Florent Domine

Comprehensive isotopic composition of atmospheric nitrate in the Atlantic Ocean boundary layer from 65°S to 79°N

The comprehensive isotopic composition of atmospheric nitrate (i.e., the simultaneous measurement of all its stable isotope ratios: 15N/14N, 17O/16O and 18O/16O) has been determined for aerosol samples collected in the marine boundary layer (MBL) over the Atlantic Ocean from 65°S (Weddell Sea) to 79°N (Svalbard), along a ship-borne latitudinal transect. In nonpolar areas, the δ 15N of nitrate mostly deriving from anthropogenically emitted NO x is found to be significantly different (from 0 to 6‰) from nitrate sampled in locations influenced by natural NO x sources (−4 ± 2)‰. The effects on δ 15N(NO3 −) of different NO x sources and nitrate removal processes associated with its atmospheric transport are discussed. Measurements of the oxygen isotope anomaly (Δ17O = δ 17O − 0.52 × δ 18O) of nitrate suggest that nocturnal processes involving the nitrate radical play a major role in terms of NO x sinks. Different Δ17O between aerosol size fractions indicate different proportions between nitrate formation pathways as a function of the size and composition of the particles. Extremely low δ 15N values (down to −40‰) are found in air masses exposed to snow-covered areas, showing that snowpack emissions of NO x from upwind regions can have a significant impact on the local surface budget of reactive nitrogen, in conjunction with interactions with active halogen chemistry. The implications of the results are discussed in light of the potential use of the stable isotopic composition of nitrate to infer atmospherically relevant information from nitrate preserved in ice cores.

Atmospheric chemistry of formaldehyde in the Arctic troposphere at Polar Sunrise, and the influence of the snowpack

The role of formaldehyde in the atmospheric chemistry of the Arctic marine boundary layer has been studied during both polar day and night at Alert, Nunavut, Canada. Formaldehyde concentrations were determined during two separate field campaigns (PSE 1998 and ALERT2000) from polar night to the light period. The large differences in the predominant chemistry and transport issues in the dark and light periods are examined here. Formaldehyde concentrations during the dark period were found to be dependent on the transport of air masses to the Alert site. Three regimes were identified during the dark period, including background (free-tropospheric) air, transported polluted air from Eurasia, and halogen-processed air transported across the dark Arctic Ocean. In the light period, background formaldehyde levels were compared to a calculation of the steady-state formaldehyde concentrations under background and low-ozone conditions. We found that, for sunlit conditions, the ambient formaldehyde concentrations cannot be reproduced by known gas-phase chemistry. We suggest that snowpack photochemistry contributes to production and emission of formaldehyde in the light period, which could account for the high concentrations observed at Alert.

Halogens in the coastal snow pack near Barrow, Alaska: Evidence for active bromine air-snow chemistry during springtime

Received 14 October 2004; revised 11 January 2005; accepted 25 January 2005; published 26 February 2005. [1] We measured halide concentrations of snow and frost flowers in the vicinity of Barrow, Alaska. We find that the ratio of bromide to sodium in frost flowers is slightly enhanced (10%) as compared to sea water. In contrast, the ratio of bromide to sodium in some snow samples is more than an order of magnitude enhanced, and in other samples is more than an order of magnitude depleted. We interpret the bromide depleted snow as having been processed by heterogeneous chemistry and providing reactive halogen compounds to the atmosphere. The eventual end product of reactive bromine chemistry is HBr that is then deposited over a wide region, enhancing bromide in inland snow samples. Although frost flowers or open leads are likely to be the original source of halides that become reactive halogen gases, we find that the bromide release often occurs subsequent to production of aerosol from marine sources. Citation: Simpson, W. R., L. Alvarez-Aviles, T. A. Douglas, M. Sturm, and F. Domine (2005), Halogens in the coastal snow pack near Barrow, Alaska: Evidence for active bromine air-snow chemistry during springtime, Geophys. Res. Lett., 32, L04811,

High pressure pyrolysis of n-hexane, 2,4-dimethylpentane and 1-phenylbutane. Is pressure an important geochemical parameter?

