Quick measurement of CH<Subscript>4</Subscript>, CO<Subscript>2</Subscript> and N<Subscript>2</Subscript>O emissions from a short-plant ecosystem

Combining improved injector, gas line and valve-driving models, a gas chromatograph (GC) equipped with Hydrogen Flame Ionization Detector (FID) and Electron Capture Detector (ECD), can measure CH4, CO2, and N2O simultaneously in an air sample in four minutes. Test results show that the system has high sensitivity, resolution, and precision; the linear response range of the system meets the requirement of flux measurements in situ. The system is suitable for monitoring fluxes of the main greenhouse gases in a short-plant field since it is easy to use, efficacious, and constant and reliable in collecting data.     

The millennial atmospheric lifetime of anthropogenic CO<Subscript>2</Subscript>

The notion is pervasive in the climate science community and in the public at large that the climate impacts of fossil fuel CO₂ release will only persist for a few centuries. This conclusion has no basis in theory or models of the atmosphere/ocean carbon cycle, which we review here. The largest fraction of the CO₂ recovery will take place on time scales of centuries, as CO₂ invades the ocean, but a significant fraction of the fossil fuel CO₂, ranging in published models in the literature from 20–60%, remains airborne for a thousand years or longer. Ultimate recovery takes place on time scales of hundreds of thousands of years, a geologic longevity typically associated in public perceptions with nuclear waste. The glacial/interglacial climate cycles demonstrate that ice sheets and sea level respond dramatically to millennial-timescale changes in climate forcing. There are also potential positive feedbacks in the carbon cycle, including methane hydrates in the ocean, and peat frozen in permafrost, that are most sensitive to the long tail of the fossil fuel CO₂ in the atmosphere.     

DOAS observations of formaldehyde and its impact on the HO<Subscript>x</Subscript> balance in the tropical Atlantic marine boundary layer

Measurements of formaldehyde (HCHO) were made at the Cape Verde Atmospheric Observatory between November 2006 and June 2007 using the Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) technique. Observations show that typical HCHO mixing ratios ranged between 350 and 550 pptv (with typical 2-σ uncertainties of ~110 pptv), with several events of high HCHO, the maximum being 1,885?±?149 pptv. The observations indicate a lack of strong seasonal or diurnal variations, within the uncertainty of the measurements. A box model is employed to test whether the observations can be explained using known hydrocarbon photochemistry; the model replicates well the typical diurnal profile and monthly mean values. The model results indicate that on average 20% of HO₂ production and 10% of OH destruction can be attributed to the mean HCHO levels, suggesting that even at these low average mixing ratios HCHO plays an important role in determining the HO_x (HO₂+OH) balance of the remote marine boundary layer.     

Radiative destabilization of the nocturnal stable atmospheric boundary layer over the desert

Radiative destabilization of the nocturnal stable atmospheric boundary layer (NSABL) over homogeneous desert terrain is predicted by an analytical model based on a modified diffusion equation. The model applies late at night under calm, dry conditions when long-wave radiative transfer dominates the NSABL evolution. A three-layer structure for the NSABL is proposed: a shear sub-layer closest to the surface, a radiative sub-layer which contains the inversion top, and a coupling sub-layer which matches the NSABL with the residual layer aloft. A sub-sub-layer called the nocturnal internal boundary layer (NIBL) is nested within the radiative sub-layer and comprises the temperature maximum. The model can explain: (1) maximum cooling in the NIBL, (2) deepening of the NIBL, (3) radiative destabilization of the NSABL, and (4) possible surface warming before sunrise. An example from the Mohave Desert, USA is presented, and the observed temperature profile compares favorably with the model solution.     

A pseudo-Lagrangian study of the sulfur budget in the remote Arctic marine boundary layer

The atmospheric sulfur cycle of the remote Arctic marine boundary layer is studied using trajectories and measurements of sulfur compounds from the International Arctic Ocean Expedition 1991, along with a pseudo-Lagrangian approach and an analytical model. The dimethyl sulfide [DMS(g)] turnover time was  h. Only  % of DMS(g) followed reaction paths to sulfur dioxide [SO₂(g)], sub-micrometre aerosol non-seasalt sulfate (nss-SO₄²⁻) or methane sulfonate (MSA). During the first 3 d of transport over the pack ice, fog deposition and drizzle resulted in short turnover times;  h for SO₂(g),  h for MSA and  h for nss-SO₄²⁻. Therefore, DMS(g) will, owing to its origin along or south of the ice edge and longer turnover time, survive the original sub-micrometre sulfur aerosol mass and gradually replace it with new biogenic sulfur aerosol mass. The advection of DMS(g) along with heat and moisture will influence the clouds and fogs over the Arctic pack ice through the formation of cloud condensation nuclei (CCN). If the pack ice cover were to decrease owing to a climate change, the total Arctic Ocean DMS production would change, and potentially there could be an ice–DMS–cloud–albedo climate feedback effect, but it would be accompanied by changes in the fog aerosol sink.     

