http://www.atmos-chem-phys.net/ Atmospheric Chemistry and Physics Latest Articlesenhttp://www.atmos-chem-phys.net/13/5907/2013/<b>A functional group oxidation model (FGOM) for SOA formation and aging</b><br /><br />Atmospheric Chemistry and Physics, 13, 5907-5926, 2013<br /><br />Author(s): X. Zhang and J. H. Seinfeld<br /><br />Secondary organic aerosol (SOA) formation from a volatile organic compound (VOC) involves multiple generations of oxidation that include functionalization and fragmentation of the parent carbon backbone and likely particle-phase oxidation and/or accretion reactions. Despite the typical complexity of the detailed molecular mechanism of SOA formation and aging, a relatively small number of functional groups characterize the oxidized molecules that constitute SOA. Given the carbon number and set of functional groups, the volatility of the molecule can be estimated. We present here a functional group oxidation model (FGOM) that represents the process of SOA formation and aging. The FGOM contains a set of parameters that are to be determined by fitting of the model to laboratory chamber data: total organic aerosol concentration, and O : C and H : C atomic ratios. The sensitivity of the model prediction to variation of the adjustable parameters allows one to assess the relative importance of various pathways involved in SOA formation. An analysis of SOA formation from the high- and low-NO_x photooxidation of four C₁₂ alkanes (n-dodecane, 2-methylundecane, hexylcyclohexane, and cyclododecane) using the FGOM is presented, and comparison with the statistical oxidation model (SOM) of Cappa et al. (2013) is discussed.Tue, 18 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5887/2013/<b>Estimation of volatile organic compound emissions for Europe using data assimilation</b><br /><br />Atmospheric Chemistry and Physics, 13, 5887-5905, 2013<br /><br />Author(s): M. R. Koohkan, M. Bocquet, Y. Roustan, Y. Kim, and C. Seigneur<br /><br />The emissions of non-methane volatile organic compounds (VOCs) over western Europe for the year 2005 are estimated via inverse modelling by assimilation of in situ observations of concentration and then subsequently compared to a standard emission inventory. The study focuses on 15 VOC species: five aromatics, six alkanes, two alkenes, one alkyne and one biogenic diene. The inversion relies on a validated fast adjoint of the chemical transport model used to simulate the fate and transport of these VOCs. The assimilated ground-based measurements over Europe are provided by the European Monitoring and Evaluation Programme (EMEP) network. The background emission errors and the prior observational errors are estimated by maximum-likelihood approaches. The positivity assumption on the VOC emission fluxes is pivotal for a successful inversion, and this maximum-likelihood approach consistently accounts for the positivity of the fluxes. For most species, the retrieved emissions lead to a significant reduction of the bias, which underlines the misfit between the standard inventories and the observed concentrations. The results are validated through a forecast test and a cross-validation test. An estimation of the posterior uncertainty is also provided. It is shown that the statistically consistent non-Gaussian approach based on a reliable estimation of the errors offers the best performance. The efficiency in correcting the inventory depends on the lifetime of the VOCs and the accuracy of the boundary conditions. In particular, it is shown that the use of in situ observations using a sparse monitoring network to estimate emissions of isoprene is inadequate because its short chemical lifetime significantly limits the spatial radius of influence of the monitoring data. For species with a longer lifetime (a few days), successful, albeit partial, emission corrections can reach regions hundreds of kilometres away from the stations. Domain-wide corrections of the emission inventories of some VOCs are significant, with underestimations of the order of a factor of 2 for propane, ethane, ethylene and acetylene.Tue, 18 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5873/2013/<b>Modeling air pollution in Lebanon: evaluation at a suburban site in Beirut during summer</b><br /><br />Atmospheric Chemistry and Physics, 13, 5873-5886, 2013<br /><br />Author(s): A. Waked, C. Seigneur, F. Couvidat, Y. Kim, K. Sartelet, C. Afif, A. Borbon, P. Formenti, and S. Sauvage<br /><br />Beirut, the capital of Lebanon, which is located on the eastern shore of the Mediterranean basin, experiences high air pollution episodes. Annual average concentrations of coarse and fine particulate matter (PM_2.5) as well as nitrogen oxides (NO_x) often exceed the World Health Organization (WHO) guidelines. Therefore, improving air quality in this region is essential. The Polyphemus/Polair3D modeling system is used here to investigate air pollution episodes in Beirut during 2 to 18 July 2011. The modeling domain covers two nested grids of 1 and 5 km horizontal resolution over greater Beirut and Lebanon, respectively. The anthropogenic emission inventory was developed earlier (Waked et al., 2012). The Weather and Research Forecasting (WRF) model is used to generate the meteorological fields and the Model of Emissions of Gases and Aerosols from Nature (MEGAN) is used for biogenic emissions. The results of the study are compared to measurements from a field campaign conducted in the suburb of Beirut during 2–18 July 2011. The model reproduces satisfactorily the concentrations of most gaseous pollutants, the total mass of PM_2.5 as well as PM_2.5 elemental carbon (EC), organic carbon (OC), and sulfate. Ozone concentrations are overestimated and it appears that this overestimation results mainly from the boundary conditions.Mon, 17 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5857/2013/<b>Formation of aqueous-phase