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Review
. 2020 Jan 20;25(2):423.
doi: 10.3390/molecules25020423.

Photochemistry of the Cloud Aqueous Phase: A Review

Angelica Bianco  1 , Monica Passananti  1   2 , Marcello Brigante  3 , Gilles Mailhot  3
Affiliations
Review

Photochemistry of the Cloud Aqueous Phase: A Review

Angelica Bianco et al. Molecules. .

Abstract

This review paper describes briefly the cloud aqueous phase composition and deeply its reactivity in the dark and mainly under solar radiation. The role of the main oxidants (hydrogen peroxide, nitrate radical, and hydroxyl radical) is presented with a focus on the hydroxyl radical, which drives the oxidation capacity during the day. Its sources in the aqueous phase, mainly through photochemical mechanisms with H2O2, iron complexes, or nitrate/nitrite ions, are presented in detail. The formation rate of hydroxyl radical and its steady state concentration evaluated by different authors are listed and compared. Finally, a paragraph is also dedicated to the sinks and the reactivity of the HO• radical with the main compounds found in the cloud aqueous phase. This review presents an assessment of the reactivity in the cloud aqueous phase and shows the significant potential impact that this medium can have on the chemistry of the atmosphere and more generally on the climate.

Keywords: atmosphere; cloud; hydroxyl radical; oxidant capacity; photochemistry.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Concentration of the main ions in cloud water for different air mass origin (highly marine, marine, continental, and polluted) measured in nine different sites (Mt. Brocken, Germany; Vosges, France; Mt. Rax, Austria; Puy de Dôme, France; Szrenica, Poland; East Peak, Puerto Rico; Whiteface Mt., US; Mt. Tai, China; Mont Smücke, Germany). The full line represents the median values. The bottom and top lines correspond to the 25th and 75th percentiles, respectively. The ends of the whiskers are the 10th and 90th percentiles. Medians and percentiles are calculated on the average of the measurement reported in each work.
Figure 2
Figure 2
Main HO• generation and destruction pathways. Double circular arrow (light green) means radical recombination. DOM (dissolved organic matter); VOC (volatile organic compounds); hν means light.
Figure 3
Figure 3
(a) Iron chemistry and photochemistry (adapted from Long et al. [120]); (b) Pourbaix diagram of iron: speciation as a function of pH and redox potential.
Figure 4
Figure 4
HO• reaction pathway (left) and becoming of organic compounds after reaction (right).
See this image and copyright information in PMC

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