A method for the selective measurement of HO<Subscript>2</Subscript> and RO<Subscript>2</Subscript> radical concentrations using a Nafion-PERCA system

An instrument is developed for the measurement of peroxy radical using chemical amplification coupled with NO₂-luminol chemiluminescence detection. The chain length of 147 ± 10 (1σ) is determined by an HO₂ source that uses the photolysis of water vapor under 184.9 nm in air. A Nafion system equipped with a Nafion tube of ~2.2 mm external diameter and 350 mm length is employed in the PERCA instrument (Nafion-PERCA system). When flowing an air sample containing HO₂ through the Nafion system, it is found that - 94.6 % of HO₂ is removed. In contrast, only 17.8 % of RO₂ radicals (a mixtures of CH₃O₂ and CH₃C(O)O₂ with a ratio of 1.1:0.9) are removed. The results indicate the Nafion system has a good selective removing performance of HO₂ radical during the PERCA measurement. Therefore, the method could be applied to ambient and laboratory measurements of absolute concentrations of RO₂ as well as the sum of HO₂ and RO₂.     

A model study of the nocturnal boundary layer

In this paper, a second-order model is proposed for the study of the evolution of the nocturnal boundary layer (NBL). The model is tested against the Wangara data on atmospheric boundary layer. The computer results show ihat the model can simulate some important characters observed in the NBL, and that a kind of sudden change may occur in the developing process of NBL.     

Observations in the nocturnal boundary layer

Low-latitude observations of the stably-stratified planetary boundary layer (SBL) above rough terrain are compared to observations of the mid-latitude SBL mainly through the depth h and its dependence upon surface fluxes. This involves the quantity h/L and the similarity prediction h = (u _* L/f)^1/2.Mid-latitude observations are consistent with model calculations for nighttime-averaged quantities and their deviations, as functions of latitude and surface roughness, from the equilibrium values found at large t. The above applies to horizontally-homogeneous terrain.Low-latitude observations of % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGafq4SdCMbae% baaaa!37AB!\[\bar \gamma \] and h/L are significantly smaller than mid-latitude values, apparently the result of katabatic flows at the site and not the differences in latitude. This is consistent with model calculations for non-zero slope terrain.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Multi-Phase Chemistry of C<Subscript>2</Subscript> and C<Subscript>3</Subscript> Organic Compounds in the Marine Atmosphere

A box model is used to explore the detailed chemistry of C₂ and C₃ organic compounds in the marine troposphere by tracing the individual reaction paths resulting from the oxidation of ethane, ethene, acetylene, propane, propene and acetic acid. The mechanisms include chemical reactions in the gas phase and in the aqueous phase of clouds and aerosol particles at cloud level under conditions resembling those in the northern hemisphere. Organic hydroperoxides are found to be important intermediate products, with subsequent reactions leading partly to the formation of mixed hydroxy or carbonyl hydroperoxides that are readily absorbed into cloud water, where they contribute significantly to the formation of multifunctional organic compounds and organic acids. Organic hydroperoxides add little to the oxidation of sulfur dioxide dissolved in the aqueous phase, which is dominated by H₂O₂. Next to acetaldehyde and acetone, glycol aldehyde, glyoxal, methyl glyoxal and hydroxy propanone are prominent oxidation products in the gas and the aqueous phase. Acetaldehyde is not efficiently converted to acetic acid in clouds; the major local sources of acetic acid are gas-phase reactions. Other acids produced include hydroperoxy acetic, glycolic, glyoxylic, oxalic, pyruvic, and lactic acid. The mechanism of Schuchmann et al. (1985), which derives glycolic and glyoxylic acid from the oxidation of acetate, is found unimportant in the marine atmosphere. The principal precursors of glyoxylic acid are glyoxal and glycolic acid. The former derives mainly from acetylene and ethene, the latter from glycolaldehyde, also an oxidation product of ethene. The oxidation of glyoxylic acid leads to oxalic acid, which accumulates and is predicted to reach steady state concentrations in the range 30–90 ng m⁻³. This is greater, yet of the same magnitude, than the concentrations observed over the remote Pacific Ocean.     

