http://www.atmos-chem-phys-discuss.net/Atmospheric Chemistry and Physics Discussions Latest Articlesenhttp://www.atmos-chem-phys-discuss.net/12/4165/2012/<b>Atmospheric histories and growth trends of C₄F₁₀, C₅F₁₂, C₆F₁₄, C₇F₁₆ and C₈F₁₈</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 4165-4184, 2012<br /><br />Author(s): D. J. Ivy, T. Arnold, C. M. Harth, L. P. Steele, J. Mühle, M. Rigby, P. K. Salameh, M. Leist, P. B. Krummel, P. J. Fraser, R. F. Weiss, and R. G. Prinn<br /><br />The first atmospheric observations and trends are presented for the high molecular weight perfluorocarbons (PFCs): decafluorobutane (C₄F₁₀), dodecafluoropentane (C₅F₁₂), tetradecafluorohexane (C₆F₁₄), hexadecafluoroheptane (C₇F₁₆) and octadecafluorooctane (C₈F₁₈). Their atmospheric histories are based on measurements of 38 Northern Hemisphere and 46 Southern Hemisphere archived air samples collected between 1973 to 2011 using the Advanced Global Atmospheric Gases Experiment (AGAGE) "Medusa" preconcentration gas chromatography-mass spectrometry systems. A new calibration scale was prepared for each PFC, with estimated accuracies of 6.8% for C₄F₁₀, 7.8% for C₅F₁₂, 4.0% for C₆F₁₄, 6.6% for C₇F₁₆ and 7.9% for C₈F₁₈. Based on our observations the 2011 globally averaged dry air mole fractions of these heavy PFCs are: 0.18 parts-per-trillion (ppt, i.e., parts per 10¹²) for C₄F₁₀, 0.12 ppt for C₅F₁₂, 0.28 ppt for C₆F₁₄, 0.12 ppt for C₇F₁₆ and 0.09 ppt for C₈F₁₈. These atmospheric mole fractions combine to contribute to a global average radiative forcing of 0.35 mW m⁻², which is 3.6% of the total PFC radiative forcing. The globally averaged mean atmospheric growth rates of these PFCs during 1973–2011 are 4.58 parts per quadrillion (ppq, i.e., parts per 10¹⁵) per year (yr) for C₄F₁₀, 3.29 ppq yr⁻¹ for C₅F₁₂, 7.50 ppq yr⁻¹ for C₆F₁₄, 3.19 ppq yr⁻¹ for C₇F₁₆ and 2.51 ppq yr⁻¹ for C₈F₁₈. The growth rates of the heavy perfluorocarbons were largest in the early 1990s for C₄F₁₀ and C₅F₁₂ and in the mid-to-late 1990s for C₆F₁₄, C₇F₁₆ and C₈F₁₈. The more recent slow down in the growth rates of the high molecular weight PFCs suggests that emissions are declining as compared to the 1980s and 1990s. Nevertheless continued monitoring of these potent, extremely long-lived greenhouse gases is necessary to verify that global PFC emissions continue to decline.Fri, 03 Feb 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/4101/2012/<b>Central Arctic atmospheric summer conditions during the Arctic Summer Cloud Ocean Study (ASCOS): contrasting to previous expeditions</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 4101-4164, 2012<br /><br />Author(s): M. Tjernström, C. E. Birch, I. M. Brooks, M. D. Shupe, P. O. G. Persson, J. Sedlar, T. Mauritsen, C. Leck, J. Paatero, M. Szczodrak, and C. R. Wheeler<br /><br />Understanding the rapidly changing climate in the Arctic is limited by a lack of understanding of underlying strong feedback mechanisms that are specific to the Arctic. Progress in this field can only be obtained by process-level observations; this is the motivation for intensive ice-breaker-based campaigns such as that described in this paper: the Arctic Summer Cloud-Ocean Study (ASCOS). However, detailed field observations also have to be put in the context of the larger-scale meteorology, and short field campaigns have to be analysed within the context of the underlying climate state and temporal anomalies from this. <br></br> To aid in the analysis of other parameters or processes observed during this campaign, this paper provides an overview of the synoptic-scale meteorology and its climatic anomaly during the ASCOS field deployment. It also provides a statistical analysis of key features during the campaign, such as some key meteorological variables, the vertical structure of the lower troposphere and clouds, and energy fluxes at the surface. In order to assess the representativity of the ASCOS results, we also compare these features to similar observations obtained during three earlier summer experiments in the Arctic Ocean, the AOE-96, SHEBA and AOE-2001 expeditions. <br></br> We find that these expeditions share many key features of the summertime lower troposphere. Taking ASCOS and the previous expeditions together, a common picture emerges with a large amount of low-level cloud in a well-mixed shallow boundary layer, capped by a weak to moderately strong inversion where moisture, and sometimes also cloud top, penetrate into the lower parts of the inversion. Much of the boundary-layer mixing is due to cloud-top cooling and subsequent buoyant overturning of the cloud. The cloud layer may, or may not, be connected with surface processes depending on the depths of the cloud and surface-based boundary layers and on the relative strengths of surface-shear and cloud-buoyancy turbulence generation. The latter also implies a connection between the cloud layer and the free troposphere through entrainment at cloud top.Fri, 03 Feb 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/4073/2012/<b>Distributions, long term trends and emissions of four perfluorocarbons in remote parts of the atmosphere and firn air</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 4073-4100, 2012<br /><br />Author(s): J. C. Laube, C. Hogan, M. J. Newland, F. S. Mani, P. J. Fraser, C. A. M. Brenninkmeijer, P. Martinerie, D. E. Oram, T. Röckmann, J. Schwander, E. Witrant, G. P. Mills, C. E. Reeves, and W. T. Sturges<br /><br />We report the first data set of atmospheric abundances for the following four perfluoroalkanes: <i>n</i>-decafluorobutane (<i>n</i>-C₄F₁₀), <i>n</i>-dodecafluoropentane (<i>n</i>-C₅F₁₂), <i>n</i>-tetradecafluorohexane (<i>n</i>-C₆F₁₄) and <i>n</i>-hexadecafluoroheptane (<i>n</i>-C₇F₁₆). All four compounds could be detected and quantified in air samples from remote locations in the Southern Hemisphere (at Cape Grim, Tasmania, archived samples dating back to 1978) and the upper troposphere (a passenger aircraft flying from Germany to South Africa). Further observations originate from air samples extracted from deep firn in Greenland and allow trends of atmospheric abundances in the earlier 20th century to be inferred. All four compounds were not present in the atmosphere prior to the 1960s. <i>n</i>-C₄F₁₀ and <i>n</i>-C₅F₁₂ were also measured in samples collected in the stratosphere confirming their very long atmospheric lifetimes. Emissions were inferred from these observations and found to be comparable with emissions from the EDGAR database for <i>n</i>-C₆F₁₄. However, emissions of <i>n</i>-C₄F₁₀, <i>n</i>-C₅F₁₂ and <i>n</i>-C₇F₁₆ were found to differ by up to five orders of magnitude. Although the abundances of the four perfluorocarbons reported here are currently small (less than 0.3 ppt, parts per trillion) they have strong Global Warming Potentials several thousand times higher than carbon dioxide and continue to increase in the atmosphere. The sum of their cumulative emissions reached 325 mt (million metric tonnes) CO₂ equivalent at the end of 2009.Fri, 03 Feb 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/4031/2012/<b>Retrieval of aerosol optical depth over land based on a time series technique using MSG/SERIVI data</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 4031-4071, 2012<br /><br />Author(s): L. Mei, Y. Xue, G. de Leeuw, T. Holzer-Popp, J. Guang, Y. Li, L. Yang, H. Xu, X. Xu, C. Li, Y. Wang, C. Wu, T. Hou, X. He, J. Liu, J. Dong, and Z. Chen<br /><br />A novel approach for the joint retrieval of aerosol optical depth (AOD) and surface reflectance, using Meteosat Second Generation – Spinning Enhanced Visible and Infrared Imagers (MSG/SEVIRI) observations in two solar channels, is presented. The retrieval is based on a time series (TS) technique, which makes use of the two visible bands at 0.6 μm and 0.8 μm in three orderly scan times (15 min interval between two scans) to retrieve the AOD over land. Using the radiative transfer equation for plane-parallel atmospheres two coupled differential equations for the upward and downward fluxes are derived. The boundary conditions for the upward and downward fluxes at the top and at the bottom of the atmosphere are used in these equations to provide an analytic solution for the surface reflectance. To derive these fluxes, the aerosol single scattering albedo (SSA) and asymmetry factor are required to provide a solution. These are provided from a set of six pre-defined aerosol types with the SSA and asymmetry factor (<i>g</i>). We assume one aerosol type for a grid of 1° × 1° and the surface reflectance changes little between two consequent scans. A <i>k</i> approximation was used in the inversion to find the best solution of atmospheric properties and surface reflectance. The algorithm makes use of numerical minimisation routines to obtain the optimal solution of atmospheric properties and surface reflectance by selection of the most suitable aerosol type from pre-defined sets. Also, it is assumed that the surface reflectance is little influenced by aerosol scattering at 1.6 μm and therefore the ratio of surface reflectances in the solar band for two consequent scans can be well-approximated by the ratio of the reflectances at 1.6 μm. A further assumption is that the surface reflectance varies only slightly over a period of 30 min. <br><br> A detailed analysis of the retrieval results show that it is suitable for AOD retrieval over land. Six Aerosol Robotic Network (AERONET) sites with different surface types were used for detailed analysis and 42 other AERONET sites were used for validation. From 445 collocations representing stable and homogeneous aerosol type, we found that >75% of MSG-retrieved AOD values compared to AERONET observed values with an error envelope of &plusmn;0.05 &plusmn; 0.15&tau; and a high correlation (<i>R</i> > 0.86). The AOD datasets derived using the TS method with SEVIRI data was also compared with collocated AOD products derived from the NASA TERRA and AQUA MODIS data using the dark dense vegetation (DDV) method and the Deep Blue algorithms. Using the TS method, AOD could be retrieved for more pixels than with the NASA Deep Blue algorithm. The AOD values derived compare favourably.Fri, 03 Feb 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3983/2012/<b>MAX-DOAS measurements of NO₂, HCHO and CHOCHO at a rural site in Southern China</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3983-4029, 2012<br /><br />Author(s): X. Li, T. Brauers, A. Hofzumahaus, K. Lu, Y. P. Li, M. Shao, T. Wagner, and A. Wahner<br /><br />We performed MAX-DOAS measurements during the PRIDE-PRD2006 campaign in the Pearl River Delta region (PRD), China, for 4 weeks in July 2006 at a site located 60 km north of Guangzhou. The vertical distributions of NO₂, HCHO, and CHOCHO were independently retrieved by an automated iteration method. The MAX-DOAS measured NO₂ mixing ratios showed reasonable agreement with the simultaneous, ground based in-situ data. While the tropospheric NO₂ vertical column densities (VCDs) observed by OMI on board EOS-Aura satellite agreed with those by MAX-DOAS, the 3-D chemical transport model CMAQ overestimated the NO₂ VCDs as well as the surface concentrations by about 40%. From this observation, a reduction of NO_X emission strength in CMAQ seems to be necessary in order to well reproduce the NO₂ observations. The average mixing ratios of HCHO and CHOCHO were 12 ppb and 1.6 ppb, respectively, substantially higher than in other rural or semirural environments. The high ratio of 0.135 between CHOCHO and HCHO corresponds to the high VOCs reactivity and high HO_X turnover rate consistent with other observations during the campaign.Fri, 03 Feb 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3941/2012/<b>Tropospheric methanol observations from space: retrieval evaluation and constraints on the seasonality of biogenic emissions</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3941-3982, 2012<br /><br />Author(s): K. C. Wells, D. B. Millet, L. Hu, K. E. Cady-Pereira, Y. Xiao, M. W. Shephard, C. L. Clerbaux, L. Clarisse, P.