Supercooled liquid fogs over the central Greenland Ice Sheet

Abstract. Radiation fogs at Summit Station, Greenland (72.58&thinsp;N,38.48&thinsp;W; 3210&thinsp;m&thinsp;a.s.l.), are frequently reported by observers. Thefogs are often accompanied by fogbows, indicating the particles are composedof liquid; and because of the low temperatures at Summit, this liquid issupercooled. Here we analyze the formation of these fogs as well as theirphysical and radiative properties. In situ observations of particle size anddroplet number concentration were made using scattering spectrometers near 2 and 10&thinsp;m height from 2012 to 2014. These data are complemented bycolocated observations of meteorology, turbulent and radiative fluxes, andremote sensing. We find that liquid fogs occur in all seasons with thehighest frequency in September and a minimum in April. Due to thecharacteristics of the boundary-layer meteorology, the fogs are elevated,forming between 2 and 10&thinsp;m, and the particles then fall toward the surface.The diameter of mature particles is typically 20–25&thinsp;µm in summer.Number concentrations are higher at warmer temperatures and, thus, higher insummer compared to winter. The fogs form at temperatures as warm as −5&thinsp;C, while the coldest form at temperatures approaching −40&thinsp;C. Facilitated by the elevated condensation, in winter two-thirds offogs occurred within a relatively warm layer above the surface when thenear-surface air was below more »

Authors:
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Award ID(s):
Publication Date:
NSF-PAR ID:
10108872
Journal Name:
Atmospheric Chemistry and Physics
Volume:
19
Issue:
11
Page Range or eLocation-ID:
7467 to 7485
ISSN:
1680-7324
2. Abstract. Triplet excited states of organic matter are formed when colored organicmatter (i.e., brown carbon) absorbs light. While these “triplets” can beimportant photooxidants in atmospheric drops and particles (e.g., theyrapidly oxidize phenols), very little is known about their reactivity towardmany classes of organic compounds in the atmosphere. Here we measure thebimolecular rate constants of the triplet excited state of benzophenone(3BP), a model species, with 17 water-solubleC3C6 alkenes that have either been found in theatmosphere or are reasonable surrogates for identified species. Measured rateconstants (${k}_{\mathrm{ALK}+\mathrm{3}{\mathrm{BP}}^{\ast }}$) vary by a factor of 30 and are in therange of (0.24–7.5)&thinsp;×109&thinsp;M−1&thinsp;s−1. Biogenic alkenesfound in the atmosphere – e.g., cis-3-hexen-1-ol, cis-3-hexenyl acetate, andmethyl jasmonate – react rapidly, with rate constants above 1×109&thinsp;M−1&thinsp;s−1. Rate constants depend on alkene characteristicssuch as the location of the double bond, stereochemistry, and alkylsubstitution on the double bond. There is a reasonable correlation between${k}_{\mathrm{ALK}+\mathrm{3}{\mathrm{BP}}^{\ast }}$ and the calculated one-electron oxidation potential(OP) of the alkenes (more »); in contrast, rate constants are notcorrelated with bond dissociation enthalpies, bond dissociation freeenergies, or computed energy barriers for hydrogen abstraction. Using the OPrelationship, we estimate aqueous rate constants for a number of unsaturatedisoprene and limonene oxidation products with 3BP: values are inthe range of (0.080–1.7)&thinsp;×109&thinsp;M−1&thinsp;s−1, withgenerally faster values for limonene products. Rate constants with lessreactive triplets, which are probably more environmentally relevant, arelikely roughly 25 times slower. Using our predicted rate constants, alongwith values for other reactions from the literature, we conclude thattriplets are probably minor oxidants for isoprene- and limonene-relatedcompounds in cloudy or foggy atmospheres, except in cases in which the tripletsare very reactive.