Abstract Pure hexane, 2,4-dimethylpentane and 1-phenylbutane have been pyrolysed in closed gold reactors between 290 and 365°C, in the 210–15,600 bar pressure range. Reaction products at low conversion are alkanes and alkenes (sometimes with a substituted phenyl group in the case of 1-phenylbutane) lighter than the reactant, alkanes (phenyl substituted in the case of 1-phenylbutane) heavier than the reactant and, in the case of 2,4-dimethylpentane, alkenes and cyclic compounds heavier than the reactant. The formation of these products is explained by two types of mechanism, both involving chain reactions that yield alkanes and alkenes lighter than the reactant. The first one is typical of low temperature-high pressure pyrolysis, and radicals formed by H-transfer reactions are predominant. The second one is typical of high temperature-low pressure pyrolysis, and radicals formed by decomposition reactions are predominant. Hexane pyrolysis follows the first pattern, and 1-phenylbutane pyrolysis is similar. 2,4-Dimethylpentane pyrolysis is intermediate between both mechanisms. The effect of pressure on both mechanisms is estimated. The pyrolysis of hydrocarbons following the first type of mechanism is predicted to be greatly hindered by higher pressure while the pyrolysis of hydrocarbons following the second one is slightly faster at higher pressures. It is predicted that under geochemically relevant conditions, these hydrocarbons, and similar ones, will follow a low temperature-high pressure mechanism and their pyrolysis rate will be greatly reduced by high pressures. This effect is difficult or impossible to observe under laboratory conditions, because the pyrolysis mechanism may then be in part, or totally, of the high temperature-low pressure type. In conclusion, it is suggested that high pressures probably considerably hinder the thermal evolution of geological organic matter.

Snow-pile and chamber experiments during the Polar Sunrise Experiment ‘Alert 2000’: exploration of nitrogen chemistry

Abstract Snow chamber and snow-pile experiments performed during the ‘Alert 2000’ campaign show significant release of NO, NO 2 , and HONO in steady ratios under the influence of irradiation. Both light and a minimal degree of heating are required to produce this effect. We suggest diffusion and re-distribution of NO 3 − in the form of HNO 3 as an important step in the mechanism of active nitrogen release from the snowpack.

Structure, microphysics, and surface area of the Arctic snowpack near Alert during the ALERT 2000 campaign

The seasonal snowpackat Alert (North coast of Ellesmere Island, 82 129.94 0 N, 62120.55 0 W) was studied in February and April 2000, on land and on sea ice. The stratigraphy was studied, and the density and specific surface area (SSA) of each snow layer were measured. SSA was measured by CH4 adsorption at 77 K using a volumetric method. On land, the snowpackwas 10–50 cm thickand consisted of a depth hoar layer covered by one or more hard wind-packed layers with densities between 0.35 and 0.52. These were sometimes separated by soft layers of more or less faceted crystals. The surface was covered by recent precipitation and surface hoar. The stratigraphy on sea ice was more variable, with numerous hard wind-packed layers alternating with soft layers of depth hoar or faceted crystals. SSA values ranged from 125 cm 2 /g for depth hoar to 1500 cm 2 /g for diamond dust and dendritic snow. The total surface area of the snowpackwas calculated from the thickness, density, and SSA of each layer, and ranged from 1160 to 3710 m 2 of snow surface area per m 2 of ground. These values were used to estimate the potential impact of the snowpackon atmospheric chemistry, by adsorption/desorption of trace gases. Using the example of acetone, whose adsorption behavior on ice is estimated, it is found that the snowpackmay sequester most of the acetone of the (snow+boundary layer) system most of the year. The release during metamorphism of trace gases dissolved in snow is also discussed. We propose that the frequency and intensity of wind storms will strongly affect the release of trace gases, as this will determine whether intense metamorphism leading to depositional depth hoar can happen. r 2002 Elsevier Science Ltd. All rights reserved.

The effects of snowpack properties and plant strategies on litter decomposition during winter in subalpine meadows

AimsClimate-induced changes in snow cover are likely to affect cold arctic and alpine ecosystems functioning and major processes such as wintertime plant litter decomposition. However, it remains poorly studied in subalpine systems where the snowpack may be irregular. In this paper we explored the dynamic of the winter plant litter decomposition process, its magnitude and its relationship with the snowpack properties.MethodsIn subalpine grasslands of the Central French Alps, we performed a litter bag experiment monitoring over a whole winter the litter decomposition from the exploitative Dactylis glomerata and the conservative Festuca paniculata, under two contiguous experimental sites with snowpacks differing in depth and physical properties.ResultsLitter decomposition rates were stable during winter and 3-fold higher under deeper and permanent snowpack with higher thermal resistance. Litter quality appeared only significant under thinner snowpack with higher decomposition rates for the exploitative species. A snowpack with higher thermal resistance created an insulating layer promoting the decomposition process.ConclusionThese results suggest that the temporal (permanence vs. intermittency) and physical (depth and thermal resistance) characteristics of the snowpack should be considered when studying the response of winter ecosystems functioning to global changes.

