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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,BrCmore »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. Supercooled liquid fogs over the central Greenland Ice Sheet

   https://doi.org/10.5194/acp-19-7467-2019  

   Cox, Christopher J. ; Noone, David C. ; Berkelhammer, Max ; Shupe, Matthew D. ; Neff, William D. ; Miller, Nathaniel B. ; Walden, Von P. ; Steffen, Konrad ( January 2019 , Atmospheric Chemistry and Physics)

   Abstract. Radiation fogs at Summit Station, Greenland (72.58^∘ N,38.48^∘ W; 3210 m 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 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 m, and the particles then fall toward the surface.The diameter of mature particles is typically 20–25 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 ^∘C, while the coldest form at temperatures approaching −40 ^∘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 belowmore »−40 ^∘C, as cold as −57 ^∘C,which is too cold to support liquid water. This implies that fog particlessettling through this layer of cold air freeze in the air column beforecontacting the surface, thereby accumulating at the surface as ice withoutriming. Liquid fogs observed under otherwise clear skies annually imparted1.5 W m⁻² of cloud radiative forcing (CRF). While this is a smallcontribution to the surface radiation climatology, individual events areinfluential. The mean CRF during liquid fog events was 26 W m⁻², andwas sometimes much higher. An extreme case study was observed toradiatively force 5 ^∘C of surface warming during the coldest partof the day, effectively damping the diurnal cycle. At lower elevations ofthe ice sheet where melting is more common, such damping could signal a rolefor fogs in preconditioning the surface for melting later in the day.

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3. Photochemistry on the bottom side of the mesospheric Na layer

   https://doi.org/10.5194/acp-19-3769-2019  

   Yuan, Tao ; Feng, Wuhu ; Plane, John M. ; Marsh, Daniel R. ( January 2019 , Atmospheric Chemistry and Physics)

   Abstract. Lidar observations of the mesospheric Na layer have revealed considerablediurnal variations, particularly on the bottom side of the layer, where morethan an order-of-magnitude increase in Na density has been observed below 80 kmafter sunrise. In this paper, multi-year Na lidar observations areutilized over a full diurnal cycle at Utah State University (USU) (41.8^∘ N,111.8^∘ W) and a global atmospheric model of Na with 0.5 kmvertical resolution in the mesosphere and lower thermosphere (WACCM-Na) to explorethe dramatic changes of Na density on the bottom side of the layer. Photolysis of the principal reservoir NaHCO₃ is shown to beprimarily responsible for the increase in Na after sunrise, amplified by theincreased rate of reaction of NaHCO₃ with atomic H, which is mainlyproduced from the photolysis of H₂O and the reaction of OH withO₃. This finding is further supported by Na lidar observation at USUduring the solar eclipse (>96 % totality) event on 21 August 2017, when a decrease and recovery of the Na density on thebottom side of the layer were observed. Lastly, the model simulation showsthat the Fe density below around 80 km increases more strongly and earlierthan observed Na changes during sunrise because of the considerably fasterphotolysis rate of its majormore »reservoir of FeOH.

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4. 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 meanmore »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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5. Detecting the permafrost carbon feedback: talik formation and increased cold-season respiration as precursors to sink-to-source transitions

   https://doi.org/10.5194/tc-12-123-2018  

   Parazoo, Nicholas C. ; Koven, Charles D. ; Lawrence, David M. ; Romanovsky, Vladimir ; Miller, Charles E. ( January 2018 , The Cryosphere)

   Abstract. Thaw and release of permafrost carbon (C) due to climate change is likely tooffset increased vegetation C uptake in northern high-latitude (NHL)terrestrial ecosystems. Models project that this permafrost C feedback mayact as a slow leak, in which case detection and attribution of the feedbackmay be difficult. The formation of talik, a subsurface layer of perenniallythawed soil, can accelerate permafrost degradation and soil respiration,ultimately shifting the C balance of permafrost-affected ecosystems fromlong-term C sinks to long-term C sources. It is imperative to understand andcharacterize mechanistic links between talik, permafrost thaw, andrespiration of deep soil C to detect and quantify the permafrost C feedback.Here, we use the Community Land Model (CLM) version 4.5, a permafrost andbiogeochemistry model, in comparison to long-term deep borehole data alongNorth American and Siberian transects, to investigate thaw-driven C sourcesin NHL (>55^∘N) from 2000 to 2300. Widespread talik at depth isprojected across most of the NHL permafrost region(14million km²) by 2300, 6.2million km² of which isprojected to become a long-term C source, emitting 10Pg C by 2100,50Pg C by 2200, and 120Pg C by 2300, with few signs ofslowing. Roughly half of the projected C source region is in predominantlywarm sub-Arctic permafrost following talik onset. This region emits only20Pg C by 2300, butmore »the CLM4.5 estimate may be biased low by notaccounting for deep C in yedoma. Accelerated decomposition of deep soilC following talik onset shifts the ecosystem C balance away from surfacedominant processes (photosynthesis and litter respiration), butsink-to-source transition dates are delayed by 20–200 years by highecosystem productivity, such that talik peaks early (∼2050s, although boreholedata suggest sooner) and C source transition peaks late(∼2150–2200). The remaining C source region in cold northern Arcticpermafrost, which shifts to a net source early (late 21st century), emits5 times more C (95Pg C) by 2300, and prior to talik formation dueto the high decomposition rates of shallow, young C in organic-rich soilscoupled with low productivity. Our results provide important clues signalingimminent talik onset and C source transition, including (1) late cold-season(January–February) soil warming at depth (∼2m),(2) increasing cold-season emissions (November–April), and (3) enhancedrespiration of deep, old C in warm permafrost and young, shallow C in organic-rich cold permafrost soils. Our results suggest a mosaic of processes thatgovern carbon source-to-sink transitions at high latitudes and emphasize theurgency of monitoring soil thermal profiles, organic C age and content, cold-season CO₂ emissions, andatmospheric ¹⁴CO₂ as key indicatorsof the permafrost C feedback.

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