-F. Coheur, E. C. Apel, J. de Gouw, C. Warneke, H. B. Singh, A. H. Goldstein, and B. C. Sive<br /><br />Methanol retrievals from nadir-viewing space-based sensors offer powerful new information for quantifying methanol emissions on a global scale. Here we apply an ensemble of aircraft observations over North America to evaluate new methanol measurements from the Tropospheric Emission Spectrometer (TES) on the Aura satellite, and combine the TES data with observations from the Infrared Atmospheric Sounding Interferometer (IASI) on the MetOp-A satellite to investigate the seasonality of methanol emissions from northern midlatitude ecosystems. Using the GEOS-Chem chemical transport model as an intercomparison platform, we find that the TES retrieval performs well when the degrees of freedom for signal (DOFS) are above 0.5, in which case the model : TES regressions are generally consistent with the model : aircraft comparisons. Including retrievals with DOFS below 0.5 degrades the comparisons, as these are excessively influenced by the a priori. The comparisons suggest DOFS > 0.5 as a minimum threshold for interpreting retrievals of trace gases with a weak tropospheric signal. We analyze one full year of satellite observations and find that GEOS-Chem, driven with MEGANv2.1 biogenic emissions, underestimates observed methanol concentrations throughout the midlatitudes in springtime, with the timing of the seasonal peak in model emissions 1–2 months too late. We attribute this discrepancy to an underestimate of emissions from new leaves in MEGAN, and apply the satellite data to better quantify the seasonal change in methanol emissions for midlatitude ecosystems. The derived parameters (relative emission factors of 11.0, 1.0, 0.05 and 8.6 for new, growing, mature, and old leaves, respectively, plus a leaf area index activity factor of 0.75 for expanding canopies with leaf area index < 2.0) provide a more realistic simulation of seasonal methanol concentrations in midlatitudes on the basis of IASI, TES, and ground-based measurements.Fri, 03 Feb 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3875/2012/<b>Projections of mid-century summer air-quality for North America: effects of changes in climate and precursor emissions</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3875-3940, 2012<br /><br />Author(s): J. Kelly, P. A. Makar, and D. A. Plummer<br /><br />Ten year simulations of North American current and future air-quality were carried out using a regional air-quality model driven by a regional climate model, in turn driven by a general circulation model. Three separate summer scenarios were performed: a scenario representing the years 1997 to 2006, and two SRES A2 climate scenarios for the years 2041 to 2050. The first future climate scenario makes use of 2002 anthropogenic precursor emissions, and the second applied emissions scaling factors derived from the IPCC Representative Concentration Pathway 6 (RCP 6) scenario to estimate emissions for 2050 from existing 2020 projections. Ten-year averages of ozone and PM_2.5 at North American monitoring network stations were used to evaluate the model's current chemical climatology. The model was found to have a similar performance for ozone as when driven by an operational weather forecast model. The PM_2.5 predictions had larger negative biases, likely resulting from the absence of rainwater evaporation, and from sub-regional negative biases in the surface temperature fields, in the version of the climate model used here. <br><br> The differences between the two future climate simulations and the current climate simulation were used to predict the changes to air-quality that might be expected in a future warmer climate, if anthropogenic precursor emissions remain constant at their current levels, versus if the RCP 6 emissions controls were adopted. Metrics of concentration, human health, and ecosystem damage were compared for the simulations. The scenario with future climate and current anthropogenic emissions resulted in worse air-quality than for current conditions – that is, the effect of climate-change alone, all other factors being similar, would be a worsening of air-quality. These effects are spatially inhomogeneous, with the magnitude and sign of the changes varying with region. The scenario with future climate and RCP 6 emissions for 2050 resulted in an improved air-quality, with decreases in key pollutant concentrations, in acute human mortality associated with air-pollution, and in sulphur and ozone deposition to the ecosystem. The positive outcomes of the RCP 6 emissions reductions were found to be of greater magnitude than the negative outcomes of climate change alone. The RCP 6 scenario however resulted in an increase in the deposition of nitrogen, as a result of increased ammonia emissions expected in that scenario, compared to current ammonia emissions levels. <br><br> The results of the study raise the possibility that simultaneous reductions of greenhouse gases and air pollution precursors may further reduce air pollution levels, with the added benefits of an immediate reduction in the impacts of air pollution on human and ecosystem health. Further scenarios to investigate this possibility are therefore recommended.Fri, 03 Feb 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3781/2012/<b>The EMEP MSC-W chemical transport model &ndash; Part 1: Model description</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3781-3874, 2012<br /><br />Author(s): D. Simpson, A. Benedictow, H. Berge, R. Bergström, L. D. Emberson, H. Fagerli, G. D. Hayman, M. Gauss, J. E. Jonson, M. E. Jenkin, A. Nyíri, C. Richter, V. S. Semeena, S. Tsyro, J.