Measurement of vertical profiles of snow specific surface area with a 1 cm resolution using infrared reflectance: instrument description and validation

The specific surface area (SSA), defined as the surface area of ice per unit mass, is an important variable characterizing the complex microstructure of snow. Its application range covers the physical evolution of snow (metamorphism), photochemistry and optical and microwave remote sensing. This paper presents a new device, POSSSUM (Profiler Of Snow Specific Surface area Using SWIR reflectance Measurement), designed to allow the rapid acquisition of SSA profiles down to ∼20 m depth and with an effective vertical resolution of 10―20 mm. POSSSUM is based on the infrared (IR) reflectance technique: A laser diode operating at 1310 nm illuminates the snow at nadir incidence angle along the face of a drilled hole. The reflected radiance is measured at three zenith angles (20°, 40° and 60°) each for two azimuth angles (0° and 180°). A second laser operating at a shorter wavelength (635 nm), which is almost insensitive to SSA, allows the distance to the snow face to be estimated. The reflected IR radiance and the distance are combined to estimate bidirectional reflectances. These reflectances are converted into hemispherical reflectances and in turn into SSA using a theoretical formulation based on an asymptotic solution of the radiative transfer equation. The evaluation and validation of POSSSUMs SSA measurements took place in spring 2009 in the French Alps. The new method was compared with the methane adsorption technique and DUFISSS, another well-validated instrument based on the IR technique. The overall measurement error is in the range 10―15%.

Rate of decrease of the specific surface area of dry snow : Isothermal and temperature gradient conditions

The specific surface area (SSA) of snow is the surface area available to gases per unit mass. It is an important variable for quantifying air-snow exchange of chemical species, and it is closely related to other variables such as albedo. Snow SSA decreases during metamorphism, but few data are available to quantify its rate of decrease. We have performed laboratory experiments under isothermal and temperature gradient conditions during which the SSA of snow samples was monitored for several months. We have also monitored the SSA of snowfalls subjected to large temperature gradients at a field site in the central Alaskan taiga. The same snow layers were also monitored in a manipulated snowpack where the temperature gradient was greatly reduced. In all cases, the SSA decay follows a logarithmic equation with three adjustable variables that are parameterized using the initial snow SSA and the time-averaged temperature of the snow. Two parameterizations of the three adjustable variables are found: One applies to the isothermal experiments and to the quasi-isothermal cases studied in Alaska (equitemperature (ET) metamorphism), and the other is applicable to both the laboratory experiments performed under temperature gradients and to the natural snowpack in Alaska (temperature gradient (TG) metamorphism). Higher temperatures accelerate the decrease in SSA, and this decrease is faster under TG than ET conditions. We discuss the conditions of applicability of these parameterizations and use them to speculate on the effect of climate change on snow SSA. Depending on the climate regime, changes in the rate of decay of snow SSA and hence in snow albedo may produce either negative or positive feedbacks on climate change.

Microorganisms in Dry Polar Snow Are Involved in the Exchanges of Reactive Nitrogen Species with the Atmosphere

The snowpack is a complex photochemical reactor that emits a wide variety of reactive molecules to the atmosphere. In particular, the photolysis of nitrate ions, NO(3)(-), produces NO, NO(2), and HONO, which affects the oxidative capacity of the atmosphere. We report measurements in the European High Arctic where we observed for the first time emissions of NO, NO(2), and HONO by the seasonal snowpack in winter, in the complete or near-complete absence of sunlight and in the absence of melting. We also detected unusually high concentrations of nitrite ions, NO(2)(-), in the snow. These results suggest that microbial activity in the snowpack is responsible for the observed emissions. Isotopic analysis of NO(2)(-) and NO(3)(-) in the snow confirm that these ions, at least in part, do not have an atmospheric origin and are most likely produced by the microbial oxidation of NH(4)(+) coming from clay minerals into NO(2)(-) and NO(3)(-). These metabolic pathways also produce NO. Subsequent dark abiotic reactions lead to NO(2) and HONO production. The snow cover is therefore not only an active photochemical reactor but also a biogeochemical reactor active in the cycling of nitrogen and it can affect atmospheric composition all year round.