α-hydroxyhydroperoxides (α-HHP): potential atmospheric impacts</b><br /><br />Atmospheric Chemistry and Physics, 13, 5857-5872, 2013<br /><br />Author(s): R. Zhao, A. K. Y. Lee, R. Soong, A. J. Simpson, and J. P. D. Abbatt<br /><br />The focus of this work is on quantifying the degree of the aqueous-phase formation of α-hydroxyhydroperoxides (α-HHPs) via reversible nucleophilic addition of H₂O₂ to aldehydes. Formation of this class of highly oxygenated organic hydroperoxides represents a poorly characterized aqueous-phase processing pathway that may lead to enhanced SOA formation and aerosol toxicity. Specifically, the equilibrium constants of α-HHP formation have been determined using proton nuclear-magnetic-resonance (¹H NMR) spectroscopy and proton-transfer-reaction mass spectrometry (PTR-MS). Significant α-HHP formation was observed from formaldehyde, acetaldehyde, propionaldehyde, glycolaldehyde, glyoxylic acid, and methylglyoxal, but not from methacrolein and ketones. Low temperatures enhanced the formation of α-HHPs but slowed their formation rates. High inorganic salt concentrations shifted the equilibria toward the hydrated form of the aldehydes and slightly suppressed α-HHP formation. Using the experimental equilibrium constants, we predict the equilibrium concentration of α-HHPs to be in the μM level in cloud water, but it may also be present in the mM level in aerosol liquid water (ALW), where the concentrations of H₂O₂ and aldehydes can be high. Formation of α-HHPs in ALW may significantly affect the effective Henry's law constants of H₂O₂ and aldehydes but may not affect their gas-phase levels. The photochemistry and reactivity of this class of atmospheric species have not been studied.Mon, 17 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5831/2013/<b>Black carbon physical properties and mixing state in the European megacity Paris</b><br /><br />Atmospheric Chemistry and Physics, 13, 5831-5856, 2013<br /><br />Author(s): M. Laborde, M. Crippa, T. Tritscher, Z. Jurányi, P. F. Decarlo, B. Temime-Roussel, N. Marchand, S. Eckhardt, A. Stohl, U. Baltensperger, A. S. H. Prévôt, E. Weingartner, and M. Gysel<br /><br />Aerosol hygroscopicity and refractory black carbon (rBC) properties were characterised during wintertime at a suburban site in Paris, one of the biggest European cities. Hygroscopic growth factor (GF) frequency distributions, characterised by distinct modes of more-hygroscopic background aerosol and non- or slightly hygroscopic aerosol of local (or regional) origin, revealed an increase of the relative contribution of the local sources compared to the background aerosol with decreasing particle size. BC-containing particles in Paris were mainly originating from fresh traffic emissions, whereas biomass burning only gave a minor contribution. The mass size distribution of the rBC cores peaked on average at an rBC core mass equivalent diameter of <i>D</i>_MEV ~ 150 nm. The BC-containing particles were moderately coated (coating thickness &Delta;_coat ~ 33 nm on average for rBC cores with </i>D</i>_MEV = 180–280 nm) and an average mass absorption coefficient (MAC) of ~ 8.6 m² g^&minus;1 at the wavelength λ = 880 nm was observed. <br><br> Different time periods were selected to investigate the properties of BC-containing particles as a function of source and air mass type. The traffic emissions were found to be non-hygroscopic (GF ≈ 1.0), and essentially all particles with a dry mobility diameter (<i>D</i>₀) larger than <i>D</i>₀ = 110 nm contained an rBC core. rBC from traffic emissions was further observed to be uncoated within experimental uncertainty (&Delta;_coat ~ 2 nm ± 10 nm), to have the smallest BC core sizes (maximum of the rBC core mass size distribution at <i>D</i>_MEV ~ 100 nm) and to have the smallest MAC (~ 7.3 m²g^&minus;1 at λ = 880 nm). <br><br> The biomass burning aerosol was slightly more hygroscopic than the traffic emissions (with a distinct slightly-hygroscopic mode peaking at GF ≈ 1.1–1.2). Furthermore, only a minor fraction (&leq; 10%) of the slightly-hygroscopic particles with 1.1 &leq; GF &leq; 1.2 (and <i>D</i>₀ = 265 nm) contained a detectable rBC core. The BC-containing particles from biomass burning were found to have a medium coating thickness as well as slightly larger mean rBC core sizes and MAC values compared to traffic emissions. <br><br> The aerosol observed under the influence of aged air masses and air masses from Eastern Continental Europe was dominated by a~more-hygroscopic mode peaking at GF ≈ 1.6. Most particles (95%), in the more-hygroscopic mode at <i>D</i>₀ = 265 nm, did not contain a detectable rBC core. A significant fraction of the BC-containing particles had a substantial coating with non-refractory aerosol components. MAC values of ~ 8.8 m²g^&minus;1 and ~ 8.3 m²g^&minus;1 at λ = 880 nm and mass mean rBC core diameters of 150 nm and 200 nm were observed for the aged and continental air mass types, respectively. The reason for the larger rBC core sizes compared to the fresh emissions – transport effects or a different rBC source – remains unclear. <br><br> The dominant fraction of the BC-containing particles was found to have no or very little coating with non-refractory matter. The lack of coatings is consistent with the observation that the BC-containing particles are non- or slightly-hygroscopic, which makes them poor cloud condensation nuclei. It can therefore be expected that wet removal through nucleation scavenging is inefficient for fresh BC-containing particles in urban plumes. The mixing-state-specific cloud droplet activation behaviour of BC-containing particles including the effects of atmospheric aging processes should be considered in global simulations of atmospheric BC, as the wet removal efficiency remains a major source of uncertainty in its life-cycle.Fri, 14 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5813/2013/<b>Ozone and fine particle in the western Yangtze River Delta: an overview of 1 yr data at the SORPES station</b><br /><br />Atmospheric Chemistry and Physics, 13, 5813-5830, 2013<br /><br />Author(s): A. J. Ding, C. B. Fu, X. Q. Yang, J. N. Sun, L. F. Zheng, Y. N. Xie, E. Herrmann, W. Nie, T. Petäjä, V.