http://www.atmos-chem-phys-discuss.net/Atmospheric Chemistry and Physics Discussions Latest Articlesenhttp://www.atmos-chem-phys-discuss.net/13/13285/2013/<b>Air/sea DMS gas transfer in the North Atlantic: evidence for limited interfacial gas exchange at high wind speed</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 13285-13322, 2013<br /><br />Author(s): T. G. Bell, W. De Bruyn, S. D. Miller, B. Ward, K. Christensen, and E. S. Saltzman<br /><br />Shipboard measurements of eddy covariance DMS air/sea fluxes and seawater concentration were carried out in the North Atlantic bloom region in June/July 2011. Gas transfer coefficients (<i>k</i>₆₆₀) show a linear dependence on mean horizontal wind speed at wind speeds up to 11 m s^&minus;1. At higher wind speeds the relationship between <i>k</i>₆₆₀ and wind speed weakens. At high winds, measured DMS fluxes were lower than predicted based on the linear relationship between wind speed and interfacial stress extrapolated from low to intermediate wind speeds. In contrast, the transfer coefficient for sensible heat did not exhibit this effect. The apparent suppression of air/sea gas flux at higher wind speeds appears to be related to sea state, as determined from shipboard wave measurements. These observations are consistent with the idea that long waves suppress near surface water side turbulence, and decrease interfacial gas transfer. This effect may be more easily observed for DMS than for less soluble gases, such as CO₂, because the air/sea exchange of DMS is controlled by interfacial rather than bubble-mediated gas transfer under high wind speed conditions.Tue, 21 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/13245/2013/<b>Quantifying tracer transport in the tropical lower stratosphere using WACCM</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 13245-13283, 2013<br /><br />Author(s): M. Abalos, W. J. Randel, D. E. Kinnison, and E. Serrano<br /><br />The zonal mean transport of ozone and carbon monoxide (CO) near the tropical tropopause is investigated using the Whole-Atmosphere Community Climate Model version 4 (WACCM4). The variability in temperature, ozone and CO in the model shows good agreement with satellite and balloon observations. Modeled temperature and tracers exhibit large and closely coupled annual cycles in the tropical lower stratosphere, as in the observations. The thermodynamic and tracer budgets in the model are analyzed based on the Transformed Eulerian Mean (TEM) framework on log-pressure coordinates and also using the isentropic formulation. Results show that the coupled seasonal cycles are mainly forced by tropical upwelling over altitudes with large vertical tracer gradients, in agreement with previous observational studies. The model also allows explicit calculation of eddy transport terms, which make an important contribution to ozone tendencies in the tropical lower stratosphere. The character of the eddy fluxes changes with altitude. At higher levels (~2 km above the cold point tropopause), isentropic eddy transport occurs during winter and spring in each hemisphere in the sub-tropics, associated with transient Rossby waves acting on strong background latitudinal gradients. At lower altitudes, close to the tropical tropopause, there is a maximum in horizontal eddy transport during boreal summer associated with the Asian monsoon anticyclone. Sub-seasonal variability in ozone and CO, tied to fluctuations in temperature, is primarily driven by transient tropical upwelling. In isentropic coordinates, the overall tracer budgets are similar to the log-pressure results, highlighting cross-isentropic mean advection as the main term in the balance. However, in isentropic coordinates the tracer variability is largely reduced on both seasonal and sub-seasonal timescales, because the tracer and temperature fluctuations are highly correlated (as a response to upwelling).Tue, 21 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/13191/2013/<b>Cloud and boundary layer interactions over the Arctic sea-ice in late summer</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 13191-13244, 2013<br /><br />Author(s): M. D. Shupe, P. O. G. Persson, I. M. Brooks, M. Tjernström, J. Sedlar, T. Mauritsen, S. Sjogren, and C. Leck<br /><br />Observations from the Arctic Summer Cloud Ocean Study (ASCOS), in the central Arctic sea-ice pack in late summer 2008, provide a detailed view of cloud-atmosphere-surface interactions and vertical mixing processes over the sea–ice environment. Measurements from a suite of ground-based remote sensors, near surface meteorological and aerosol instruments, and profiles from radiosondes and a helicopter are combined to characterize a week-long period dominated by low-level, mixed-phase, stratocumulus clouds. Detailed case studies and statistical analyses are used to develop a conceptual model for the cloud and atmosphere structure and their interactions in this environment. <br><br> Clouds were persistent during the period of study, having qualities that suggest they were sustained through a combination of advective influences and in-cloud processes, with little contribution from the surface. Radiative cooling near cloud top produced buoyancy-driven, turbulent eddies that contributed to cloud formation and created a cloud-driven mixed layer. The depth of this mixed layer was related to the amount of turbulence and condensed cloud water. Coupling of this cloud-driven mixed layer to the surface boundary layer was primarily determined by proximity. For 75% of the period of study, the primary stratocumulus