Summertime Surface N2O Concentration Observed on FildesPeninsula Antarctica： Correlation with TotalAtmospheric O3 and Solar Activity

Three-year summertime surface atmospheric N2O concentrations were observed for the first timeon the Fildes Peninsula, maritime Antarctica, and the relationships among the N2O concentration, totalatmospheric O3 amount, and sunspot number were analyzed. Solar activity had an important effecton surface N20 concentration and total O3 amount, and increases of sunspot number were followed bydecreases in the N2O concentration and total O3 amount. A corresponding relationship exists betweenthe N2O concentration and total atmospheric O3, and ozone destruction was preceded by N2O reduction.We propose that the extended solar activity in the Antarctic summer reduces the stratospheric N2O byconverting it into NOx, increases the diffusion of N2O from the troposphere to the stratosphere, decreasesthe surface atmospheric N2O, and depletes O3 via the chemical reaction between O3 and NOx. Ourobservation results are consistent with the theory of solar activity regarding the formation of the AntarcticO3 hole.     

Chemical characterization and multivariate analysis of atmospheric PM<Subscript>2.5</Subscript> particles

The new European Council Directive (PE-CONS 3696/07) frames the inhalable (PM₁₀) and fine particles (PM_2.5) on priority to chemically characterize these fractions in order to understand their possible relation with health effects. Considering this, PM_2.5 was collected during four different seasons to evaluate the relative abundance of bulk elements (Cl, S, Si, Al, Br, Cu, Fe, Ti, Ca, K, Pb, Zn, Ni, Mn, Cr and V) and water soluble ions (F⁻, Cl⁻, NO₂ ⁻, NO₃ ⁻, SO₄ ²⁻, Na⁺, NH₄ ⁺, Ca²⁺ and Mg²⁺) over Menen, a Belgian city near the French border. The air quality over Menen is influenced by industrialized regions on both sides of the border. The most abundant ionic species were NO₃ ⁻, SO₄ ²⁻ and NH₄ ⁺, and they showed distinct seasonal variation. The elevated levels of NO₃ ⁻ during spring and summer were found to be related to the larger availability of the NO_x precursor. The various elemental species analyzed were distinguished into crustal and anthropogenic source categories. The dominating elements were S and Cl in the PM_2.5 particles. The anthropogenic fraction (e.g. Zn, Pb, and Cu) shows a more scattered abundance. Furthermore, the ions and elemental data were also processed using principal component analysis and cluster analysis to identify their sources and chemistry. These approach identifies anthropogenic (traffic and industrial) emissions as a major source for fine particles. The variations in the natural/anthropogenic fractions of PM_2.5 were also found to be a function of meteorological conditions as well as of long-range transport of air masses from the industrialized regions of the continent. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

CO<Subscript>2</Subscript> and non-CO<Subscript>2</Subscript> radiative forcings in climate projections for twenty-first century mitigation scenarios

Climate is simulated for reference and mitigation emissions scenarios from Integrated Assessment Models using the Bern2.5CC carbon cycle–climate model. Mitigation options encompass all major radiative forcing agents. Temperature change is attributed to forcings using an impulse–response substitute of Bern2.5CC. The contribution of CO₂ to global warming increases over the century in all scenarios. Non-CO₂ mitigation measures add to the abatement of global warming. The share of mitigation carried by CO₂, however, increases when radiative forcing targets are lowered, and increases after 2000 in all mitigation scenarios. Thus, non-CO₂ mitigation is limited and net CO₂ emissions must eventually subside. Mitigation rapidly reduces the sulfate aerosol loading and associated cooling, partly masking Greenhouse Gas mitigation over the coming decades. A profound effect of mitigation on CO₂ concentration, radiative forcing, temperatures and the rate of climate change emerges in the second half of the century.     