-M. Kerminen, and M. Kulmala<br /><br />This work presents an overview of 1 yr measurements of ozone (O₃) and fine particular matter (PM_2.5) and related trace gases at a recently developed regional background site, the Station for Observing Regional Processes of the Earth System (SORPES), in the western part of the Yangtze River Delta (YRD) in eastern China. Ozone and PM_2.5 showed strong seasonal cycles but with contrast patterns: O₃ reached a maximum in warm seasons but PM_2.5 in cold seasons. Correlation analysis suggests a VOC-sensitive regime for O₃ chemistry and a formation of secondary aerosols under conditions of high O₃ in summer. Compared with the National Ambient Air Quality Standards in China, our measurements report 15 days of O₃ exceedance and 148 days of PM_2.5 exceedance during the 1 yr period, suggesting a severe air pollution situation in this region. Case studies for typical O₃ and PM_2.5 episodes demonstrated that these episodes were generally associated with an air mass transport pathway over the mid-YRD, i.e., along the Nanjing–Shanghai axis with its city clusters, and showed that synoptic weather played an important role in air pollution, especially for O₃. Agricultural burning activities caused high PM_2.5 and O₃ pollution during harvest seasons, especially in June. A calculation of potential source contributions based on Lagrangian dispersion simulations suggests that emissions from the YRD contributed to over 70% of the O₃ precursor CO, with a majority from the mid-YRD. North-YRD and the North China Plain are the main contributors to PM_2.5 pollution in this region. This work shows an important environmental impact from industrialization and urbanization in the YRD region, and suggests an urgent need for improving air quality in these areas through collaborative control measures among different administrative regions.Fri, 14 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5791/2013/<b>Validation of MIPAS-ENVISAT H₂O operational data collected between July 2002 and March 2004</b><br /><br />Atmospheric Chemistry and Physics, 13, 5791-5811, 2013<br /><br />Author(s): G. Wetzel, H. Oelhaf, G. Berthet, A. Bracher, C. Cornacchia, D. G. Feist, H. Fischer, A. Fix, M. Iarlori, A. Kleinert, A. Lengel, M. Milz, L. Mona, S. C. Müller, J. Ovarlez, G. Pappalardo, C. Piccolo, P. Raspollini, J.-B. Renard, V. Rizi, S. Rohs, C. Schiller, G. Stiller, M. Weber, and G. Zhang<br /><br />Water vapour (H₂O) is one of the operationally retrieved key species of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument aboard the Environmental Satellite (ENVISAT) which was launched into its sun-synchronous orbit on 1 March 2002 and operated until April 2012. Within the MIPAS validation activities, independent observations from balloons, aircraft, satellites, and ground-based stations have been compared to European Space Agency (ESA) version 4.61 operational H₂O data comprising the time period from July 2002 until March 2004 where MIPAS measured with full spectral resolution. No significant bias in the MIPAS H₂O data is seen in the lower stratosphere (above the hygropause) between about 15 and 30 km. Differences of H₂O quantities observed by MIPAS and the validation instruments are mostly well within the combined total errors in this altitude region. In the upper stratosphere (above about 30 km), a tendency towards a small positive bias (up to about 10%) is present in the MIPAS data when compared to its balloon-borne counterpart MIPAS-B, to the satellite instruments HALOE (Halogen Occultation Experiment) and ACE-FTS (Atmospheric Chemistry Experiment, Fourier Transform Spectrometer), and to the millimeter-wave airborne sensor AMSOS (Airborne Microwave Stratospheric Observing System). In the mesosphere the situation is unclear due to the occurrence of different biases when comparing HALOE and ACE-FTS data. Pronounced deviations between MIPAS and the correlative instruments occur in the lowermost stratosphere and upper troposphere, a region where retrievals of H₂O are most challenging. Altogether it can be concluded that MIPAS H₂O profiles yield valuable information on the vertical distribution of H₂O in the stratosphere with an overall accuracy of about 10 to 30% and a precision of typically 5 to 15% – well within the predicted error budget, showing that these global and continuous data are very valuable for scientific studies. However, in the region around the tropopause retrieved MIPAS H₂O profiles are less reliable, suffering from a number of obstacles such as retrieval boundary and cloud effects, sharp vertical discontinuities, and frequent horizontal gradients in both temperature and H₂O volume mixing ratio (VMR). Some profiles are characterized by retrieval instabilities.Fri, 14 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5767/2013/<b>Formation of organic aerosol in the Paris region during the MEGAPOLI summer campaign: evaluation of the volatility-basis-set approach within the CHIMERE model</b><br /><br />Atmospheric Chemistry and Physics, 13, 5767-5790, 2013<br /><br />Author(s): Q. J. Zhang, M. Beekmann, F. Drewnick, F. Freutel, J. Schneider, M. Crippa, A. S. H. Prevot, U. Baltensperger, L. Poulain, A. Wiedensohler, J. Sciare, V. Gros, A. Borbon, A. Colomb, V. Michoud, J.-F. Doussin, H. A. C. Denier van der Gon, M. Haeffelin, J.