cloud-driven mixed layer was decoupled from the surface and typically at a warmer potential temperature. Since the near-surface temperature was constrained by the ocean–ice mixture, warm temperatures aloft suggest that these air masses had not significantly interacted with the sea–ice surface. Instead, back trajectory analyses suggest that these warm airmasses advected into the central Arctic Basin from lower latitudes. Moisture and aerosol particles likely accompanied these airmasses, providing necessary support for cloud formation. On the occasions when cloud-surface coupling did occur, back trajectories indicated that these air masses advected at low levels, while mixing processes kept the mixed layer in equilibrium with the near-surface environment. Rather than contributing buoyancy forcing for the mixed-layer dynamics, the surface instead simply appeared to respond to the mixed-layer processes aloft. Clouds in these cases often contained slightly higher condensed water amounts, potentially due to additional moisture sources from below.Fri, 17 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/13119/2013/<b>The global impact of the transport sectors on atmospheric aerosol: simulations for year 2000 emissions</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 13119-13189, 2013<br /><br />Author(s): M. Righi, J. Hendricks, and R. Sausen<br /><br />We use the EMAC-MADE global aerosol model to quantify the impact of transport emissions (land transport, shipping and aviation) on global aerosol. We consider a present-day (2000) scenario and the CMIP5 emission dataset developed in support of the IPCC Fifth Assessment Report. The model takes also into account particle number emissions, which are derived from mass emissions under different assumptions on the size distribution of particles emitted by the three transport sectors. Additional sensitivity experiments are performed to quantify the effects of the uncertainties behind such assumptions. The model simulations show that the impact of the transport sectors closely matches the emission patterns. Land transport is the most important source of black carbon pollution in USA, Europe and Arabian Peninsula. Shipping strongly contributes to aerosol sulfate concentrations along the most-traveled routes of the northern Atlantic and northern Pacific oceans, with a significant impact along the coastlines. The effect of aviation is mostly confined to the upper-troposphere (7–12 km), in the northern mid-latitudes, although significant effects are also simulated at the ground, due to the emissions from landing and take-off cycles. The transport-induced perturbations to particle number concentrations are very sensitive to the assumptions on the size distribution of emitted particles, with the largest uncertainties obtained for the land transport sector. The simulated climate impacts, due to aerosol direct and indirect effects, are strongest for the shipping sector, as a consequence of the large impact of sulfate aerosol on low marine clouds and their optical properties.Fri, 17 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/13079/2013/<b>Long term in-situ observations of biomass burning aerosol at a high altitude station in Venezuela – sources, impacts and inter annual variability</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 13079-13117, 2013<br /><br />Author(s): T. Hamburger, M. Matisāns, P. Tunved, J. Ström, S. Calderon, P. Hoffmann, G. Hochschild, J. Gross, T. Schmeissner, and R. Krejci<br /><br />First long-term observations of South American biomass burning aerosol within the tropical lower free troposphere are presented. The observations were conducted between 2007 and 2009 at a high altitude station (4765 m a.s.l.) on the Pico Espejo, Venezuela. Sub-micron aerosol volume, number concentrations of primary particles and particle absorption were observed. Orographic lifting and shallow convection leads to a distinct diurnal cycle at the station. It enables measurements within the lower free troposphere during night time and observations of boundary layer air masses during day time and at their transitional regions. The seasonal cycle is defined by a wet rainy season and a dry biomass burning season. The particle load of biomass burning aerosol is dominated by fires in the Venezuelan savannah. Increases of aerosol concentrations could not be linked to long-range transport of biomass burning plumes from the Amazon basin or Africa due to effective wet scavenging of particles. Highest particle concentrations were observed within boundary layer air masses during the dry season. Ambient sub-micron aerosol volume reached 1.4 &plusmn; 1.3 &mu;m³ cm^&minus;3, heated (300 &deg;C) particle number concentrations 510 &plusmn; 420 cm^&minus;3 and the absorption coefficient 0.91 &plusmn; 1.2 Mm^&minus;1. The respective concentrations were lowest within the lower free troposphere during the wet season and averaged at 0.19 &plusmn; 0.25 &mu;m³ cm^&minus;3, 150 &plusmn; 94 cm^&minus;3 and 0.15 &plusmn; 0.26 Mm^&minus;1. A decrease of particle concentrations during the dry seasons from 2007–2009 could be connected to a decrease in fire activity in the wider region of Venezuela using MODIS satellite observations. The variability of biomass burning is most likely linked to the El Niño-Southern Oscillation (ENSO). Low biomass burning activity in the Venezuelan savannah was observed to follow La Niña conditions, high biomass burning activity followed El Niño conditions.Fri, 17 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/13045/2013/<b>Ozone weekend effects in