-P. Tuovinen, Á. Valdebenito, and P. Wind<br /><br />The Meteorological Synthesizing Centre-West (MSC-W) of the European Monitoring and Evaluation Programme (EMEP) has been performing model calculations in support of the Convention on Long Range Transboundary Air Pollution (CLRTAP) for more than 30 yr. The EMEP MSC-W chemical transport model is still one of the key tools within European air pollution policy assessments. <br><br> Traditionally, the EMEP model has covered all of Europe with a resolution of about 50 × 50 km², and extending vertically from ground level to the tropopause (100 hPa). The model has undergone substantial development in recent years, and is now applied on scales ranging from local (ca. 5 km grid size) to global (with 1 degree resolution). The model is used to simulate photo-oxidants and both inorganic and organic aerosols. <br><br> In 2008 the EMEP model was released for the first time as public domain code, along with all required input data for model runs for one year. Since then, many changes have been made to the model physics, and input data. The second release of the EMEP MSC-W model became available in mid 2011, and a new release is targeted for early 2012. This publication is intended to document this third release of the EMEP MSC-W model. The model formulations are given, along with details of input data-sets which are used, and brief background on some of the choices made in the formulation are presented. The model code itself is available at <a href="http://www.emep.int"target="_blank">www.emep.int</a>, along with the data required to run for a full year over Europe.Thu, 02 Feb 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3731/2012/<b>Lessons learnt from the first EMEP intensive measurement periods</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3731-3780, 2012<br /><br />Author(s): W. Aas, S. Tsyro, E. Bieber, R. Bergström, D. Ceburnis, T. Ellermann, H. Fagerli, M. Frölich, R. Gehrig, U. Makkonen, E. Nemitz, R. Otjes, N. Perez, C. Perrino, A. S. H. Prévôt, J.-P. Putaud, D. Simpson, G. Spindler, M. Vana, and K. E. Yttri<br /><br />The first EMEP intensive measurement periods were held in June 2006 and January 2007. The measurements aimed to characterize the aerosol chemical compositions, including the gas/aerosol partitioning of inorganic compounds. The measurement program during these periods included daily or hourly measurements of the secondary inorganic components, with additional measurements of elemental- and organic carbon (EC and OC) and mineral dust in PM₁, PM_2.5 and PM₁₀. These measurements have provided extended knowledge regarding the composition of particulate matter and the temporal and spatial variability of PM, as well as an extended database for the assessment of chemical transport models. This paper summarise the first experiences of making use of measurements from the first EMEP intensive measurement periods along with EMEP model results from the updated model version to characterise aerosol composition. We investigated how the PM chemical composition varies between the summer and the winter month and geographically. <br><br> The observation and model data are in general agreement regarding the main features of PM₁₀ and PM_2.5 composition and the relative contribution of different components, though the EMEP model tends to slightly underestimate PM₁₀ and PM_2.5 compared to measurements. The intensive measurement data has identified areas where improvements are needed. In particular, the model description of formation of coarse nitrate on sea salt and dust particles requires further attention. Hourly concurrent measurements of gaseous and particulate components for the first time facilitated testing of modelled diurnal variability of the gas/aerosol partitioning of nitrogen species. In general, the modelled diurnal cycles of nitrate and ammonium aerosols are in good agreement with the measurements. As expected, the diurnal variability of ammonia is not very well captured, but this will probably improve if the EMEP model is coupled to a dynamic, mechanistic ammonia emission module. The largest underestimations of aerosol mass are seen in Italy during winter, which to a large extent may be explained by an underestimation of residential wood burning source. It should be noted that both primary and secondary OC has been included in the calculations for the first time, showing promising results. Mineral dust is important, especially in southern Europe, and the model seems to capture the dust episodes well. The lack of measurements of mineral dust hampers the possibility for model evaluation for this highly uncertain PM component. <br><br> There are also lessons learnt regarding improved measurements for future intensive periods. There is a need for increased comparability between the measurements at different sites. For the nitrogen compounds it is clear that more measurements using artefact free methods based on continuous measurement methods and/or denuders are needed. For EC/OC, a reference methodology (both in field and laboratory) was lacking during these periods giving problems