http://www.atmos-chem-phys.net/Atmospheric Chemistry and Physics Latest Articlesenhttp://www.atmos-chem-phys.net/13/5103/2013/<b>Evaluation of seasonal atmosphere–biosphere exchange estimations with TCCON measurements</b><br /><br />Atmospheric Chemistry and Physics, 13, 5103-5115, 2013<br /><br />Author(s): J. Messerschmidt, N. Parazoo, D. Wunch, N. M. Deutscher, C. Roehl, T. Warneke, and P. O. Wennberg<br /><br />We evaluate three estimates of the atmosphere-biosphere exchange against total column CO₂ observations from the Total Carbon Column Observing Network (TCCON). Using the GEOS-Chem transport model, we produce forward simulations of atmospheric CO₂ concentrations for the 2006–2010 time period using the Carnegie-Ames-Stanford Approach (CASA), the Simple Biosphere (SiB) and the GBiome-BGC models. Large differences in the CO₂ simulations result from the choice of the atmosphere-biosphere model. We evaluate the seasonal cycle phase, amplitude and shape of the simulations. The version of CASA currently used as the a priori model by the GEOS-Chem carbon cycle community poorly represents the season cycle in total column CO₂. Consistent with earlier studies, enhancing the CO₂ uptake in the boreal forest and shifting the onset of the growing season earlier significantly improve the simulated seasonal CO₂ cycle using CASA estimates. The SiB model gives a better representation of the seasonal cycle dynamics. The difference in the seasonality of net ecosystem exchange (NEE) between these models is not the absolute gross primary productivity (GPP), but rather the differential phasing of ecosystem respiration (RE) with respect to GPP between these models.Fri, 17 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5089/2013/<b>Effects of internal mixing and aggregate morphology on optical properties of black carbon using a discrete dipole approximation model</b><br /><br />Atmospheric Chemistry and Physics, 13, 5089-5101, 2013<br /><br />Author(s): B. V. Scarnato, S. Vahidinia, D. T. Richard, and T. W. Kirchstetter<br /><br />According to recent studies, internal mixing of black carbon (BC) with other aerosol materials in the atmosphere alters its aggregate shape, absorption of solar radiation, and radiative forcing. These mixing state effects are not yet fully understood. In this study, we characterize the morphology and mixing state of bare BC and BC internally mixed with sodium chloride (NaCl) using electron microscopy and examine the sensitivity of optical properties to BC mixing state and aggregate morphology using a discrete dipole approximation model (DDSCAT). DDSCAT is flexible in simulating the geometry and refractive index of particle aggregates. DDSCAT predicts a higher mass absorption coefficient (MAC), lower single scattering albedo (SSA), and higher absorption Angstrom exponent (AAE) for bare BC aggregates that are lacy rather than compact. Predicted values of SSA at 550 nm range between 0.16 and 0.27 for lacy and compact aggregates, respectively, in agreement with reported experimental values of 0.25 ± 0.05. The variation in absorption with wavelength does not adhere precisely to a power law relationship over the 200 to 1000 nm range. Consequently, AAE values depend on the wavelength region over which they are computed. The MAC of BC (averaged over the 200–1000 nm range) is amplified when internally mixed with NaCl (100–300 nm in radius) by factors ranging from 1.0 for lacy BC aggregates partially immersed in NaCl to 2.2 for compact BC aggregates fully immersed in NaCl. The SSA of BC internally mixed with NaCl is higher than for bare BC and increases with the embedding in the NaCl. Internally mixed BC SSA values decrease in the 200–400 nm wavelength range, a feature also common to the optical properties of dust and organics. Linear polarization features are also predicted in DDSCAT and are dependent on particle size and morphology. <br><br> This study shows that DDSCAT predicts complex morphology and mixing state dependent aerosol optical properties that have been reported previously and are relevant to radiative transfer, climate modeling, and interpretation of remote sensing measurements.Thu, 16 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5063/2013/<b>Singular vector decomposition for sensitivity analyses of tropospheric chemical scenarios</b><br /><br />Atmospheric Chemistry and Physics, 13, 5063-5087, 2013<br /><br />Author(s): N. Goris and H. Elbern<br /><br />Observations of the chemical state of the atmosphere typically provide only sparse snapshots of the state of the system due to their insufficient temporal and spatial density. One possibility for optimisation of the state estimate is