the Beijing–Tianjin–Hebei metropolitan area, China</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 13045-13078, 2013<br /><br />Author(s): Y. H. Wang, B. Hu, and Y. S. Wang<br /><br />The ozone weekend effect (OWE) was first investigated in the metropolitan area of Beijing–Tianjin–Hebei (BTH), China, using in situ measurements from the Atmospheric Environment Monitoring Network from July 2009 to August 2011. The results indicate that there is an obvious weekly periodical variation in the surface ozone concentration based on 24 h averaged value. There is a lower ozone concentration from Wednesday to Friday (weekday) and a higher concentration from Saturday to Monday (weekend) over the entire study area. NO_x also displays weekly cycle, with the maximum level occurring on weekdays and the minimum level on weekends, especially later on Sunday night and early Monday morning. This pattern may be responsible for the higher concentration of ozone on weekends. Additionally, the vertical variations in O₃ and NO_x from the 8 m 47 m, 120 m and 280 m observation platforms on the 325 m Beijing meteorological tower displayed obvious weekly cycles that corresponded to the surface results. <br><br> A smaller decrease in VOCs (a proxy for CO) and much lower NO_x concentrations on the weekend may lead to higher VOC/NO_x ratio, which can enhance the ozone production efficiency in VOC-regime areas. Additionally, a clear weekly cycle in the fine aerosol concentration was observed, with maximum values occurring on weekdays and minimum values occurring on weekends. Higher concentrations of aerosol on weekdays can reduce the UV radiation flux by absorption or scattering, which leads to a decrease in the ozone production efficiency. A significant weekly cycle in UV radiation, in consistent with the aerosol concentration, was discovered at the BJT site, validating the assumption. A comprehensive understanding of the ozone weekend effect in the BTH area can provide deep insights into controlling photochemical pollution.Fri, 17 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/12991/2013/<b>Reconstruction of Northern Hemisphere 1950–2010 atmospheric non-methane hydrocarbons</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 12991-13043, 2013<br /><br />Author(s): D. Helmig, V. Petrenko, P. Martinerie, E. Witrant, T. Röckmann, A. Zuiderweg, R. Holzinger, J. Hueber, C. Stephens, J. White, W. Sturges, A. Baker, T. Blunier, D. Etheridge, M. Rubino, and P. Tans<br /><br />The short-chain non-methane hydrocarbons (NMHC) are mostly emitted into the atmosphere by anthropogenic processes. Recent studies have pointed out a tight linkage between the atmospheric mole fractions of the NMHC ethane to the atmospheric growth rate of methane. Consequently, atmospheric NMHC are valuable indicators for tracking changes in anthropogenic emissions, photochemical ozone production, and greenhouse gases. This study investigates the 1950–2010 Northern Hemisphere atmospheric C₂-C₅ NMHC ethane, propane, <i>i</i>-butane, <i>n</i>-butane, <i>i</i>-pentane, and <i>n</i>-pentane. Atmospheric mole fractions of these trace gases were constructed from (a) air samples of these trace gases from air samples extracted from three firn boreholes in 2008 and 2009 at the North Greenland Eemian Ice Drilling (NEEM) site using state of the art models of trace gas transport in firn, and by (b) considering eight years of ambient NMHC monitoring data from five Arctic sites within the NOAA Global Monitoring Division (GMD) Cooperative Air Sampling Network. Results indicate that these NMHC increased by ~ 40–120% after 1950, peaked around 1980 (with the exception of ethane, which peaked approximately 10 years earlier), and have since dramatically decreased to be now back close to 1950 levels. The earlier peak time of ethane versus the C₃-C₅ NMHC suggests that different processes and emissions mitigation measures contributed to the decline in these NMHC. The 60 yr record also illustrates notable increases in the ratios of the isomeric <i>iso-/n</i>-butane and <i>iso-/n</i>-pentane ratios. Comparison of the reconstructed NMHC histories with 1950–2000 volatile organic compounds (VOC) emissions data and with other recently published ethane trend analyses from ambient air Pacific transect data showed (a) better agreement with North America and Western Europe emissions than with total Northern Hemisphere emissions data, and (b) better agreement with other Greenland firn air data NMHC history reconstructions than with the Pacific region trends. These analyses emphasize that for NMHC, having atmospheric lifetimes on the order of < 2 months, the Greenland firn air records are primarily a representation of Western Europe and North America emission histories.Wed, 15 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/12913/2013/<b>The balances of mixing ratios and segregation intensity: a case study from the field (ECHO 2003)</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 12913-12989, 2013<br /><br />Author(s): R. Dlugi, M. Berger, M. Zelger, A. Hofzumahaus, F. Rohrer, F. Holland, K. Lu, and G. Kramm<br /><br />An inhomogeneous mixing of reactants causes a reduction of their chemical removal compared to the homogeneously mixed case in turbulent atmospheric flows. This can be described by the intensity of segregation <i>I</i>_S being the covariance of the mixing ratios of two species divided by the product of their means. Both terms appear in the balance equation of the mixing ratio and are discussed for the reaction between isoprene and OH for data of the field study ECHO 