with comparability, though presently measurement protocols have been established and these should be followed by the Parties to the EMEP Protocol. For measurements with no defined protocols, it might be a good solution to use centralised laboratories to ensure comparability across the network. To cope with the introduction of these new measurements new reporting guidelines have been developed to ensure that all proper information about the methodologies and data quality is given.Thu, 02 Feb 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3695/2012/<b>Regional air-quality forecasting for the Pacific Northwest using MOPITT/TERRA assimilated carbon monoxide MOZART-4 forecasts as a near real-time boundary condition</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3695-3730, 2012<br /><br />Author(s): F. L. Herron-Thorpe, G. H. Mount, L. K. Emmons, B. K. Lamb, S. H. Chung, and J. K. Vaughan<br /><br />Results from a regional air quality forecast model, AIRPACT-3, were compared to AIRS carbon monoxide column densities for the spring of 2010 over the Pacific Northwest. AIRPACT-3 column densities showed high correlation (<i>R</i>>0.9) but were significantly biased (~25 %) with significant under-predictions for spring months with significant transport from Asia. The AIRPACT-3 CO bias relative to AIRS was eliminated by incorporating dynamic boundary conditions derived from NCAR's MOZART forecasts with assimilated MOPITT carbon monoxide. Changes in ozone-related boundary conditions derived from MOZART forecasts are also discussed and found to affect background levels by ±10 ppb but not found to significantly affect peak ozone surface concentrations.Thu, 02 Feb 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3655/2012/<b>Biogenic influence on cloud microphysics over the global ocean</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3655-3694, 2012<br /><br />Author(s): A. Lana, R. Simó, S. M. Vallina, and J. Dachs<br /><br />Aerosols have a large potential to influence climate through their effects on the microphysics and optical properties of clouds and, hence, on the Earth's radiation budget. Aerosol-cloud interactions have been intensively studied in polluted air, but the possibility that the marine biosphere plays a role in regulating cloud brightness in the pristine oceanic atmosphere remains largely unexplored. We used 9 yr of global satellite data and ocean climatologies to derive parameterizations of (a) production fluxes of sulfur aerosols formed by the oxidation of the biogenic gas dimethylsulfide emitted from the sea surface; (b) production fluxes of secondary organic aerosols from biogenic organic volatiles; (c) emission fluxes of biogenic primary organic aerosols ejected by wind action on sea surface; and (d) emission fluxes of sea salt also lifted by the wind upon bubble bursting. Series of global weekly estimates of these fluxes were correlated to series of cloud droplet effective radius data derived from satellite (MODIS). Similar analyses were conducted in more detail at 6 locations spread among polluted and clean regions of the oceanic atmosphere. The outcome of the statistical analysis was that negative correlation was common at mid and high latitude for sulfur and organic secondary aerosols, indicating both might be important in seeding cloud droplet activation. Conversely, primary aerosols (organic and sea salt) showed more variable, non-significant or positive correlations, indicating that, despite contributing to large shares of the marine aerosol mass, they are not major drivers of the variability of cloud microphysics. Uncertainties and synergisms are discussed, and recommendations of research needs are given.Thu, 02 Feb 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3619/2012/<b>Signals of El Niño Modoki in the tropical tropopause layer and stratosphere</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3619-3653, 2012<br /><br />Author(s): F. Xie, J. Li, W. Tian, and J. Feng<br /><br />The effects of El Niño Modoki events on the tropical tropopause layer (TTL) and on the stratosphere were investigated using European Center for Medium Range Weather Forecasting (ECMWF) reanalysis data, satellite observations from the Aura satellite Microwave Limb Sounder (MLS), oceanic El Niño indices, and general climate model outputs. El Niño Modoki events tend to depress convective activities in the western and eastern Pacific but enhance convective activities in the central and northern Pacific. Consequently, during Modoki events, negative water vapor anomalies occur in the western and eastern Pacific upper troposphere, whereas there are positive anomalies in the central and northern Pacific upper troposphere. The spatial patterns of the outgoing longwave radiation (OLR) and upper tropospheric water vapor anomalies exhibit a tripolar form. The empirical orthogonal function (EOF) analysis of the OLR and upper tropospheric water vapor anomalies reveals that canonical El Niño events are associated with the leading mode of the EOF, while El Niño Modoki events correspond to the second mode. El Niño Modoki activities tend to moisten the lower and middle stratosphere, but dry the upper stratosphere. It was also found that the canonical El Niño signal can overlay linearly on the QBO signal in the stratosphere, whereas the interaction between the El Niño Modoki and QBO signals is non-linear. Because of these non-linear interactions, El Niño Modoki events have a reverse effect on high latitudes stratosphere, as compared with the effects of typical Modoki events, i.e. the northern polar vortex is stronger and colder but the southern polar vortex is weaker and warmer during El Niño Modoki events. However, simulations