-C. Dupont, G. Siour, H. Petetin, B. Bessagnet, S. N. Pandis, A. Hodzic, O. Sanchez, C. Honoré, and O. Perrussel<br /><br />Simulations with the chemistry transport model CHIMERE are compared to measurements performed during the MEGAPOLI (Megacities: Emissions, urban, regional and Global Atmospheric POLlution and climate effects, and Integrated tools for assessment and mitigation) summer campaign in the Greater Paris region in July 2009. The volatility-basis-set approach (VBS) is implemented into this model, taking into account the volatility of primary organic aerosol (POA) and the chemical aging of semi-volatile organic species. Organic aerosol is the main focus and is simulated with three different configurations with a modified treatment of POA volatility and modified secondary organic aerosol (SOA) formation schemes. In addition, two types of emission inventories are used as model input in order to test the uncertainty related to the emissions. Predictions of basic meteorological parameters and primary and secondary pollutant concentrations are evaluated, and four pollution regimes are defined according to the air mass origin. Primary pollutants are generally overestimated, while ozone is consistent with observations. Sulfate is generally overestimated, while ammonium and nitrate levels are well simulated with the refined emission data set. As expected, the simulation with non-volatile POA and a single-step SOA formation mechanism largely overestimates POA and underestimates SOA. Simulation of organic aerosol with the VBS approach taking into account the aging of semi-volatile organic compounds (SVOC) shows the best correlation with measurements. High-concentration events observed mostly after long-range transport are well reproduced by the model. Depending on the emission inventory used, simulated POA levels are either reasonable or underestimated, while SOA levels tend to be overestimated. Several uncertainties related to the VBS scheme (POA volatility, SOA yields, the aging parameterization), to emission input data, and to simulated OH levels can be responsible for this behavior. Despite these uncertainties, the implementation of the VBS scheme into the CHIMERE model allowed for much more realistic organic aerosol simulations for Paris during summertime. The advection of SOA from outside Paris is mostly responsible for the highest OA concentration levels. During advection of polluted air masses from northeast (Benelux and Central Europe), simulations indicate high levels of both anthropogenic and biogenic SOA fractions, while biogenic SOA dominates during periods with advection from Southern France and Spain.Fri, 14 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5751/2013/<b>Immersion freezing of ice nucleation active protein complexes</b><br /><br />Atmospheric Chemistry and Physics, 13, 5751-5766, 2013<br /><br />Author(s): S. Hartmann, S. Augustin, T. Clauss, H. Wex, T. Šantl-Temkiv, J. Voigtländer, D. Niedermeier, and F. Stratmann<br /><br />Utilising the Leipzig Aerosol Cloud Interaction Simulator (LACIS), the immersion freezing behaviour of droplet ensembles containing monodisperse particles, generated from a Snomax™ solution/suspension, was investigated. Thereto ice fractions were measured in the temperature range between −5 °C to −38 °C. Snomax™ is an industrial product applied for artificial snow production and contains <i>Pseudomonas</i> syringae} bacteria which have long been used as model organism for atmospheric relevant ice nucleation active (INA) bacteria. The ice nucleation activity of such bacteria is controlled by INA protein complexes in their outer membrane. <br><br> In our experiments, ice fractions increased steeply in the temperature range from about −6 °C to about −10 °C and then levelled off at ice fractions smaller than one. The plateau implies that not all examined droplets contained an INA protein complex. Assuming the INA protein complexes to be Poisson distributed over the investigated droplet populations, we developed the CHESS model (stoCHastic modEl of similar and poiSSon distributed ice nuclei) which allows for the calculation of ice fractions as function of temperature and time for a given nucleation rate. Matching calculated and measured ice fractions, we determined and parameterised the nucleation rate of INA protein complexes exhibiting class III ice nucleation behaviour. Utilising the CHESS model, together with the determined nucleation rate, we compared predictions from the model to experimental data from the literature and found good agreement. <br><br> We found that (a) the heterogeneous ice nucleation rate expression quantifying the ice nucleation behaviour of the INA protein complex is capable of describing the ice nucleation behaviour observed in various experiments for both, Snomax™ and <i>P. syringae</i> bacteria, (b) the ice nucleation rate, and its temperature dependence, seem to be very similar regardless of whether the INA protein complexes inducing ice nucleation are attached to the outer membrane of intact bacteria or membrane fragments, (c) the temperature range in which heterogeneous droplet freezing occurs, and the fraction of droplets being able to freeze, both depend on the actual number of INA protein complexes present in the droplet ensemble, and (d) possible artifacts suspected to occur in connection with the drop freezing method, i.e., the method frequently used by biologist for quantifying ice nucleation behaviour, are of minor importance, at least for substances such as <i>P. syringae</i>, which induce freezing at comparably high temperatures. The last statement implies that for single ice nucleation entities such as INA protein complexes, it is the number of entities present in the droplet population, and the entities' nucleation rate, which control the freezing behaviour of the droplet population. Quantities such as ice active surface site density are not suitable in this context. <br><br> The results obtained in this study allow a different perspective on the quantification of the immersion freezing behaviour of bacterial ice nucleation.Fri, 14 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5731/2013/<b>Diel