2003 above a deciduous forest. For most of these data, <i>I</i>_S is negatively correlated with the fraction of mean OH mixing ratio reacting with isoprene. <i>I</i>_S is also negatively correlated with the isoprene standard deviation. Both findings agree with model results discussed by Patton et al. (2001) and others. The correlation coefficient between OH and isoprene, and, therefore, <i>I</i>_S increases with increasing mean reaction rate. In addition, the balance equation of the covariance between isoprene and OH is applied for the analysis of the same field data. The storage term is small, and, therefore, a diagnostic equation for this covariance can be derived. The chemical reaction term <i>R</i>_<i>ij</i> is dominated by the variance of isoprene times the quotient of mixing ratios of OH and isoprene. In addition a diagnostic equation for <i>I</i>_S is formulated. Comparing different terms of this equation, <i>I</i>_S and <i>R</i>_<i>ij</i> show a relation also to the normalized isoprene standard deviation. It is shown that not only chemistry, but also turbulent and convective mixing and advection – considered in a residual term – influence <i>I</i>_S. Despite this finding, a detection of the influence of coherent eddy transport above the forest according to Katul et al. (1997) on <i>I</i>_S fails, but a relation with the turbulent transport of isoprene variance is determined. In addition, largest values of <i>I</i>_S are found for most unstable conditions with increasing buoyancy. These results are compared to model results by Ouwersloot et al. (2011).Wed, 15 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/12867/2013/<b>Chemical evolution of organic aerosol in Los Angeles during the CalNex 2010 study</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 12867-12911, 2013<br /><br />Author(s): R. Holzinger, A. H. Goldstein, P. L. Hayes, J. L. Jimenez, and J. Timkovsky<br /><br />During the CalNex study (15 May to 16 June 2010) a large suite of instruments was operated at the Los Angeles area ground supersite to characterize the sources and atmospheric processing of atmospheric pollution. The thermal-desorption proton-transfer-reaction mass-spectrometer (TD-PTR-MS) was deployed to an urban area for the first time and detected 691 organic ions in aerosol samples, the mean total concentration of which was estimated as 3.3 &mu;g m^&minus;3. Based on comparison to total organic aerosol (OA) measurements, we estimate that approximately 50% of the OA mass at this site was directly measured by the TD-PTR-MS. Based on correlations with aerosol mass spectrometer (AMS) OA components, the ions were grouped to represent hydrocarbon-like OA (HOA), local OA (LOA), semi-volatile oxygenated OA (SV-OOA), and low volatility oxygenated OA (LV-OOA). Mass spectra and thermograms of the ion groups are mostly consistent with the assumed sources and/or photochemical origin of the OA components. The mass spectra of ions representing the primary components HOA and LOA included the highest <i>m/z</i>, consistent with their higher resistance to thermal decomposition, and they were volatilized at lower temperatures. Photochemical ageing weakens C-C bond strengths (also resulting in chemical fragmentation), and produces species of lower volatility (through the addition of functional groups). Accordingly the mass spectra of ions representing the oxidized OA components (SV-OOA, and LV-OOA) lack the highest masses and they are volatilized at higher temperatures. Chemical parameters like mean carbon number (<span style="border-top: 1px solid #000; color: #000;"><i>n</i><sub>C</span>), mean carbon oxidation state (<span style="border-top: 1px solid #000; color: #000;">OS_C</span>), and the atomic ratios O/C and H/C of the ion groups are consistent with the expected sources and photochemical processing of the aerosol components. Our data suggest that chemical fragmentation gains importance over functionalization as photochemical age of OA increases. Surprisingly, the photochemical age of OA decreases during the daytime hours, demonstrating the importance of rapid production of new (photochemically young) SV-OOA during daytime. The PTR detects higher organic N concentrations than the AMS, the reasons for which are not well understood and cannot be explained by known artifacts related to PTR or the AMS. The median atomic N/C ratio (6.4%) of the ion group representing LV-OOA is a factor 2 higher than N/C of any other ion group. This suggests a multiphase chemical source involving ammonium ions is contributing to LV-OOA.Wed, 15 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/12825/2013/<b>Comparison of ensemble Kalman filter and variational approaches for CO₂ data assimilation</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 12825-12865, 2013<br /><br />Author(s): A. Chatterjee and A. M. Michalak<br /><br />Data assimilation (DA) approaches, such as the variational and the ensemble Kalman filter, provide a computationally efficient framework for solving the CO₂ source-sink estimation problem. Unlike DA applications for weather prediction and constituent assimilation, however, the advantages and disadvantages of alternative DA approaches for CO₂ flux estimation have not been extensively explored. In this study, we compare and assess estimates from two advanced DA methods (an ensemble square root filter and a variational technique) using a simple 1-dimensional advection-diffusion inverse problem that has been designed to capture the nuances of a real CO₂ flux estimation problem. Experiments are specifically designed to