Spatial and temporal variability of snowfall over Greenland from CloudSat observations

Abstract. We use the CloudSat 2006–2016 data record to estimate snowfall over theGreenland Ice Sheet (GrIS). We first evaluate CloudSat snowfall retrievalswith respect to remaining ground-clutter issues. Comparing CloudSatobservations to the GrIS topography (obtained from airborne altimetrymeasurements during IceBridge) we find that at the edges of the GrISspurious high-snowfall retrievals caused by ground clutter occasionallyaffect the operational snowfall product. After correcting for this effect,the height of the lowest valid CloudSat observation is about 1200&thinsp;mabove the local topography as defined by IceBridge. We then use ground-based millimeter wavelength cloud radar (MMCR) observations obtained from the Integrated Characterization of Energy, Clouds, Atmospheric state, and Precipitation at Summit, Greenland (ICECAPS) experiment to devise a simple,empirical correction to account for precipitation processes occurringbetween the height of the observed CloudSat reflectivities and the snowfallnear the surface. Using the height-corrected, clutter-cleared CloudSatreflectivities we next evaluate various ZS relationships in terms ofsnowfall accumulation at Summit through comparison with weekly stake fieldobservations of snow accumulation available since 2007. Using a set of threeZS relationships that best agree with the observed accumulation at Summit,we then calculate the annual cycle snowfall over the entire GrIS as well asover different drainage areas and compare the derived mean more »

Authors:
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Award ID(s):
Publication Date:
NSF-PAR ID:
10108871
Journal Name:
Atmospheric Chemistry and Physics
Volume:
19
Issue:
12
Page Range or eLocation-ID:
8101 to 8121
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.
4. Abstract. Chemical ionization massspectrometry (CIMS) instruments routinely detect hundreds of oxidized organic compoundsin the atmosphere. A major limitation of these instruments is the uncertaintyin their sensitivity to many of the detected ions. We describe thedevelopment of a new high-resolution time-of-flight chemical ionization massspectrometer that operates in one of two ionization modes: using eitherammonium ion ligand-switching reactions such as for ${\mathrm{NH}}_{\mathrm{4}}^{+}$ CIMS orproton transfer reactions such as for proton-transfer-reaction massspectrometer (PTR-MS). Switching between the modes can be done within 2&thinsp;min.The ${\mathrm{NH}}_{\mathrm{4}}^{+}$ CIMS mode of the new instrument has sensitivities of upto 67&thinsp;000&thinsp;dcps&thinsp;ppbv−1 (duty-cycle-corrected ion counts per second perpart per billion by volume) and detection limits between 1 and 60&thinsp;pptv at2σ for a 1&thinsp;s integration time for numerous oxygenated volatileorganic compounds. We present a mass spectrometric voltage scanning procedurebased on collision-induced dissociation that allows us to determine thestability of ammonium-organic ions detected by the ${\mathrm{NH}}_{\mathrm{4}}^{+}$more » CIMS instrument.Using this procedure, we can effectively constrain the sensitivity of theammonia chemical ionization mass spectrometer to a wide range of detectedoxidized volatile organic compounds for which no calibration standards exist.We demonstrate the application of this procedure by quantifying thecomposition of secondary organic aerosols in a series of laboratoryexperiments.