to target the observation of those parameters that have the largest potential of resulting in forecast improvements. In the present work, the technique of singular vector analysis is applied to atmospheric chemical modelling in order to identify the most sensitive chemical compounds during a given time period and prioritise them for measurement. Novel to the current work is the fact that, in the application of singular vector analysis, not only the initial values but also the emissions are considered as target variables for adaptive observation strategies. This specific application of singular vector analysis is studied in the context of a chemistry box model allowing for validation of its new features for two chemical regimes. The time and regime dependence of the ozone (O₃) and peroxyacetyl nitrate (PAN) formation potential of individual volatile organic compounds (VOCs) is investigated. Results show that the combined sensitivity of O₃ and PAN to individual VOCs is strongly dependent on the photochemical scenario and simulation interval used. Particularly the alkanes show increasing sensitivities with increasing simulation length. Classifying the VOCs as being of high, medium, little or negligible importance for the formation of O₃ and PAN allows for the identification of those VOCs that may be omitted from measurement. We find that it is possible to omit 6 out of 18 VOCs considered for initial value measurement and 4 out of 12 VOCs considered for emission measurement. The omission of these VOCs is independent of photochemical regime and simulation length. The VOCs selected for measuring account for more than 96% and 90% of the O₃ and PAN sensitivity to VOCs, respectively.Wed, 15 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5049/2013/<b>Aerosol mixing state, hygroscopic growth and cloud activation efficiency during MIRAGE 2006</b><br /><br />Atmospheric Chemistry and Physics, 13, 5049-5062, 2013<br /><br />Author(s): S. Lance, T. Raatikainen, T. B. Onasch, D. R. Worsnop, X.-Y. Yu, M. L. Alexander, M. R. Stolzenburg, P. H. McMurry, J. N. Smith, and A. Nenes<br /><br />Observations of aerosol hygroscopic growth and CCN activation spectra for submicron particles are reported for the T1 ground site outside of Mexico City during the MIRAGE 2006 campaign. <i>κ</i>-Köhler theory is used to evaluate the characteristic hygroscopicity parameter, &kappa;*, for the CCN active aerosol population using both size-resolved HTMDA and size-resolved CCNc measurements. Organic mass fractions (<i>f</i>_org) are evaluated from size-resolved aerosol mass spectrometer (AMS) measurements, from which predictions of the hygroscopicity parameter are compared against &kappa;*. <br></br> Strong diurnal changes in aerosol water uptake parameters and aerosol composition are observed. We find that new particle formation (NPF) events are correlated with an increased &kappa;* and CCN-active fraction during the daytime, with greater impact on smaller particles. During NPF events, the number concentration of 40 nm particles acting as CCN at 0.51% ± 0.06% supersaturation can surpass by more than a factor of two the corresponding concentrations of 100 nm particles. We also find that at 06:00–08:00 LT throughout the campaign, fresh traffic emissions result in substantial changes to the chemical distribution of the aerosol, with on average 65% externally mixed fraction for 40 nm particles and 30% externally mixed fraction for 100 nm particles, whereas at midday nearly all particles of both sizes can be described as "internally mixed". <br></br> Average activation spectra and growth factor distributions are analyzed for different time periods characterizing the daytime (with and without NPF events), the early morning "rush hour" and the entire campaign. We show that &kappa;* derived from CCNc measurements decreases as a function of size during all time periods, while the CCN-active fraction increases as a function of size. Size-resolved AMS measurements do not predict the observed trend for &kappa;* versus particle size, which can be attributed to unresolved mixing state and the presence of refractory material not measured by the AMS. Measured &kappa;* typically ranges from 0.2 to 0.35, and organics typically make up 60–85 % of the aerosol mass in the size range studied. We show that &kappa;_AMS is able to describe CCN concentrations reasonably well, provided mixing-state information is available, especially at the highest CCN concentrations. This is consistent with other CCN studies carried out in urban environments, and is partly due to the fact that the highest CCN concentrations occur during the daytime when the aerosol is internally