂ ⁻ (+ O₂)

Photochemical reduction (+oxidation)

No facilitator

Iron(III) oxide, soil, organic matter; TiO₂ plus humic acids

[103, 306,307,308,309]

NO₂ → HONO

Photochemical reduction

Humic acids, soot, soil

Observed in ice

[157, 310, 311]

N₂O → N₂

Photochemical reduction

ZnO, Fe₂O₃, sand

Humic and fulvic acids

[94, 95, 151, 312]

N₂O → N₂ + O₂

Photochemical dissociation

ZnO, Cu(I) zeolites

[313, 314]

N₂ → NH₃

Photochemical reduction/(reductive) photochemical fixation

ZnO, Al₂O₃, Fe₂O₃, Ni₂O₃, CoO, CuO, Fe(III) in TiO₂, Fe₂O₃-Fe₃O₄, MnO₂,

Sand, soil

Aqueous suspensions of TiO₂, ZnO, CdS, SrTiO₃, Ti(III) zeolites

Hydrous iron(III) oxide in the absence of O₂

[2, 229, 315,316,317,318,319,320,321]

N₂ + H₂O → NH₃ + O₂

Photochemical reduction + oxidation

TiO₂ in the absence of O₂, α-Fe₂O₃,

Fe(III)-doped TiO₂

[58, 321, 322]

N₂ → N₂H₄

Photochemical reduction

Sand

[2]

N₂ + H₂O → N₂H₄ + O₂

Photochemical reduction + oxidation

TiO₂ in the absence of O₂

[322]

N₂ + O₂ → NO

Photochemical oxidation (oxidative) photochemical fixation

TiO₂ in air

[323]

N₂ → NO₂ ⁻

N₂ → NO₃ ⁻

Photochemical oxidation (oxidative) photochemical fixation

Suspension of ZnO in the absence of O₂

Aerated suspension of hydrous iron(III) oxide

TiO₂, soil

[320, 324, 325]

N₂ + H₂O → NO₂ ⁻ + H₂

Photochemical oxidation + reduction

ZnO-Fe₂O₃ under N₂

[326]

Metal compounds

Organic complexes of Fe, Al, Co, Ni (aq) → ionic Fe, Al, Co, Ni (aq)

Photochemical decomposition + decomplexation

[327, 328]

Organic complexes of Fe, Cu, Cr, Pb, V (aq) → colloidal Fe, Cu, Cr, Pb, V

Photochemical decomposition + precipitation

[328]

Organic matter (aq) + iron (aq) → organic matter + iron (s)

Photochemical flocculation

[193, 329]

FeOH⁺ (aq) → FeOOH

Photochemical oxidation

[330]

Fe(III) (hydr)oxides (s) →

Fe(II) (aq)

(Reductive) photochemical dissolution of FeOOH + photochemical oxidation of organic matter (if present)

No facilitator

Coprecipitated or dissolved organic matter, HSO₃ ⁻, montmorillonite

Accelerated in ice

[70, 71, 92, 122, 331,332,333,334,335,336,337,338]

Fe(II) (aq)/Fe(OH)₂ + H₂O → Fe(III) + H₂

Photochemical oxidation + reduction

No facilitator

Chromophores such as chlorophyll

[339, 340]

Fe(III)-carboxylate complexes (aq) → Fe(II) (aq)

Photochemical reduction + decomplexation

[66, 70, 341, 342]

Mn(IV) oxide → Mn(II) (aq)

(Reductive) photochemical dissolution

Dissolved organic matter

Accelerated in ice

[337, 343,344,345,346,347]

Mn(II) (aq) → MnO_x (x = 1 to 2)

Photochemical oxidation

Organic matter, TiO₂

[348, 349]

Cu(II) (aq) → Cu(I)

Photochemical reduction

Amino acids

[224, 225]

Cr(VI) (aq) → Cr(III) (aq)

Photochemical reduction

Ferritin, phenol

[350, 351]

ZnS + H₂O → H₂S → H₂

Photochemical reduction + dissolution

[21, 251]

ZnS → Zn(0) + S(0)

Photochemical oxidation + reduction

[21]

CdS → Cd(II) + S(0)

Photochemical oxidation

[211]