peroxy radicals in a semi-industrial coastal area: nighttime formation of free radicals</b><br /><br />Atmospheric Chemistry and Physics, 13, 5731-5749, 2013<br /><br />Author(s): M. D. Andrés-Hernández, D. Kartal, J. N. Crowley, V. Sinha, E. Regelin, M. Martínez-Harder, V. Nenakhov, J. Williams, H. Harder, H. Bozem, W. Song, J. Thieser, M. J. Tang, Z. Hosaynali Beigi, and J. P. Burrows<br /><br />Peroxy radicals were measured by a PeRCA (Peroxy Radical Chemical Amplifier) instrument in the boundary layer during the DOMINO (Diel Oxidant Mechanisms In relation to Nitrogen Oxides) campaign at a coastal, forested site influenced by urban-industrial emissions in southern Spain in late autumn. Total peroxy radicals (RO₂* = HO₂ + &Sigma;RO₂) generally showed a daylight maximum between 10 and 50 pptv at 13:00 UTC, with an average of 18 pptv over the 15 days of measurements. Emissions from the industrial area of Huelva often impacted the measurement site at night during the campaign. The processing of significant levels of anthropogenic organics leads to an intense nocturnal radical chemistry accompanied by formation of organic peroxy radicals at comparable levels to those of summer photochemical conditions with peak events up to 60–80 pptv. The RO₂ production initiated by reactions of NO₃ with organic trace gases was estimated to be significant, but not sufficient to account for the concentrations of RO₂* observed in air masses carrying high pollutant loading. The nocturnal production of peroxy radicals in those periods seems therefore to be dominated by ozonolysis of volatile organic compounds, in particular alkenes of industrial petrochemical origin. <br><br> RO₂* diurnal variations were consistent with HO₂ measurements available at the site. HO₂/RO₂* ratios generally varied between 0.3 and 0.6, though on some occasions this ratio was likely to have been affected by instrumental artifacts (overestimated HO₂) associated with high RO₂ loads.Fri, 14 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5715/2013/<b>Production of methyl vinyl ketone and methacrolein via the hydroperoxyl pathway of isoprene oxidation</b><br /><br />Atmospheric Chemistry and Physics, 13, 5715-5730, 2013<br /><br />Author(s): Y. J. Liu, I. Herdlinger-Blatt, K. A. McKinney, and S. T. Martin<br /><br />The photo-oxidation chemistry of isoprene (ISOP; C₅H₈) was studied in a continuous-flow chamber under conditions such that the reactions of the isoprene-derived peroxyl radicals (RO₂) were dominated by the hydroperoxyl (HO₂) pathway. A proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) with switchable H₃O⁺ and NO⁺ reagent ions was used for product analysis. The products methyl vinyl ketone (MVK; C₄H₆O) and methacrolein (MACR; C₄H₆O) were differentiated using NO⁺ reagent ions. The MVK and MACR yields via the HO₂ pathway were (3.8 ± 1.3)% and (2.5 ± 0.9)%, respectively, at +25 &deg;C and < 2% relative humidity. The respective yields were (41.4 ± 5.5)% and (29.6 ± 4.2)% via the NO pathway. Production of MVK and MACR via the HO₂ pathway implies concomitant production of hydroxyl ((6.3 ± 2.1)%) and hydroperoxyl ((6.3 ± 2.1)%) radicals, meaning a HO_x recycling of (12.6 ± 4.2)% given that HO₂ was both a reactant and product. Other isoprene oxidation products, believed to be mostly organic hydroperoxides, also contributed to the ion intensity at the same mass-to-charge (<i>m/z</i>) ratios as the MVK and MACR product ions for HO₂-dominant conditions. These products were selectively removed from the gas phase by placement of a cold trap (−40 &deg;C) inline prior to the PTR-TOF-MS. When incorporated into regional and global chemical transport models, the yields of MVK and MACR and the concomitant HO_x recycling reported in this study can improve the accuracy of the simulation of the HO₂ reaction pathway of isoprene, which is believed to be the fate of approximately half of atmospherically produced isoprene-derived peroxy radicals on a global scale.Fri, 14 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5697/2013/<b>Estimating regional methane surface fluxes: the relative importance of surface and GOSAT mole fraction measurements</b><br /><br />Atmospheric Chemistry and Physics, 13, 5697-5713, 2013<br /><br />Author(s): A. Fraser, P. I. Palmer, L. Feng, H. Boesch, A. Cogan, R. Parker, E. J. Dlugokencky, P. J. Fraser, P. B. Krummel, R. L. Langenfelds, S. O'Doherty, R. G. Prinn, L. P. Steele, M. van der Schoot, and R. F. Weiss<br /><br />We use an ensemble Kalman filter (EnKF), together with the GEOS-Chem chemistry transport model, to estimate regional monthly methane (CH₄) fluxes for the period June 2009–December 2010 using proxy dry-air column-averaged mole fractions of methane (XCH₄) from GOSAT (Greenhouse gases Observing SATellite) and/or NOAA ESRL (Earth System Research Laboratory) and CSIRO GASLAB (Global Atmospheric Sampling Laboratory) CH₄ surface mole fraction measurements. Global posterior estimates using GOSAT and/or surface measurements are between 510–516 Tg yr⁻¹, which is less than, though within the uncertainty of, the prior global flux of 529 ± 25 Tg yr⁻¹. We find larger differences between regional prior and posterior fluxes, with the largest changes in monthly emissions (75 Tg yr⁻¹) occurring in Temperate Eurasia. In non-boreal regions the error reductions for inversions using the GOSAT data are at least three times larger (up to 45%) than if only surface data are assimilated, a reflection of the greater spatial coverage of GOSAT, with the two exceptions of latitudes >60° associated with a data filter and over Europe where the surface network adequately describes fluxes on our model spatial and temporal grid. We use CarbonTracker and GEOS-Chem XCO₂ model output to investigate model error on quantifying proxy GOSAT XCH₄ (involving model XCO₂) and inferring methane flux estimates from surface mole fraction data and