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1. Spatial and temporal variability of snowfall over Greenland from CloudSat observations

   https://doi.org/10.5194/acp-19-8101-2019  

   Bennartz, Ralf; Fell, Frank; Pettersen, Claire; Shupe, Matthew D.; Schuettemeyer, Dirk (January 2019, Atmospheric Chemistry and Physics)

   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 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 Z–S relationships in terms ofsnowfall accumulation at Summit through comparison with weekly stake fieldobservations of snow accumulation available since 2007. Using a set of threeZ–S 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 values and annualcycles of snowfall to ERA-Interim reanalysis. We find the annual meansnowfall over the GrIS inferred from CloudSat to be 34±7.5 cm yr⁻¹liquid equivalent (where the uncertainty is determined by the range invalues between the three different Z–S relationships used). In comparison,the ERA-Interim reanalysis product only yields 30 cm yr⁻¹ liquid equivalentsnowfall, where the majority of the underestimation in the reanalysisappears to occur in the summer months over the higher GrIS and appears to berelated to shallow precipitation events. Comparing all available estimatesof snowfall accumulation at Summit Station, we find the annually averagedliquid equivalent snowfall from the stake field to be between 20 and 24 cm yr⁻¹, depending on the assumed snowpack density and from CloudSat 23±4.5 cm yr⁻¹. The annual cycle at Summit is generally similar betweenall data sources, with the exception of ERA-Interim reanalysis, which showsthe aforementioned underestimation during summer months. 

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2. Contributions of biomass-burning, urban, and biogenic emissions to the concentrations and light-absorbing properties of particulate matter in central Amazonia during the dry season

   https://doi.org/10.5194/acp-19-7973-2019  

   de Sá, Suzane S.; Rizzo, Luciana V.; Palm, Brett B.; Campuzano-Jost, Pedro; Day, Douglas A.; Yee, Lindsay D.; Wernis, Rebecca; Isaacman-VanWertz, Gabriel; Brito, Joel; Carbone, Samara; et al (January 2019, Atmospheric Chemistry and Physics)

   Abstract. Urbanization and deforestation have important impacts on atmosphericparticulate matter (PM) over Amazonia. This study presents observations andanalysis of PM₁ concentration, composition, and opticalproperties in central Amazonia during the dry season, focusing on theanthropogenic impacts. The primary study site was located 70 km downwind ofManaus, a city of over 2 million people in Brazil, as part of theGoAmazon2014/5 experiment. A high-resolution time-of-flight aerosol massspectrometer (AMS) provided data on PM₁ composition, and aethalometermeasurements were used to derive the absorption coefficient b_abs,BrC ofbrown carbon (BrC) at 370 nm. Non-refractory PM₁ mass concentrationsaveraged 12.2 µg m⁻³ at the primary study site, dominated byorganics (83 %), followed by sulfate (11 %). A decrease inb_abs,BrC was observed as the mass concentration of nitrogen-containingorganic compounds decreased and the organic PM₁ O:C ratio increased,suggesting atmospheric bleaching of the BrC components. The organic PM₁was separated into six different classes by positive-matrix factorization(PMF), and the mass absorption efficiency E_abs associated with eachfactor was estimated through multivariate linear regression ofb_abs,BrC on the factor loadings. The largest E_abs values wereassociated with urban (2.04±0.14 m² g⁻¹) and biomass-burning(0.82±0.04 to 1.50±0.07 m² g⁻¹) sources. Together, these sources contributed at least 80 % ofb_abs,BrC while accounting for 30 % to 40 % of the organic PM₁ massconcentration. In addition, a comparison of organic PM₁ compositionbetween wet and dry seasons revealed that only part of the 9-foldincrease in mass concentration between the seasons can be attributed tobiomass burning. Biomass-burning factor loadings increased by 30-fold,elevating its relative contribution to organic PM₁ from about 10 % inthe wet season to 30 % in the dry season. However, most of the PM₁mass (>60 %) in both seasons was accounted for by biogenicsecondary organic sources, which in turn showed an 8-fold seasonalincrease in factor loadings. A combination of decreased wet deposition andincreased emissions and oxidant concentrations, as well as a positivefeedback on larger mass concentrations are thought to play a role in theobserved increases. Furthermore, fuzzy c-means clustering identified threeclusters, namely “baseline”, “event”, and “urban” to representdifferent pollution influences during the dry season. The baseline cluster,representing the dry season background, was associated with a mean massconcentration of 9±3 µg m⁻³. This concentration increasedon average by 3 µg m⁻³ for both the urban and the event clusters.The event cluster, representing an increased influence of biomass burningand long-range transport of African volcanic emissions, was characterized byremarkably high sulfate concentrations. The urban cluster, representing theinfluence of Manaus emissions on top of the baseline, was characterized byan organic PM₁ composition that differed from the other two clusters.The differences discussed suggest a shift in oxidation pathways as well asan accelerated oxidation cycle due to urban emissions, in agreement withfindings for the wet season. 

