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1. 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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2. 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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3. 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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4. Carbon Flux Explorer optical assessment of C, N and P fluxes

   https://doi.org/10.5194/bg-16-1249-2019  

   Bourne, Hannah L.; Bishop, James K.; Wood, Todd J.; Loew, Timothy J.; Liu, Yizhuang (January 2019, Biogeosciences)

   Abstract. The magnitude and controls of particulate carbon exported from surface watersand its remineralization at depth are poorly constrained. The Carbon FluxExplorer (CFE), a Lagrangian float-deployed imaging sediment trap, has beendesigned to optically measure the hourly variations of particle flux tokilometer depths for months to seasons while relaying data in near-real timeto shore via satellite without attending ships. The main optical proxy forparticle load recorded by the CFE, volume attenuance (VA; units ofmATN cm²), while rigorously defined and highly precise, has not beenrobustly calibrated in terms of particulate organic carbon (POC), nitrogen(PN) and phosphorus (PP). In this study, a novel 3-D-printed particle samplerusing cutting edge additive manufacturing was developed and integrated withthe CFE. Two such modified floats (CFE-Cals) were deployed a total of15 times for 18–24 h periods to gain calibration imagery and samples atdepths near 150 m in four contrasting productivity environments during theJune 2017 California Current Ecosystem Long-Term Ecological Research (LTER)process study. Regression slopes for VA : POC and VA : PN (unitsmATN cm²: mmol; R², n, p value in parentheses) were1.01×10⁴ (0.86, 12, < 0.001) and 1.01×10⁵(0.86, 15, < 0.001), respectively, and were not sensitive toparticle size classes or the contrasting environments encountered. PP was notwell correlated with VA, reflecting the high lability of P relative to C andN. The volume attenuance flux (VAF) to POC flux calibration is compared toprevious estimates. 

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5. Ice shelf basal melt rates from a high-resolution DEM record for Pine Island Glacier, Antarctica

   https://doi.org/10.5194/tc-2018-209  

   Shean, David E.; Joughin, Ian R.; Dutrieux, Pierre; Smith, Benjamin E.; Berthier, Etienne (October 2018, The Cryosphere Discussions)

   Abstract. Ocean-induced basal melting is directly and indirectly responsible for much of the Amundsen Sea Embayment ice loss in recent decades, but the total magnitude and spatiotemporal evolution of this melt is poorly constrained. To address this problem, we generated a record of high-resolution Digital Elevation Models (DEMs) for Pine Island Glacier (PIG) using commercial sub-meter satellite stereo imagery and integrated additional 2002–2015 DEM/altimetry data. We implemented a Lagrangian elevation change (Dh/Dt) framework to estimate ice shelf basal melt rates at 32–256-m resolution. We describe this methodology and consider basal melt rates and elevation change over the PIG shelf and lower catchment from 2008–2015. We document the evolution of Eulerian elevation change (dh/dt) and upstream propagation of thinning signals following the end of rapid grounding line retreat around 2010. Mean full-shelf basal melt rates for the 2008–2015 period were ~82–93 Gt/yr, with ~ 200–250 m/yr basal melt rates within large channels near the grounding line, ~ 10–30 m/yr over the main shelf, and ~ 0–10 m/yr over the North and South shelves, with the notable exception of a small area with rates of ~ 50–100 m/yr near the grounding line of a fast-flowing tributary on the South shelf. The observed basal melt rates show excellent agreement with, and provide context for, in situ basal melt rate observations. We also document the relative melt rates for km-scale basal channels and keels at different locations on the shelf and consider implications for ocean circulation and heat content. These methods and results offer new indirect observations of ice-ocean interaction and constraints on the processes driving sub-shelf melting beneath vulnerable ice shelves in West Antarctica. 

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