show similar resulting fluxes, with differences reflecting initial differences in the proxy value. Using a series of observing system simulation experiments (OSSEs) we characterize the posterior flux error introduced by non-uniform atmospheric sampling by GOSAT. We show that clear-sky measurements can theoretically reproduce fluxes within 10% of true values, with the exception of tropical regions where, due to a large seasonal cycle in the number of measurements because of clouds and aerosols, fluxes are within 15% of true fluxes. We evaluate our posterior methane fluxes by incorporating them into GEOS-Chem and sampling the model at the location and time of surface CH₄ measurements from the AGAGE (Advanced Global Atmospheric Gases Experiment) network and column XCH₄ measurements from TCCON (Total Carbon Column Observing Network). The posterior fluxes modestly improve the model agreement with AGAGE and TCCON data relative to prior fluxes, with the correlation coefficients (<i>r</i>²) increasing by a mean of 0.04 (range: −0.17 to 0.23) and the biases decreasing by a mean of 0.4 ppb (range: −8.9 to 8.4 ppb).Thu, 13 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5685/2013/<b>Analysis of a winter regional haze event and its formation mechanism in the North China Plain</b><br /><br />Atmospheric Chemistry and Physics, 13, 5685-5696, 2013<br /><br />Author(s): X. J. Zhao, P. S. Zhao, J. Xu, W. Meng,, W. W. Pu, F. Dong, D. He, and Q. F. Shi<br /><br />A regional haze episode occurred in the Beijing, Tianjin and Hebei province (BTH) area in the North China Plain (NCP) from 16 to 19 January 2010. Data were collected and analyzed during the time frame of 14 through 23 January 2010 to include the haze event. The increase of secondary inorganic pollutants (SO₄^2&minus;, NO₃^&minus;, NH₄⁺) in PM_2.5 was observed simultaneously at four sites, especially in the plain area of the BTH, which could be identified as a common characteristic of pollution haze in east China. The sulfate and nitrate in PM_2.5 were mainly formed through the heterogeneous reaction process in the urban area. The organic matter (OM) increased more significantly at the Chengde (CD) site than the other three sites in the plain area. The secondary organic aerosols only existed during haze days at CD but in both haze and non-haze days at the other three sites, which suggested the greater regional impact of secondary formation process during the haze episode. The secondary formation of aerosol was one important formation mechanism of haze. The strong temperature inversion and descending air motions in the planetary boundary layer (PBL) allowed pollutants to accumulate in a shallow layer. The weak surface wind speed produced high pollutants concentration within source regions. The accumulation of pollutants was one main factor in the haze formation. The enhanced southwest wind in the last period of this episode transported pollutants to the downwind area and expanded the regional scope of the haze.Thu, 13 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5671/2013/<b>Newly observed peroxides and the water effect on the formation and removal of hydroxyalkyl hydroperoxides in the ozonolysis of isoprene</b><br /><br />Atmospheric Chemistry and Physics, 13, 5671-5683, 2013<br /><br />Author(s): D. Huang, Z. M. Chen, Y. Zhao, and H. Liang<br /><br />The ozonolysis of alkenes is considered to be an important source of atmospheric peroxides, which serve as oxidants, reservoirs of HO_x radicals, and components of secondary organic aerosols (SOAs). Recent laboratory investigations of this reaction identified hydrogen peroxide (H₂O₂) and hydroxymethyl hydroperoxide (HMHP) in ozonolysis of isoprene. Although larger hydroxyalkyl hydroperoxides (HAHPs) were also expected, their presence is not currently supported by experimental evidence. In the present study, we investigated the formation of peroxides in the gas phase ozonolysis of isoprene at various relative humidities on a time scale of tens of seconds, using a quartz flow tube reactor coupled with the online detection of peroxides. We detected a variety of conventional peroxides, including H₂O₂, HMHP, methyl hydroperoxide, bis-hydroxymethyl hydroperoxide, and ethyl hydroperoxide, and interestingly found three unknown peroxides. The molar yields of the conventional peroxides fell within the range of values provided in the literature. The three unknown peroxides had a combined molar yield of ~ 30% at 5% relative humidity (RH), which was comparable with that of the conventional peroxides. Unlike H₂O₂ and HMHP, the molar yields of these three unknown peroxides were inversely related to the RH. On the basis of experimental kinetic and box model analysis, we tentatively assigned these unknown peroxides to C2−C4 HAHPs, which are produced by the reactions of different Criegee intermediates with water. Our study provides experimental evidence for the formation of large HAHPs in the ozonolysis of isoprene (one of the alkenes). These large HAHPs have a sufficiently long lifetime, estimated as tens of minutes, which allows them to become involved in atmospheric chemical processes, e.g., SOA formation and radical recycling.Wed, 12 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5655/2013/<b>Megacity impacts on regional ozone formation: observations and WRF-Chem modeling for the MIRAGE-Shanghai field campaign</b><br /><br />Atmospheric Chemistry and Physics, 13, 5655-5669, 2013<br /><br />Author(s): X. Tie, F. Geng, A. Guenther, J. Cao, J. Greenberg, R. Zhang, E. Apel, G. Li, A. Weinheimer, J. Chen, and C. Cai<br /><br />The MIRAGE-Shanghai experiment was designed to characterize the factors controlling regional air pollution near a Chinese megacity (Shanghai) and was conducted during September 2009. This paper provides information on the measurements conducted for this study. In order to have some deep analysis of the