The Heterogeneous Reaction of NO<Subscript>2</Subscript> with NH<Subscript>4</Subscript>Cl: A Molecular Diffusion Tube Study

The heterogeneous interaction of nitrogen dioxide with ammonium chloride was investigated in a molecular diffusion tube experiment at 295–335 K and interpreted using Monte Carlo trajectory calculations. The surface residence time (τ_surf) of NO₂ on NH₄Cl is equal to 15 μs at 295 K, increases with temperature up to 323 K (τ_surf = 45 μs) and probably decreases beyond 323 K. The same experiment also yields uptake coefficients, γ, which are derived from the absolute number of surviving molecules effusing out of the diffusion tube. The rate of uptake of NO₂ on NH₄Cl followed a rate law first order in [NO₂] and the uptake coefficient γ is equal to 7 × 10⁻⁵ at 295 K, increases with temperature up to 323 K (γ = 2.1 × 10⁻⁴) and probably decreases beyond 323 K. Nitrous acid, water and nitrogen were detected as products. From these products, it is concluded that the reaction of NO₂ with NH₄Cl is a reverse disproportionation reaction where two moles of NO₂ result in ammonium nitrite, NH₄NO₂, as an intermediate, and nitryl chloride, NO₂Cl. NH₄NO₂ decomposes in two pathways, one to nitrous acid, HONO and NH₃, the other to nitrogen and water. The branching ratio for the production of HONO + NH₃ to that of N₂ + H₂O is approximately 20 at 298 K and increases with increasing temperature.     

A climatological study of the nocturnal planetary boundary layer

Seventy-five nights of fast-response wind and temperature data taken from a 300 m tower near Augusta, GA, were analyzed to determine the time-height structure of the nocturnal planetary boundary layer. The nights were selected from all four seasons over a wide range of synoptic conditions. Statistical summaries of Pasquill-Gifford stability, boundary-layer depth, nocturnal jet height, directional shear, gravity wave occurrence, and azimuthal meandering were obtained. The diversity of nocturnal conditions for the 75 cases resulted in histograms with broad peaks and slowly-varying distributions.To reduce the overall variance, we grouped the nights into two classes: steady nights and unsteady nights. Nights classified as steady maintained relatively uniform wind conditions. The data base was large enough to permit a further breakdown of the steady nights into three subclasses based on the height and strength of the wind maximum. Unsteady nights were more disturbed, showing time-dependent features in the wind field and were also divided into three subclasses, depending on the predominant features observed: microfrontal passage, trend, or variable conditions. Although the subclasses were based mainly on wind structure, they correlated well with other NPBL properties, such as mixed-layer depth and inversion strength. Thus, the classification procedure tended to group together nights with similar dispersion characteristics.     

Estimation of hydroxyl radical concentrations in the marine atmospheric boundary layer using a reactive atmospheric tracer

Hydroxyl radical (OH) concentrations in the atmospheric boundary layer over a number of remote ocean locations are calculated from the measured diurnal variation in atmospheric dimethylsulfide (DMS). By using averaged DMS data sets from extended periods, the calculation yields OH concentrations averaged over periods from several days to weeks. These average OH concentrations range from 7×10⁵ to 2.9×10⁶ molecules cm^-3, corresponding to midday maxima of 3 to 12×10⁶ molecules cm^-3. The lowest values correspond to studies with the lowest light intensity (Antarctic summer and South Atlantic winter), and the highest values to regions with probable anthropogenic influence. In addition to the long term averages, daily average OH levels can be calculated for most days in a two week period from a cruise in the tropical eastern Pacific. These calculations are in good argeement with global average OH levels derived from other tracers, and are consistent with model OH calculations when allowance is made for variation in ambient ozone levels between the studies. Estimates of gas exchange made from the diurnal variation of DMS suggest that either the gas exchange coefficient of DMS or the boundary layer mixing depth may have been overestimated in past analyses.     

Urban Atmospheric Chemistry During the PUMA Campaign 2: Radical Budgets for OH,HO<Subscript>2</Subscript> and RO<Subscript>2</Subscript>

A detailed photochemical box model was used to investigate the key reaction pathways between OH, HO₂ and RO₂ radicals during the summer and winter PUMA field campaigns in the urban city-centre of Birmingham in the UK. The model employed the most recent version of the Master Chemical Mechanism and was constrained to 15-minute average measurements of long-lived species determined in situ at the site. The results showed that in the summer, OH initiation was dominated by the reactions of ozone with alkenes, nitrous acid (HONO) photolysis and the reaction of excited oxygen atoms atoms with water. In the winter, ozone+alkene reactions were the primary initiation route, with a minor contribution from HONO photolysis. Photolysis of aldehydes was the main initiation route for HO₂, in both summer and winter. RO₂ initiation was dominated by the photolysis of aldehydes in the summer with a smaller contribution from ozone+alkenes, a situation that was reversed in the winter. At night, ozone+alkene reactions were the main radical source. Termination, under all conditions, primarily involved reactions with NO (OH) and NO₂ (OH and RCO₃). These results demonstrate the importance of ozone+alkene reactions in urban atmospheres, particularly when photolysis reactions were less important during winter and at nighttime. The implications for urban atmospheric chemistry are discussed.     