identify the impact of the observational density, heterogeneity, and uncertainty, as well as operational constraints (i.e., ensemble size, number of descent iterations) in order to isolate the degradation in the DA estimates relative to the estimates from a batch inverse modeling scheme. No dynamical model is explicitly specified for the DA methods to keep the problem setup analogous to a real CO₂ flux estimation problem. Results demonstrate that the performance of the DA approaches depends on a complex interplay between the measurement network and the operational constraints imposed to make the DA algorithms practically feasible. The overall advantages/disadvantages of the two examined DA approaches are complementary and highlight that, specifically for CO₂ applications, selection of one method over the other should be dictated by the carbon science questions being asked, and the inversion conditions under which the approaches are being applied.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/12797/2013/<b>Direct estimation of the rate constant of the reaction ClO + HO₂ → HOCl + O₂ from SMILES atmospheric observations</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 12797-12823, 2013<br /><br />Author(s): K. Kuribayashi, H. Sagawa, R. Lehmann, T. O. Sato, and Y. Kasai<br /><br />Diurnal variations of ClO, HO₂, and HOCl were simultaneously observed by the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) between 12 October 2009 and 21 April 2010. These were the first global observations of the diurnal variation of HOCl in the upper atmosphere. A major reaction to produce HOCl is ClO + HO₂ → HOCl + O₂ (R1) in extra polar region. A model study suggested that in the mesosphere during night this is the only reaction influencing the amount of HOCl and ClO. The evaluation of the pure reaction period, where only reaction (R1) occurred in Cl_y chemical system, was performed by the consistency between two reaction rates, HOCl production and ClO loss, from SMILES observation data. It turned out that the SMILES data at the pressure level of 0.28 hPa (about 58 km) during night (between local time 18:30 and 04:00) in the autumn mid-latitude region (20–40&deg; February–April 2010) were suitable for the estimation of <i>k</i>₁. The rate constant was obtained to be <i>k</i>₁(245 K) = 7.73 &pm; 0.26 (1&sigma;) [&times; 10^–12 cm³/molecule s] from SMILES atmospheric observations. This result was consistent with that from both the laboratory experiment and the ab initio calculations for similar low-pressure conditions. The 1&sigma; precision of <i>k</i>₁ obtained was 2–10 times better than those of previous laboratory measurements.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/12771/2013/<b>Method development estimating ambient mercury concentration from monitored mercury wet deposition</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 12771-12796, 2013<br /><br />Author(s): S. M. Chen, X. Qiu, L. Zhang, F. Yang, and P. Blanchard<br /><br />Speciated atmospheric mercury data have recently been monitored at multiple locations in North America; but the spatial coverage is far less than the long-established mercury wet deposition network. The present study describes a first attempt linking ambient concentration with wet deposition using Beta distribution fitting of a ratio estimate. The mean, median, mode, standard deviation, and skewness of the fitted Beta distribution parameters were generated using data collected in 2009 at 11 monitoring stations. Comparing the normalized histogram and the fitted density function, the empirical and fitted Beta distribution of the ratio shows a close fit. The estimated ambient mercury concentration was further partitioned into reactive gaseous mercury and particulate bound mercury using linear regression model developed by Amos et al. (2012). The method presented here can be used to roughly estimate mercury ambient concentration at locations and/or times where such measurement is not available but where wet deposition is monitored.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/12743/2013/<b>Particle partitioning potential of organic compounds is highest in the Eastern US and driven by anthropogenic water</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 12743-12770, 2013<br /><br />Author(s): A. G. Carlton and B. J. Turpin<br /><br />Gas phase water-soluble organic matter (WSOM_g) is ubiquitous in the troposphere. In the summertime, the potential for these gases to partition to particle phase liquid water (H₂O_ptcl) where they can form secondary organic aerosol (SOA_AQ) is high in the Eastern US and low elsewhere, with the exception of an area near Los Angeles, CA. This spatial pattern is driven by mass concentrations of H₂O_ptcl, not WSOM_g. H₂O_ptcl mass concentrations are predicted to be high in the Eastern US, largely due to sulfate. The ability of sulfate to increase H₂O_ptcl is well-established and routinely included in atmospheric models, however WSOM_g partitioning to this water and subsequent SOA formation is not. The high mass concentrations of H₂O_ptcl in the southeast (SE) US but not the Amazon, may help explain why biogenic SOA mass concentrations are high in the SE US, but low in the Amazon. Furthermore, during the summertime in the Eastern US, the potential for organic gases to partition into liquid water is greater than their potential to partition into organic matter (OM) because concentrations of WSOM_g and H₂O_ptcl are higher than semi-volatile gases and OM. Thus, unless condensed phase yields are substantially different (> ~ order of magnitude), we expect that SOA formed through aqueous phase pathways (SOA_AQ) will dominate in the Eastern US. These findings also suggest that H₂O_ptcl is largely anthropogenic and provide a previously unrecognized mechanism by which anthropogenic pollutants impact the amount of SOA mass formed from biogenic organic emissions. The previously reported estimate of the controllable fraction of biogenic SOA in the Eastern US (50%) is likely too low.