suggest that canonical El Niño and El Niño Modoki activities actually have the same influence on high latitudes stratosphere, in the absence of interactions between QBO and ENSO signals. The present results also reveal that canonical El Niño events have a greater impact on the high-latitude Northern Hemisphere stratosphere than on the high-latitude Southern Hemisphere stratosphere. However, El Niño Modoki events can more profoundly influence the high-latitude Southern Hemisphere stratosphere than the high-latitude Northern Hemisphere stratosphere.Thu, 02 Feb 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3595/2012/<b>Effects of cosmic ray decreases on cloud microphysics</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3595-3617, 2012<br /><br />Author(s): J. Svensmark, M. B. Enghoff, and H. Svensmark<br /><br />Using cloud data from MODIS we investigate the response of cloud microphysics to sudden decreases in galactic cosmic radiation – Forbush decreases – and find responses in effective emissivity, cloud fraction, liquid water content, and optical thickness above the 2–3 sigma level 6–9 days after the minimum in atmospheric ionization and less significant responses for effective radius and cloud condensation nuclei (<2 sigma). The magnitude of the signals agree with derived values, based on simple equations for atmospheric parameters. Furthermore principal components analysis gives a total significance of the signal of 3.1 sigma. We also see a correlation between total solar irradiance and strong Forbush decreases but a clear mechanism connecting this to cloud properties is lacking. There is no signal in the UV radiation. The responses of the parameters correlate linearly with the reduction in the cosmic ray ionization. These results support the suggestion that ions play a significant role in the life-cycle of clouds.Wed, 01 Feb 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3555/2012/<b>A mechanistic model of global soil nitric oxide emissions: implementation and space based-constraints</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3555-3594, 2012<br /><br />Author(s): R. C. Hudman, N. E. Moore, R. V. Martin, A. R. Russell, A. K. Mebust, L. C. Valin, and R. C. Cohen<br /><br />Soil emissions have been identified as a major source (~15%) of global nitrogen oxide (NO_x) emissions. Parameterizations of soil NO_x emissions (<i>S</i>_{NO_x}) for use in the current generation of chemical transport models were designed to capture mean seasonal behaviour. These parameterizations do not, however, respond quantitatively to the meteorological triggers that result in pulsed <i>S</i>_{NO_x} as are widely observed. Here we present a new mechanistic parameterization of <i>S</i>_{NO_x} implemented into a global chemical transport model (GEOS-Chem). The parameterization represents available nitrogen (N) in soils using biome specific emission factors, online wet- and dry-deposition of N as well as fertilizer and manure N derived from a spatially explicit dataset distributed using seasonality derived from data obtained by the Moderate Resolution Imaging Spectrometer. Moreover, it represents the functional form of emissions derived from point measurements and ecosystem scale experiments including pulsing following soil wetting by rain or irrigation, and emissions that are a smooth function of soil moisture. This parameterization yields global above-soil <i>S</i>_{NO_x} of 10.7 Tg N yr⁻¹, including 1.8 Tg N yr⁻¹ from fertilizer N input (0.68% of applied N) and 0.5 Tg N yr⁻¹ from atmospheric N deposition. Over the United States Great Plains, <i>S</i>_{NO_x} are predicted to comprise 15–40% of the tropospheric NO₂ column and increase column variability by a factor of 2–4 during the summer months due to chemical fertilizer application and warm temperatures. <i>S</i>_{NO_x} enhancements of 50–80% of the simulated NO₂ column are predicted over the African Sahel during the monsoon onset (April–June). In this region the day-to-day variability of column NO₂ is increased by a factor of 5 due to pulsed-N emissions. We evaluate the model by comparison to observations of the NO₂ column from the OMI instrument. We find the model is able to reproduce observations of pulsed-N induced interannual variability over the US Great Plains. We also show that the OMI mean (median) NO₂ on the overpass following first rainfall over the Sahel is 49% (23%) higher than in the five days preceding. The measured NO₂ on the day after rainfall is still 23% (5%) higher, providing a direct measure of the pulse's decay time of 1–2 days. This is consistent with the pulsing representation used in our parameterization and much shorter than 5–14 day pulse decay length used in current models.Wed, 01 Feb 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3509/2012/<b>Black carbon from ships: a review of the effects of ship speed, fuel quality and exhaust gas scrubbing</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3509-3554, 2012<br /><br />Author(s): D. A. Lack and J. J. Corbett<br /><br />The International Maritime Organization (IMO) has moved to address the health and climate impact of the emissions from the combustion of low-quality residual fuels within the commercial shipping industry. Fuel sulfur content (<i>F</i>_S) limits and an efficiency design index for future ships are examples of such IMO actions. The impacts of black carbon (BC) emissions from shipping are now under review by the IMO, with a particular focus on the potential impacts of future Arctic shipping. <br><br> Recognizing that associating impacts with BC emissions requires both ambient and onboard observations, we provide recommendations for the measurement of BC. We also evaluate current