measurements, a regional chemical/dynamical model (version 3 of Weather Research and Forecasting Chemical model – WRF-Chemv3) is applied for this study. The model results are intensively compared with the measurements to evaluate the model capability for calculating air pollutants in the Shanghai region, especially the chemical species related to ozone formation. The results show that the model is able to calculate the general distributions (the level and the variability) of air pollutants in the Shanghai region, and the differences between the model calculation and the measurement are mostly smaller than 30%, except the calculations of HONO (nitrous acid) at PD (Pudong) and CO (carbon monoxide) at DT (Dongtan). <br><br> The main scientific focus is the study of ozone chemical formation not only in the urban area, but also on a regional scale of the surrounding area of Shanghai. The results show that during the experiment period, the ozone photochemical formation was strongly under the VOC (volatile organic compound)-limited condition in the urban area of Shanghai. Moreover, the VOC-limited condition occurred not only in the city, but also in the larger regional area. There was a continuous enhancement of ozone concentrations in the downwind of the megacity of Shanghai, resulting in a significant enhancement of ozone concentrations in a very large regional area in the surrounding region of Shanghai. The sensitivity study of the model suggests that there is a threshold value for switching from VOC-limited condition to NO_x (nitric oxide and nitrogen dioxide)-limited condition. The threshold value is strongly dependent on the emission ratio of NO_x / VOCs. When the ratio is about 0.4, the Shanghai region is under a strong VOC-limited condition over the regional scale. In contrast, when the ratio is reduced to about 0.1, the Shanghai region is under a strong NO_x-limited condition. The estimated threshold value (on the regional scale) for switching from VOC-limited to NO_x-limited condition ranges from 0.1 to 0.2. This result has important implications for ozone production in this region and will facilitate the development of effective O₃ control strategies in the Shanghai region.Mon, 10 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5647/2013/<b>Estimate of surface direct radiative forcing of desert dust from atmospheric modulation of the aerosol optical depth</b><br /><br />Atmospheric Chemistry and Physics, 13, 5647-5654, 2013<br /><br />Author(s): A. di Sarra, D. Fuà, and D. Meloni<br /><br />Measurements carried out on the island of Lampedusa, in the central Mediterranean, on 7 September 2005, show the occurrence of a quasi-periodic oscillation of aerosol optical depth, column water vapour, and surface irradiance in different spectral bands. The oscillation has a period of about 13 min and is attributed to the propagation of a gravity wave able to modify the vertical structure of the planetary boundary layer, as also confirmed by satellite images. The wave occurred during a Saharan dust event. The oscillation amplitude is about 0.1 for the aerosol optical depth, and about 0.4 cm for the column water vapour. The modulation of the downward surface irradiances is in opposition of phase with respect to aerosol optical depth and water vapour column variations. The perturbation of the downward irradiance produced by the aerosols is determined by comparing the measured irradiances with estimated irradiances at a fixed value of the aerosol optical depth, and by correcting for the effect of the water vapour in the shortwave spectral range. The direct radiative forcing efficiency, i.e., the radiative perturbation of the net surface irradiance produced by a unit of optical depth aerosol layer, is determined at different solar zenith angles as the slope of the irradiance perturbation versus the aerosol optical depth. The estimated direct surface forcing efficiency at about 60° solar zenith angle is −(181 ± 17) W m⁻² in the shortwave, and −(83 ± 7) W m⁻² in the photosynthetic spectral range. The estimated daily average forcing efficiencies are of about −79 and −46 W m⁻² for the shortwave and photosynthetic spectral range, respectively.Mon, 10 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5631/2013/<b>Large-eddy simulation of organized precipitating trade wind cumulus clouds</b><br /><br />Atmospheric Chemistry and Physics, 13, 5631-5645, 2013<br /><br />Author(s): A. Seifert and T. Heus<br /><br />Trade wind cumulus clouds often organize in along-wind cloud streets and across-wind mesoscale arcs. We present a benchmark large-eddy simulation which resolves the individual clouds as well as the mesoscale organization on scales of O(10 km). Different methods to quantify organization of cloud fields are applied and discussed. Using perturbed physics large-eddy simulation experiments, the processes leading to the formation of cloud clusters and the mesoscale arcs are revealed. We find that both cold pools as well as the sub-cloud layer moisture field are crucial to understand the organization of precipitating shallow convection. Further sensitivity studies show that microphysical assumptions can have a pronounced impact on the onset of cloud organization.Mon, 10 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5629/2013/<b>Corrigendum to "Tropospheric NO₂ vertical column densities over Beijing: results of the first three years of ground-based MAX-DOAS measurements (2008–2011) and satellite validation" published in Atmos. Chem. Phys., 13, 1547&ndash;1567, 2013</b><br /><br />Atmospheric Chemistry and Physics, 13, 5629-5629, 2013<br /><br />Author(s): J. Z. Ma, S. Beirle, J. L. Jin, R. Shaiganfar, P. Yan, and T. Wagner<br /><br />No abstract available.Mon, 10 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5615/2013/<b>State transformations and ice nucleation in amorphous (semi-)solid organic aerosol</b><br /><br />Atmospheric Chemistry and Physics, 13, 5615-5628, 2013<br /><br />Author(s): K. J. Baustian, M. E. Wise, E. J. Jensen, G. P. Schill, M. A. Freedman, and M. A. Tolbert<br /><br />Amorphous (semi-)solid organic aerosol particles have the potential to serve as surfaces for heterogeneous ice nucleation in cirrus clouds. Raman spectroscopy and optical microscopy have been used in conjunction with a cold stage to examine water uptake and ice nucleation on individual amorphous (semi-)solid particles at atmospherically relevant temperatures (200–273 K). Three organic compounds considered proxies for atmospheric secondary organic aerosol (SOA) were used in this investigation: sucrose, citric acid and glucose. Internally mixed particles consisting of each organic and ammonium sulfate were also investigated. <br><br> Results from water uptake experiments followed the shape of a humidity-induced glass transition (<i>T</i>_g(RH)) curve and were used to construct state diagrams for each organic and corresponding mixture. Experimentally derived <i>T</i>_g(RH) curves are in good agreement with theoretical predictions of <i>T</i>_g(RH) following the approach of Koop et al. (2011). A unique humidity-induced glass transition point on each state diagram, <i>T</i>_g'(RH), was used to quantify and compare results from this study to previous works. Values of <i>T</i>_g'(RH) determined for sucrose, glucose and citric acid glasses were 236, 230 and 220 K, respectively. Values of <i>T</i>_g'(RH) for internally mixed organic/sulfate particles were always significantly lower; 210, 207 and 215 K for sucrose/sulfate, glucose/sulfate and citric acid/sulfate, respectively. <br><br> All investigated SOA proxies were observed to act as heterogeneous ice nuclei at tropospheric temperatures. Heterogeneous ice nucleation on pure organic particles occurred at <i>S</i>_ice = 1.1–1.4 for temperatures below 235 K. Particles consisting of 1:1 organic-sulfate mixtures took up water over a greater range of conditions but were in some cases also observed to heterogeneously nucleate ice at temperatures below 202 K (<i>S</i>_ice= 1.25–1.38). <br><br> Polynomial curves were fitted to experimental water uptake data and then incorporated into the Community Aerosol Radiation Model for Atmospheres (CARMA) along with the predicted range of humidity-induced glass transition temperatures for atmospheric SOA from Koop et al. (2011). Model results suggest that organic and organic/sulfate aerosol could be glassy more than 60% of the time in the midlatitude upper troposphere and more than 40% of the time in the tropical tropopause region (TTL). At conditions favorable for ice formation (<i>S</i>_ice > 1), particles in the TTL are expected to be glassy more than 50% of the time for temperatures below 200 K. Results from this study suggests that amorphous (semi-)solid organic particles are often present in the upper troposphere and that heterogeneous ice formation on this type of particle may play an important role in cirrus cloud formation.Fri, 07 Jun 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5601/2013/<b>Observations of peroxyacetyl nitrate (PAN) in the upper troposphere by the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS)</b><br /><br />Atmospheric Chemistry and Physics, 13, 5601-5613, 2013<br /><br />Author(s): K. A. Tereszchuk, D. P. Moore, J. J. Harrison, C. D. Boone, M. Park, J. J. Remedios, W. J. Randel, and P. F. Bernath<br /><br />Peroxyacetyl nitrate (CH₃CO·O₂NO₂, abbreviated as PAN) is a trace molecular species present in the troposphere and lower stratosphere due primarily to pollution from fuel combustion and the pyrogenic outflows from biomass burning. In the lower troposphere, PAN has a relatively short lifetime and is principally destroyed within a few hours through thermolysis, but it can act as a reservoir and carrier of NO_x in the colder temperatures of the upper troposphere, where UV photolysis becomes the dominant loss mechanism. Pyroconvective updrafts from large biomass burning events can inject PAN into the upper troposphere and lower stratosphere (UTLS), providing a means for the long-range transport of NO_x. Given the extended lifetimes at these higher altitudes, PAN is readily detectable via satellite remote sensing. <br><br> A new PAN data product is now available for the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) version 3.0 data set. We report observations of PAN in boreal biomass burning plumes recorded during the BORTAS (quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites) campaign (12 July to 3 August 2011). The retrieval method employed by incorporating laboratory-recorded absorption cross sections into version 3.0 of the ACE-FTS forward model and retrieval software is described in full detail. The estimated detection limit for ACE-FTS PAN is 5 pptv, and the total systematic error contribution to the ACE-FTS PAN retrieval is ~ 16%. <br><br> The retrieved volume mixing ratio (VMR) profiles are compared to coincident measurements made by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument on the European Space Agency (ESA) Environmental Satellite (ENVISAT). The MIPAS measurements demonstrated good agreement with the ACE-FTS VMR profiles for PAN, where the measured VMR values are well within the associated measurement errors for both instruments and comparative measurements differ no more than 70 pptv. <br><br> The ACE-FTS PAN data set is used to obtain zonal mean distributions of seasonal averages from ~ 5–20 km. A strong seasonality is clearly observed for PAN concentrations in the global UTLS. Since the principal source of PAN in the UTLS is due to lofted biomass burning emissions from the pyroconvective updrafts created by large fires, the observed seasonality in enhanced PAN coincides with fire activity in different geographical regions throughout the year.Thu, 06 Jun 2013 00:00:00 +0200