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/12725/2013/<b>The impact of horizontal heterogeneities, cloud fraction, and cloud dynamics on warm cloud effective radii and liquid water path from CERES-like Aqua MODIS retrievals</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 12725-12742, 2013<br /><br />Author(s): D. Painemal, P. Minnis, and S. Sun-Mack<br /><br />The impact of horizontal heterogeneities, liquid water path (LWP from AMSR-E), and cloud fraction (CF) on MODIS cloud effective radius (<i>r</i>_e), retrieved from the 2.1 &mu;m (<i>r</i>_e2.1) and 3.8 &mu;m (<i>r</i>_e3.8) channels, is investigated for warm clouds over the southeast Pacific. Values of <i>r</i>_e retrieved using the CERES Edition 4 algorithms are averaged at the CERES footprint resolution (~ 20 km), while heterogeneities (<i>H</i>_&sigma;) are calculated as the ratio between the standard deviation and mean 0.64 &mu;m reflectance. The value of <i>r</i>_e2.1 strongly depends on CF, with magnitudes up to 5 &mu;m larger than those for overcast scenes, whereas <i>r</i>_e3.8 remains insensitive to CF. For cloudy scenes, both <i>r</i>_e2.1 and <i>r</i>_e3.8 increase with <i>H</i>_&sigma; for any given AMSR-E LWP, but <i>r</i>_e2.1 changes more than for <i>r</i>_e3.8. Additionally, <i>r</i>_e3.8 &ndash; <i>r</i>_e2.1 differences are positive (< 1 &mu;m) for homogeneous scenes (<i>H</i>_&sigma; < 0.2) and LWP > 50 g m^&minus;2, and negative (up to −4 &mu;m) for larger <i>H</i>_&sigma;. Thus, <i>r</i>_e3.8 &ndash; <i>r</i>_e2.1 differences are more likely to reflect biases associated with cloud heterogeneities rather than information about the cloud vertical structure. The consequences for MODIS LWP are also discussed.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/12541/2013/<b>Online coupled regional meteorology-chemistry models in Europe: current status and prospects</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 12541-12724, 2013<br /><br />Author(s): A. Baklanov, K. H. Schluenzen, P. Suppan, J. Baldasano, D. Brunner, S. Aksoyoglu, G. Carmichael, J. Douros, J. Flemming, R. Forkel, S. Galmarini, M. Gauss, G. Grell, M. Hirtl, S. Joffre, O. Jorba, E. Kaas, M. Kaasik, G. Kallos, X. Kong, U. Korsholm, A. Kurganskiy, J. Kushta, U. Lohmann, A. Mahura, A. Manders-Groot, A. Maurizi, N. Moussiopoulos, S. T. Rao, N. Savage, C. Seigneur, R. Sokhi, E. Solazzo, S. Solomos, B. Sørensen, G. Tsegas, E. Vignati, B. Vogel, and Y. Zhang<br /><br />The simulation of the coupled evolution of atmospheric dynamics, pollutant transport, chemical reactions and atmospheric composition is one of the most challenging tasks in environmental modelling, climate change studies, and weather forecasting for the next decades as they all involve strongly integrated processes. Weather strongly influences air quality (AQ) and atmospheric transport of hazardous materials, while atmospheric composition can influence both weather and climate by directly modifying the atmospheric radiation budget or indirectly affecting cloud formation. Until recently, however, due to the scientific complexities and lack of computational power, atmospheric chemistry and weather forecasting have developed as separate disciplines, leading to the development of separate modelling systems that are only loosely coupled. <br></br> The continuous increase in computer power has now reached a stage that enables us to perform online coupling of regional meteorological models with atmospheric chemical transport models. The focus on integrated systems is timely, since recent research has shown that meteorology and chemistry feedbacks are important in the context of many research areas and applications, including numerical weather prediction (NWP), AQ forecasting as well as climate and Earth system modelling. However, the relative importance of online integration and its priorities, requirements and levels of detail necessary for representing different processes and feedbacks can greatly vary for these related communities: (i) NWP, (ii) AQ forecasting and assessments, (iii) climate and earth system modelling. Additional applications are likely to benefit from online modelling, e.g.: simulation of volcanic ash or forest fire plumes, pollen warnings, dust storms, oil/gas fires, geo-engineering tests involving changes in the radiation balance. <br></br> The COST Action ES1004 – European framework for online integrated air quality and meteorology modelling (EuMetChem) – aims at paving the way towards a new generation of online integrated atmospheric chemical transport and meteorology modelling with two-way interactions between different atmospheric processes including dynamics, chemistry, clouds, radiation, boundary layer and emissions. As its first task, we summarise the current status of European modelling practices and experience with online coupled modelling of meteorology with atmospheric chemistry including feedback mechanisms and attempt reviewing the various issues connected to the different modules of such online coupled models but also providing recommendations for coping with them for the benefit of the modelling community at large.