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3. Long-term lidar observations of the gravity wave activity near the mesopause at Arecibo

   https://doi.org/10.5194/acp-19-3207-2019  

   Yue, Xianchang; Friedman, Jonathan S.; Zhou, Qihou; Wu, Xiongbin; Lautenbach, Jens (January 2019, Atmospheric Chemistry and Physics)

   Abstract. Using 11-year-long K Doppler lidar observations of temperatureprofiles in the mesosphere and lower thermosphere (MLT) between 85 and100 km, conducted at the Arecibo Observatory, Puerto Rico(18.35^∘ N, 66.75^∘ W), seasonalvariations of mean temperature, the squared Brunt–Väisäläfrequency, N², and the gravity wave potential energy (GWPE) are estimated in a compositeyear. The following unique features are obtained. (1) The mean temperaturestructure shows similar characteristics to an earlier report based on a smallerdataset. (2) Temperature inversion layers (TILs) occur at 94–96 km inspring, at ∼92 km in summer, and at ∼91 km in early autumn.(3) The first complete range-resolved climatology of GWPE derived from temperature data in the tropical MLT exhibits analtitude-dependent combination of annual oscillation (AO) and semiannualoscillation (SAO). The maximum occurs in spring and the minimum in summer, and asecond maximum is in autumn and a second minimum in winter. (4) The GWPE perunit volume reduces below ∼97 km altitude in all seasons. Thereduction of GWPE is significant at and below the TILs but becomes faintabove; this provides strong support for the mechanism that the formation ofupper mesospheric TILs is mainly due to the reduction of GWPE. The climatologyof GWPE shows an indeed pronounced altitudinal and temporal correlation withthe wind field in the tropical mesopause region published in the literature.This suggests the GW activity in the tropical mesopause region should bemanifested mainly by the filtering effect of the critical level of the localbackground wind and the energy conversion due to local dynamical instability. 

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4. Physical and optical characteristics of heavily melted “rotten” Arctic sea ice

   https://doi.org/10.5194/tc-13-775-2019  

   Frantz, Carie M.; Light, Bonnie; Farley, Samuel M.; Carpenter, Shelly; Lieblappen, Ross; Courville, Zoe; Orellana, Mónica V.; Junge, Karen (January 2019, The Cryosphere)

   Abstract. Field investigations of the properties of heavily melted “rotten” Arcticsea ice were carried out on shorefast and drifting ice off the coast ofUtqiaġvik (formerly Barrow), Alaska, during the melt season. While noformal criteria exist to qualify when ice becomes rotten, the objectiveof this study was to sample melting ice at the point at which its structural andoptical properties are sufficiently advanced beyond the peak of the summerseason. Baseline data on the physical (temperature, salinity, density,microstructure) and optical (light scattering) properties of shorefast icewere recorded in May and June 2015. In July of both 2015 and 2017, smallboats were used to access drifting rotten ice within ∼32 km of Utqiaġvik. Measurements showed that pore space increased as icetemperature increased (−8 to 0 ^∘C), ice salinitydecreased (10 to 0 ppt), and bulk density decreased (0.9 to0.6 g cm⁻³). Changes in pore space were characterized with thin-sectionmicrophotography and X-ray micro-computed tomography in the laboratory. Theseanalyses yielded changes in average brine inclusion number density (whichdecreased from 32 to 0.01 mm⁻³), mean pore size (whichincreased from 80 µm to 3 mm), and total porosity (increased from0 % to > 45 %) and structural anisotropy (variable, withvalues of generally less than 0.7). Additionally, light-scattering coefficientsof the ice increased from approximately 0.06 to > 0.35 cm⁻¹ as the ice melt progressed. Together, these findings indicate thatthe properties of Arctic sea ice at the end of melt season are significantlydistinct from those of often-studied summertime ice. If such rotten ice wereto become more prevalent in a warmer Arctic with longer melt seasons, thiscould have implications for the exchange of fluid and heat at the oceansurface. 

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5. Aqueous reactions of organic triplet excited states with atmospheric alkenes

   https://doi.org/10.5194/acp-19-5021-2019  

   Kaur, Richie; Hudson, Brandi M.; Draper, Joseph; Tantillo, Dean J.; Anastasio, Cort (January 2019, Atmospheric Chemistry and Physics)

   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-solubleC₃–C₆ alkenes that have either been found in theatmosphere or are reasonable surrogates for identified species. Measured rateconstants ( $k_{\mathrm{ALK}+3{\mathrm{BP}}^{\ast }}$) vary by a factor of 30 and are in therange of (0.24–7.5) ×10⁹ M⁻¹ 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 M⁻¹ 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 $k_{\mathrm{ALK}+3{\mathrm{BP}}^{\ast }}$ and the calculated one-electron oxidation potential(OP) of the alkenes (R²=0.58); 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) ×10⁹ M⁻¹ 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. 

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