insights regarding the effect of ship speed (engine load), fuel quality and exhaust gas scrubbing on BC emissions from ships. Observations demonstrate that BC emission factors (EF_BC) increases 3 to 6 times at very low engine loads (<25% compared to EF_BC at 85–100% load); absolute BC emissions (per nautical mile of travel) also increase up to 100% depending on engine load, even with reduced load fuel savings. If fleets were required to operate at lower maximum engine loads, presumably associated with reduced speeds, then engines could be re-tuned, which would reduce BC emissions. <br><br> Ships operating in the Arctic are likely running at highly variable engine loads (25–100%) depending on ice conditions and ice breaking requirements. The ships operating at low load may be emitting up to 50% more BC than they would at their rated load. Such variable load conditions make it difficult to assess the likely emissions rate of BC. <br><br> Current fuel sulfur regulations have the effect of reducing EF_BC by an average of 30% and potentially up to 80% regardless of engine load; a removal rate similar to that of scrubbers. <br><br> Uncertainties among current observations demonstrate there is a need for more information on (a) the impact of fuel quality on EF_BC using robust measurement methods and (b) the efficacy of scrubbers for the removal of particulate matter by size and composition.Tue, 31 Jan 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3467/2012/<b>Interaction of anthropogenic and natural emission sources during a wild-land fire event – influence on ozone formation</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3467-3507, 2012<br /><br />Author(s): E. Bossioli, M. Tombrou, A. Karali, A. Dandou, D. Paronis, and M. Sofiev<br /><br />The objective of this study is to investigate the contribution of biomass burning in the formation of tropospheric O₃. Furthermore, the impact of biogenic emissions under fire and no fire conditions is examined. This is achieved by applying the CAMx chemistry transport model for a wild-land fire event over Western Russia (24 April–10 May 2006). The model results are compared with O₃ and isoprene observations from 117 and 9 stations of the EMEP network, respectively. <br><br> Model computations show that the fire episode altered the O₃ sensitivity in the area. In particular, the fire emissions increased surface O₃ over Northern and Eastern Europe by up to 80% (40–45 ppb). In case of adopting a high fire NO_x/CO emission ratio (0.06), the area (Eastern Europe and Western Russia) is characterized by VOC-sensitive O₃ production and the impact of biogenic emissions is proven significant, contributing up to 8 ppb. Under a lower ratio (0.025), total surface O₃ is almost doubled due to higher O₃ production at the fire spots and lower fires' NO emissions. In this case as well as in the absence of fires, the impact of biogenic emissions is almost negligible. Injection height of the fire emissions accounted for O₃ differences of the order of 10%, both at surface and over the planetary boundary layer (PBL).Tue, 31 Jan 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3409/2012/<b>The scavenging processes controlling the seasonal cycle in Arctic sulphate and black carbon aerosol</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3409-3465, 2012<br /><br />Author(s): J. Browse, K. S. Carslaw, S. R. Arnold, K. Pringle, and O. Boucher<br /><br />The seasonal cycle in Arctic aerosol is typified by high concentrations of large aged anthropogenic particles transported from lower latitudes in the late Arctic winter and early spring followed by a sharp transition to low concentrations of locally sourced smaller particles in the summer. However, multi-model assessments show that many models fail to simulate a realistic cycle. Here, we use a global aerosol microphysics model and surface-level aerosol observations to understand how wet scavenging processes control the seasonal variation in Arctic black carbon (BC) and sulphate aerosol concentrations. We show that the transition from high wintertime to low summertime Arctic aerosol concentrations is caused by the change from inefficient scavenging in ice clouds to the much more efficient scavenging in warm liquid clouds. This seasonal cycle is amplified further by the appearance of warm drizzling cloud in late spring and summer at a time when aerosol transport shifts mainly to low levels. Implementing these processes in a model greatly improves the agreement between the model and observations at the three Arctic ground-stations Alert, Barrow and Zeppelin Mountain on Svalbard. The SO₄ model-observation correlation coefficient (<i>R</i>) increases from: &minus;0.33 to 0.71 at Alert (82.5&deg; N), from &minus;0.16 to 0.70 at Point Barrow (71.0&deg; N) and from &minus;0.42 to 0.40 at Zeppelin Mountain (78&deg; N) while, the BC model-observation correlation coefficient increases from &minus;0.68 to 0.72 at Alert and from &minus;0.42 to 0.44 at Barrow. Observations at three marginal Arctic sites (Janiskoski, Oulanka and Karasjok) indicate a far weaker aerosol seasonal cycle, which we show is consistent with the much smaller seasonal changes in ice clouds compared to the higher latitude sites. Our results suggest that the seasonal cycle in Arctic aerosol is driven by temperature-dependent scavenging processes that may be susceptible to modification in a future climate.Tue, 31 Jan 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3357/2012/<b>Uncertainties of parameterized near-surface downward longwave and clear-sky direct radiation</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3357-3407, 2012<br /><br />Author(s): S. Gubler, S. Gruber, and R. S. Purves<br /><br />As many environmental models rely on simulating the energy balance at the Earth's surface based on parameterized radiative fluxes, knowledge of the inherent uncertainties is important. In this study we evaluate one parameterization of clear-sky incoming shortwave radiation (SDR) and diverse parameterizations of clear-sky and all-sky incoming longwave radiation (LDR). In a first step, the clear-sky global SDR is estimated based measured input variables and mean parameter values for hourly time steps during the year 1996 to 2008, and validated using the high quality measurements of seven Alpine Surface Radiation Budget (ASRB) stations in Switzerland covering different elevations. Then, twelve clear-sky LDR parameterizations are fitted to the ASRB measurements. One of the best performing LDR parameterizations is chosen to estimate the all-sky LDR based on cloud transmissivity. Cloud transmissivity is estimated using measured and modeled global SDR during daytime. For the night, the performance of several interpolation methods is evaluated. <br><br> Input variable and parameter uncertainties are assigned to estimate the total output uncertainty of the mentioned models, resulting in a mean relative uncertainty of 10% for the clear-sky direct, 15% for diffuse and 2.5% for global SDR, and 2.5% for the fitted all-sky LDR. Further, a function representing the uncertainty in dependence of the radiation is assigned for each model. Validation of the model outputs shows that direct SDR is underestimated (the mean error (ME) is around &minus;33 W m^&minus;2), while diffuse radiation is overestimated (ME around 19 W m^&minus;2). The root mean squared error (RMSE) scatters around 60 W m^&minus;2 for direct, and 40 W m^&minus;2 for diffuse SDR. The best behaviour is found, due to the compensating effects of direct and diffuse SDR, for global SDR with MEs around &minus;13 W m^&minus;2 and RMSEs around 40 W m^&minus;2. The ME of the fitted all-sky LDR is around &plusmn;10 W m^&minus;2, and the RMSE goes up to 40 W m^&minus;2. This is obtained by linearly interpolating the average of the cloud transmissivity of the four hours of the preceeding afternoon and the following morning.Tue, 31 Jan 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3295/2012/<b>Multi-generation gas-phase oxidation, equilibrium partitioning, and the formation and evolution of secondary organic aerosol</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3295-3356, 2012<br /><br />Author(s): C. D. Cappa and K. R. Wilson<br /><br />A new statistical model of secondary organic aerosol (SOA) formation is developed that explicitly takes into account multi-generational oxidation as well as fragmentation of gas-phase compounds. The model framework requires three tunable parameters to describe the kinetic evolution of SOA mass, the average oxygen-to-carbon atomic ratio and the mean particle volatility as oxidation proceeds. These parameters describe (1) the relationship between oxygen content and volatility, (2) the probability of fragmentation and (3) the amount of oxygen added per reaction. The time-evolution and absolute value of the SOA mass depends sensitively on all three tunable parameters. Of the tunable parameters, the mean O:C is most sensitive to the oxygen/volatility relationship, exhibiting only a weak dependence on the other two. The mean particle O:C produced from a given compound is primarily controlled by the number of carbon atoms comprising the SOA precursor. It is found that gas-phase compounds with larger than 11 carbon atoms are unlikely to form SOA with O:C values >0.4, which suggests that so-called "intermediate-volatility" organic compounds (IVOCs) and "semi-volatile" organic compounds (SVOCs) are not major contributors to the ambient SOA burden when high O:C ratios are observed, especially at short atmospheric times. The model is tested against laboratory measurements of SOA formation from the photooxidation of α-pinene and <i>n</i>-pentadecane and performs well (after tuning). This model may provide a generalized framework for the interpretation of laboratory SOA formation experiments in which explicit consideration of multiple-generations of products is required, which is true for all photo-oxidation experiments.Tue, 31 Jan 2012 00:00:00 +0100http://www.atmos-chem-phys-discuss.net/12/3263/2012/<b>Stochastic parameterization of dust emission and application to convective atmospheric conditions</b><br /><br />Atmospheric Chemistry and Physics Discussions, 12, 3263-3293, 2012<br /><br />Author(s): M. Klose and Y. Shao<br /><br />We develop a parameterization scheme of convective dust emission for regional and global atmospheric models. Convective dust emission occurs in the absence of saltation as large eddies intermittently produce strong shear stresses on the surface and thereby entrain dust particles into the air. The new scheme presented here takes account of the stochastic nature of convective dust emission. It consists of the statistical representations of soil particle size, inter-particle cohesion and instantaneous surface shear stress. A method of determining the probability density function of the latter quantity is proposed. Dust emission is then estimated from the overlap of the probability density functions of the aerodynamic lifting and inter-particle cohesive forces. The new scheme is implemented into the WRF/Chem model and applied to dust modeling in the Taklimakan Desert. A comparison with lidar data shows that the model can reproduce the main features of the dust patterns and their diurnal variations. For the case studied, convective dust emission is typically several μg m⁻² s⁻¹ and at times up to 50 μg m⁻² s⁻¹.Mon, 30 Jan 2012 00:00:00 +0100