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/12485/2013/<b>Evaluating global emission inventories of biogenic bromocarbons</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 12485-12539, 2013<br /><br />Author(s): R. Hossaini, H. Mantle, M. P. Chipperfield, S. A. Montzka, P. Hamer, F. Ziska, B. Quack, K. Krüger, S. Tegtmeier, E. Atlas, S. Sala, A. Engel, H. Bönisch, T. Keber, D. Oram, G. Mills, C. Ordóñez, A. Saiz-Lopez, N. Warwick, Q. Liang, W. Feng, F. Moore, B. R. Miller, V. Marécal, N. A. D. Richards, M. Dorf, and K. Pfeilsticker<br /><br />Emissions of halogenated very short-lived substances (VSLS) are poorly constrained. However, their inclusion in global models is required to simulate a realistic inorganic bromine (Br_y) loading in both the troposphere, where bromine chemistry perturbs global oxidizing capacity, and in the stratosphere, where it is a major sink for ozone (O₃). We have performed simulations using a 3-D chemical transport model (CTM) including three <i>top-down</i> and a single <i>bottom-up</i> derived emission inventory of the major brominated VSLS bromoform (CHBr₃) and dibromomethane (CH₂Br₂). We perform the first concerted evaluation of these inventories, comparing both the magnitude and spatial distribution of emissions. For a quantitative evaluation of each inventory, model output is compared with independent long-term observations at National Oceanic and Atmospheric Administration (NOAA) ground-based stations and with aircraft observations made during the NSF HIAPER Pole-to-Pole Observations (HIPPO) project. For CHBr₃, the mean absolute deviation between model and surface observation ranges from 0.22 (38%) to 0.78 (115%) parts per trillion (ppt) in the tropics, depending on emission inventory. For CH₂Br₂, the range is 0.17 (24%) to 1.25 (167%) ppt. We also use aircraft observations made during the 2011 "<b>S</b>tratospheric Ozone: <b>H</b>alogen <b>I</b>mpacts in a <b>V</b>arying <b>A</b>tmosphere" (SHIVA) campaign, in the tropical West Pacific. Here, the performance of the various inventories also varies significantly, but overall the CTM is able to reproduce observed CHBr₃ well in the free troposphere using an inventory based on observed sea-to-air fluxes. Finally, we identify the range of uncertainty associated with these VSLS emission inventories on stratospheric bromine loading due to VSLS (Br_y^VSLS). Our simulations show Br_y^VSLS ranges from ~ 4.0 to 8.0 ppt depending on the inventory. We report an optimised estimate at the lower end of this range (~ 4 ppt) based on combining the CHBr₃ and CH₂Br₂ inventories which give best agreement with the compilation of observations in the tropics.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/12437/2013/<b>Eddy covariance fluxes and vertical concentration gradient measurements of NO and NO₂ over a ponderosa pine ecosystem: observational evidence for within canopy removal of NO_x</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 12437-12484, 2013<br /><br />Author(s): K.-E. Min, S. E. Pusede, E. C. Browne, B. W. LaFranchi, P. J. Wooldridge, and R. C. Cohen<br /><br />Exchange of NO_x (NO+NO₂) between the atmosphere and biosphere is important for air quality, climate change, and ecosystem nutrient dynamics. There are few direct ecosystem scale measurements of the direction and rate of atmosphere-biosphere exchange of NO_x. As a result, a complete description of the processes affecting NO_x following emission from soils and/or plants as they transit from within the plant/forest canopy to the free atmosphere remains poorly constrained and debated. Here, we describe measurements of NO and NO₂ fluxes and vertical concentration gradients made during the Biosphere Effects on AeRosols and Photochemistry EXperiment 2009. In general, during daytime we observe upward fluxes of NO and NO₂ with counter-gradient fluxes of NO. We find that NO_x fluxes from the forest canopy are smaller than calculated using observed flux-gradient relationships for conserved tracers and also smaller than measured soil NO emissions. We interpret these differences as evidence for the existence of a "canopy reduction factor". We suggest that at this site it is primarily due to chemistry converting NO_x to higher nitrogen oxides within the forest canopy.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/12389/2013/<b>Sulfur dioxide (SO₂) as observed by MIPAS/Envisat: temporal development and spatial distribution at 15–45 km altitude</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 12389-12436, 2013<br /><br />Author(s): M. Höpfner, N. Glatthor, U. Grabowski, S. Kellmann, M. Kiefer, A. Linden, J. Orphal, G. Stiller, T. von Clarmann, and B. Funke<br /><br />We present a climatology of monthly and 10&deg; zonal mean profiles of sulfur dioxide (SO₂) volume mixing ratios (vmr) derived from MIPAS/Envisat measurements in the altitude range 15–45 km from July 2002 until April 2012. The vertical resolution varies from 3.5–4 km in the lower stratosphere up to 6–10 km at the upper end of the profiles with estimated total errors of 5–20 pptv for single profiles of SO₂. Comparisons with few available observations of SO₂ up to high altitudes from ATMOS, for a volcanically perturbed situations from ACE-FTS and, at the lowest altitudes, with stratospheric in-situ observations reveal general consistency of the datasets. The observations are the first empirical confirmation of features of the stratospheric SO₂ distribution which have only been shown by models up to now: (1) the local maximum of SO₂ at around 25–30 km altitude which is explained by the