HgS → Hg(II) (aq) + H₂S

Photochemical dissolution

[228, 352]

HgS → Hg(0) + S(0)

Photochemical oxidation + reduction

Cl⁻

[25]

HgS → [Hg₂Cl₂ and other intermediates] → HgCl₂

Photochemical oxidation, reduction/photochemical dissolution

Cl⁻

[25]

Hg(0) (aq) → Hg(II) (aq)

Photochemical oxidation

[352, 353]

Hg(II) (aq) → Hg(0) (aq)

photochemical reduction

Fe(III) species, TiO₂, organic matter

Observed in freshwater, seawater, and snow

[352, 354,355,356,357]

Hg(II) (aq) → HgCH₃ ⁺

Photochemical methylation

[358]

HgCH₃ ⁺ → Hg(II)

Photochemical demethylation

[359, 360]

HgCH₃Cl → Hg(II) + Hg(0) + CHCl₃ + HCHO

Photochemical demethylation + reduction

[361]

Other elements

Plant material → H₂

(Reductive) photochemical decomposition

[362, 363]

Dissolved organic P → inorganic phosphate

Photochemical decomposition (mineralization)

[364]

Phosphate adsorbed to Fe(III) oxides or Fe(III)-organic matter complexes → free phosphate

Photochemical desorption

[161, 365, 366]

HS⁻/S²⁻ → H₂

Photochemical reduction

CdS, α-Fe₂O₃

[367, 368]

SO₂ → SO₄ ²⁻

Photochemical oxidation

TiO₂, Fe₂O₃, ZnO, CdS

[369,370,371,372]

Thiols and SO₃ ²⁻ → oxidized products

Photochemical oxidation

Ferritin

[255]

Alkyl sulfides + NO_x → aldehydes, sulfonic acids, SO₂, SO₄ ²⁻

Photochemical oxidation

[373]

O₂ → H₂O₂

Photochemical reduction

ZnO, TiO₂, sand in the presence of organic electron donors

Aqueous Fe(III)-carboxylic acid complexes

Tryptophan and tyrosine

Porphyrins and phthalocyanines

Algae (live or dead)

[34, 107, 246, 298, 374,375, – 376]

O₂ → H₂O

Photochemical reduction

α-Fe₂O₃

Dissolved Fe and humic substances

(a catalytic cycle)

[123, 377]

H₂O → H₂

Photochemical reduction

Numerous catalysts, usually in the absence of O₂, e.g., TiO₂, ZnS, α-Fe₂O₃, hydrated Cu₂O, tungstosilicate on TiO₂, Ti(III)-zeolite, graphite oxide

[21, 22, 262, 315, 377,378,379,380,381,382]

H₂O → O₂

Photochemical oxidation

α-Fe₂O₃ + Fe(III) (aq), BiVO₄ + electron acceptor, Mn₂O₃, λ-MnO₂, Mn₃O₄, Co₃O₄ + sensitizer, AgCl, layered double hydroxide minerals

Fe(OH)²⁺ (aq)

[383,384,385,386,387,388,389,390]

H₂O → H₂ + O₂

Photochemical water splitting (oxidation + reduction)

TiO₂, Fe₂O₃-Fe₃O₄, Fe₂O₃-FeS₂, Cu₂O, ZrO₂, Ag zeolite, diverse two-mineral systems

[60, 137, 321, 322, 391,392,393]

As(III) (aq) → As(V) (aq)

Photochemical oxidation

No facilitator

Ferrihydrite, kaolinite

[158, 394, 395]

As₄S₄ → As₄S₄ (polymorph)

Photochemical structural (crystal) modification

[396]

As₂S₃ → [As + S] + O₂ → As₂O₃

As₄S₄ → As₂O₃

Photochemical oxidation/dissolution

Water

[396, 397]

Volatile organic compounds + NO_x → O₃

Photochemical oxidation

[398]

Cl⁻ → Cl⁻ ₂ (dichloride radical anion)

Photochemical oxidation

Chlorophyll, Hg(II)

[65, 352]

Cl⁻ + O₃ → Cl₂

Photochemical oxidation

[399]

NO₃ ⁻ + Br⁻ → Br₂

Photochemical oxidation

[400]