Effects of mesoscale sea-surface temperature fronts on the marine atmospheric boundary layer

A numerical modelling study is presented focusing on the effects of mesoscale sea-surface temperature (SST) variability on surface fluxes and the marine atmospheric boundary-layer structure. A basic scenario is examined having two regions of SST anomaly with alternating warm/cold or cold/warm water regions. Conditions upstream from the anomaly region have SST values equal to the ambient atmosphere temperature, creating an upstream neutrally stratified boundary layer. Downstream from the anomaly region the SST is also set to the ambient atmosphere value. When the warm anomaly is upstream from the cold anomaly, the downstream boundary layer exhibits a more complex structure because of convective forcing and mixed layer deepening upstream from the cold anomaly. An internal boundary layer forms over the cold anomaly in this case, generating two distinct layers over the downstream region. When the cold anomaly is upstream from the warm anomaly, mixing over the warm anomaly quickly destroys the shallow cold layer, yielding a more uniform downstream boundary-layer vertical structure compared with the warm-to- cold case. Analysis of the momentum budget indicates that turbulent momentum flux divergence dominates the velocity field tendency, with pressure forcing accounting for only about 20% of the changes in momentum. Parameterization of surface fluxes and boundary-layer structure at these scales would be very difficult because of their dependence on subgrid-scale SST spatial order. Simulations of similar flow over smaller scale fronts (<5 km) suggest that small-scale SST variability might be parameterized in mesoscale models by relating the effective heat flux to the strength of the SST variance.     

A parameterization of the stable atmospheric boundary layer

Two formulations of the stable atmospheric boundary layer are proposed for use in weather forecasting or climate models. They feature the log-linear profile near the surface, but are free from the associated critical Richardson number. The diffusion coefficients in the Ekman layer are a natural extension of the surface layer. They are locally determined using wind shear in one case and turbulent kinetic energy in the other. The parameterizations are tested in a one-dimensional model simulating the evolution of the nocturnal boundary layer with and without radiative cooling. Both formulations give very similar results, except near the top of the boundary layer where the transition to the free atmosphere is smoother with the wind shear formulation. A distinctive feature of these schemes is that they retain their simulating skill when resolution is reduced. This is verified for a wide range of situations. In practice, this means that there is no need for a large-scale model to have a level below 50 m or so.     

Consequences of Uncertainties in CO<Subscript>2</Subscript> Density for Estimating Net Ecosystem CO<Subscript>2</Subscript> Exchange by Open-path Eddy Covariance

Errors in the estimation of CO₂ surface exchange by open-path eddy covariance, introduced during the removal of density terms [Webb et al. Quart J Roy Meteorol Soc 106:85–100, (1980) - WPL], can happen both because of errors in energy fluxes [Liu et al. Boundary-Layer Meteorol 120:65–85, (2006)] but also because of inaccuracies in other terms included in the density corrections, most notably due to measurements of absolute CO₂ density (ρ _c ). Equations are derived to examine the propagation of all errors through the WPL algorithm. For an open-path eddy covariance system operating in the Sierra de Gádor in south-east Spain, examples are presented of the inability of an unattended, open-path infrared gas analyzer (IRGA) to reliably report ρ _c and the need for additional instrumentation to determine calibration corrections. A sensitivity analysis shows that relatively large and systematic errors in net ecosystem exchange (NEE) can result from uncertainties in ρ _c in a semi-arid climate with large sensible heat fluxes (H _s ) and (wet) mineral deposition. When ρ_c is underestimated by 5% due to lens contamination, this implies a 13% overestimation of monthly CO₂ uptake.     

Some parameterizations of the nocturnal boundary layer

The evolution and structure of a steady barotropic nocturnal boundary layer are investigated using a higher-order turbulence closure model which includes equations for the mean quantities, turbulence convariances, and the viscous dissipation rate. The results indicate that a quasi-steady nocturnal PBL might be established in 4–10 hours after transition, depending on surface cooling rate. The latter is assumed to be constant in the model. The emphasis is on prediction of eddy viscosity, nocturnal mixing-layer depth, and the stability-dependent universal functions in the geostrophic drag and heat transfer relations. The model predictions are parameterized in the framework of the PBL similarity theory and compared with observations and results of other models.Affiliation with Oak Ridge Associated Universities (ORAU).