conversion of carbonyl sulfide (COS) as the precursor of the Junge layer, and (2) the downwelling of SO₂ rich air to altitudes of 25–30 km at high latitudes during winter and its subsequent depletion on availability of sunlight. This has been proposed as the reason for the sudden appearance of enhanced concentrations of condensation nuclei during Arctic and Antarctic spring. Further, the strong increase of SO₂ to values of 80–100 pptv in the upper stratosphere through photolysis of H₂SO₄ has been confirmed. Lower stratospheric variability of SO₂ could mainly be explained by volcanic activity and no hint for a strong anthropogenic influence has been found. Regression analysis revealed a QBO (quasi-biennial oscillation) signal of the SO₂ time series in the tropics at about 30–35 km, a SAO (semi-annual oscillation) signal at tropical and subtropical latitudes above 32 km and annual periodics predominantly at high latitudes. Further, the analysis indicates a correlation with the solar cycle in the tropics and southern subtropics above 30 km. Significant negative linear trends are found in the tropical lower stratosphere, probably due to reduced tropical volcanic activity and at southern mid-latitudes above 35 km. A positive trend is visible in the lower and middle stratosphere at polar to subtropical southern latitudes.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/12337/2013/<b>Trends in stratospheric ozone profiles using functional mixed models</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 12337-12387, 2013<br /><br />Author(s): A. Y. Park, S. Guillas, and I. Petropavlovskikh<br /><br />This paper is devoted to the modeling of altitude-dependent patterns of ozone variations over time. Umkher ozone profiles (quarter of Umkehr layer) from 1978 to 2011 are investigated at two locations: Boulder (USA) and Arosa (Switzerland). The study consists of two statistical stages. First we approximate ozone profiles employing an appropriate basis. To capture primary modes of ozone variations without losing essential information, a functional principal component analysis is performed as it penalizes roughness of the function and smooths excessive variations in the shape of the ozone profiles. As a result, data driven basis functions are obtained. Secondly we estimate the effects of covariates – month, year (trend), quasi biennial oscillation, the Solar cycle, arctic oscillation and the El Niño/Southern Oscillation cycle – on the principal component scores of ozone profiles over time using generalized additive models. The effects are smooth functions of the covariates, and are represented by knot-based regression cubic splines. Finally we employ generalized additive mixed effects models incorporating a more complex error structure that reflects the observed seasonality in the data. The analysis provides more accurate estimates of influences and trends, together with enhanced uncertainty quantification. We are able to capture fine variations in the time evolution of the profiles such as the semi-annual oscillation. We conclude by showing the trends by altitude over Boulder. The strongly declining trends over 2003–2011 for altitudes of 32–64 hPa show that stratospheric ozone is not yet fully recovering.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys-discuss.net/13/12287/2013/<b>Simulation of trace gases and aerosols over the Indian domain: evaluation of the WRF-Chem Model</b><br /><br />Atmospheric Chemistry and Physics Discussions, 13, 12287-12336, 2013<br /><br />Author(s): M. Michael, A. Yadav, S. N. Tripathi, V. P. Kanawade, A. Gaur, P. Sadavarte, and C. Venkataraman<br /><br />The "online" meteorological and chemical transport Weather Research and Forecasting/Chemistry (WRF-Chem) model has been implemented over the Indian subcontinent for three consecutive summers in 2008, 2009 and 2010 to study the aerosol properties over the domain. The initial and boundary conditions are obtained from NCAR reanalysis data. The emission rates of sulfur dioxide, black carbon, organic carbon and PM_2.5, which are developed over India at a grid resolution of 0.25&deg; × 0.25&deg; have been used in the present study. The remaining emissions are obtained from global inventories (RETRO and EDGAR). The model simulated the meteorological parameters, trace gases and particulate matter. Predicted mixing ratios of trace gases (Ozone, carbon monoxide and sulfur dioxide) are compared with ground based observations over Kanpur. Simulated aerosol optical depth are compared with those observed at nine Aerosol Robotic Network stations (AERONET). The simulations show that the aerosol optical depth of the less polluted regions is better simulated compared to that of the locations where the aerosol loading is very high. The vertical profiles of extinction coefficient observed at Kanpur Micropulse Lidar Network (MPLNET) station is in agreement with the simulated values for altitudes greater than 1.5 km and qualitatively simulate the elevated layers of aerosols. The simulated mass concentration of black carbon shows very good correlation with observations, due to the better local emission inventory used. The vertical profiles of black carbon at various locations have also been compared with observations from aircraft campaign held during pre-monsoon period of 2008 and 2009 resulting in good agreement. This study shows that WRF-Chem model captures many important features of the observations and therefore can be used for understanding and predicting regional atmospheric composition over Indian subcontinent.Tue, 14 May 2013 00:00:00 +0200