mixed. During the early morning rush hour, however, failing to account for the aerosol mixing state results in systematic overestimation of CCN concentrations by as much as 50–100% on average.Wed, 15 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5033/2013/<b>Stratospheric ozone interannual variability (1995–2011) as observed by lidar and satellite at Mauna Loa Observatory, HI and Table Mountain Facility, CA</b><br /><br />Atmospheric Chemistry and Physics, 13, 5033-5047, 2013<br /><br />Author(s): G. Kirgis, T. Leblanc, I. S. McDermid, and T. D. Walsh<br /><br />The Jet Propulsion Laboratory (JPL) lidars, at the Mauna Loa Observatory, Hawaii (MLO, 19.5° N, 155.6° W) and the JPL Table Mountain Facility (TMF, California, 34.5° N, 117.7° W), have been measuring vertical profiles of stratospheric ozone routinely since the early 1990's and late-1980s respectively. Interannual variability of ozone above these two sites was investigated using a multi-linear regression analysis on the deseasonalised monthly mean lidar and satellite time-series at 1 km intervals between 20 and 45 km from January 1995 to April 2011, a period of low volcanic aerosol loading. Explanatory variables representing the 11 yr solar cycle, the El Niño Southern Oscillation, the Quasi-Biennial Oscillation, the Eliassen-Palm flux, and horizontal and vertical transport were used. A new proxy, the mid-latitude Ozone Depleting Gas Index, which shows a decrease with time as an outcome of the Montreal Protocol, was introduced and compared to the more commonly used linear trend method. The analysis also compares the lidar time-series and a merged time-series obtained from the space-borne Stratospheric Aerosol and Gas Experiment II, Halogen Occultation Experiment, and Aura-Microwave Limb Sounder instruments. <br><br> The results from both lidar and satellite measurements are consistent with recent model simulations which propose changes in tropical upwelling. Additionally, at TMF the Ozone Depleting Gas Index explains as much variance as the Quasi-Biennial Oscillation in the upper stratosphere. Over the past 17 yr a diminishing downward trend in ozone was observed before 2000 and a net increase, and sign of ozone recovery, is observed after 2005. Our results which include dynamical proxies suggest possible coupling between horizontal transport and the 11 yr solar cycle response, although a dataset spanning a period longer than one solar cycle is needed to confirm this result.Wed, 15 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/5017/2013/<b>Modeling organic aerosol from the oxidation of α-pinene in a Potential Aerosol Mass (PAM) chamber</b><br /><br />Atmospheric Chemistry and Physics, 13, 5017-5031, 2013<br /><br />Author(s): S. Chen, W. H. Brune, A. T. Lambe, P. Davidovits, and T. B. Onasch<br /><br />A model has been developed to simulate the formation and evolution of secondary organic aerosol (SOA) and was tested against data produced in a Potential Aerosol Mass (PAM) flow reactor and a large environmental chamber. The model framework is based on the two-dimensional volatility basis set approach (2D-VBS), in which SOA oxidation products in the model are distributed on the 2-D space of effective saturation concentration (<i>C</i>_<i>i</i>*) and oxygen-to-carbon ratio (O : C). The modeled organic aerosol mass concentrations (<i>C</i>_OA) and O : C agree with laboratory measurements within estimated uncertainties. However, while both measured and modeled O : C increase with increasing OH exposure as expected, the increase of modeled O : C is rapid at low OH exposure and then slows as OH exposure increases while the increase of measured O : C is initially slow and then accelerates as OH exposure increases. A global sensitivity analysis indicates that modeled <i>C</i>_OA values are most sensitive to the assumed values for the number of <i>C</i>_<i>i</i>* bins, the heterogeneous OH reaction rate coefficient, and the yield of first-generation products. Modeled SOA O : C values are most sensitive to the assumed O : C of first-generation oxidation products, the number of <i>C</i>_<i>i</i>* bins, the heterogeneous OH reaction rate coefficient, and the number of O : C bins. All these sensitivities vary as a function of OH exposure. The sensitivity analysis indicates that the 2D-VBS model framework may require modifications to resolve discrepancies between modeled and measured O : C as a function of OH exposure.Wed, 15 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/4997/2013/<b>Characterization of urban aerosol in Cork city (Ireland) using aerosol mass spectrometry</b><br /><br />Atmospheric Chemistry and Physics, 13, 4997-5015, 2013<br /><br />Author(s): M. Dall'Osto, J. Ovadnevaite, D. Ceburnis, D. Martin, R. M. Healy, I. P. O'Connor, I. Kourtchev, J. R. Sodeau, J. C. Wenger, and C. O'Dowd<br /><br />Ambient wintertime background urban aerosol in Cork city, Ireland, was characterized using aerosol mass spectrometry. During the three-week measurement study in 2009, 93% of the ca. 1 350 000 single particles characterized by an Aerosol Time-of-Flight Mass Spectrometer (TSI ATOFMS) were classified into five organic-rich particle types, internally mixed to different proportions with elemental carbon (EC), sulphate and nitrate, while the remaining 7% was predominantly inorganic in nature. Non-refractory PM₁ aerosol was characterized using a High Resolution Time-of-Flight Aerosol Mass Spectrometer (Aerodyne HR-ToF-AMS) and was also found to comprise organic aerosol as the most abundant species (62%), followed by nitrate (15%), sulphate (9%) and ammonium (9%), and chloride (5%). <br><br> Positive matrix factorization (PMF) was applied to the HR-ToF-AMS organic matrix, and a five-factor solution was found to describe the variance in the data well. Specifically, "hydrocarbon-like" organic aerosol (HOA) comprised 20% of the mass, "low-volatility" oxygenated organic aerosol (LV-OOA) comprised 18%, "biomass burning" organic aerosol (BBOA) comprised 23%, non-wood solid-fuel combustion "peat and coal" organic aerosol (PCOA) comprised 21%, and finally a species type characterized by primary \textit{m/z}~peaks at 41 and 55, similar to previously reported "cooking" organic aerosol (COA), but possessing different diurnal variations to what would be expected for cooking activities, contributed 18%. <br><br> Correlations between the different particle types obtained by the two aerosol mass spectrometers are also discussed. Despite wood, coal and peat being minor fuel types used for domestic space heating in urban areas, their relatively low combustion efficiencies result in a significant contribution to PM₁ aerosol mass (44% and 28% of the total organic aerosol mass and non-refractory total PM₁, respectively).Wed, 15 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/4983/2013/<b>Continuous atmospheric boundary layer observations in the coastal urban area of Barcelona during SAPUSS</b><br /><br />Atmospheric Chemistry and Physics, 13, 4983-4996, 2013<br /><br />Author(s): M. Pandolfi, G. Martucci, X. Querol, A. Alastuey, F. Wilsenack, S. Frey, C. D. O'Dowd, and M. Dall'Osto<br /><br />Continuous measurements of surface mixed layer (SML), decoupled residual/convective layer (DRCL) and aerosol backscatter coefficient were performed within the Barcelona (Spain) boundary layer from September to October 2010 (30 days) in the framework of the SAPUSS (Solving Aerosol Problems by Using Synergistic Strategies) field campaign. Two near-infrared ceilometers (Jenoptik CHM15K), vertically and horizontally probing (only vertical profiles are herein discussed), were deployed. Ceilometer-based DRCLs (1761 ± 363 m a.g.l.) averaged over the campaign duration were twice as high as the mean SML (904 ± 273 m a.g.l.). Both DRCL and SML showed a marked SML diurnal cycle. Ceilometer data were compared with potential temperature profiles measured by daily radiosounding (twice a day, midnight and midday) to interpret the boundary layer structure in the coastal urban area of Barcelona. The overall agreement (<i>R</i>² = 0.80) between the ceilometer-retrieved and radiosounding-based SML heights (<i>h</i>) revealed overestimation of the SML by the ceilometer (&Delta;<i>h</i>=145 ± 145 m). After separating the data in accordance with different atmospheric scenarios, the lowest SML (736 ± 183 m) and DRCL (1573 ± 428 m) were recorded during warm North African (NAF) advected air mass. By contrast, higher SML and DRCL were observed during stagnant Regional (REG) (911 ± 234 m and 1769 ± 314 m, respectively) and cold Atlantic (ATL) (965 ± 222 m and 1878 ± 290 m, respectively) air masses. In addition to being the lowest, the SML during the NAF scenario frequently showed a flat upper boundary throughout the day possibly because of the strong winds from the Mediterranean Sea limiting the midday SML convective growth. The mean backscatter coefficients were calculated at two selected heights representative of middle and top SML portions, i.e. &beta;₅₀₀ = 0.59 ± 0.45 Mm⁻¹ sr⁻¹ and &beta;₈₀₀ = 0.87 ± 0.68 Mm⁻¹ sr⁻¹ at 500 m and 800 m a.g.l., respectively. The highest backscatter coefficients were observed during NAF (&beta;₅₀₀ = 0.77 ± 0.57 Mm⁻¹ sr⁻¹) when compared with ATL (&beta;₅₀₀ = 0.51 ± 0.44 Mm⁻¹ sr⁻¹) and REG (&beta;₅₀₀ = 0.64 ± 0.39 Mm⁻¹ sr⁻¹). The relationship between the vertical change in backscatter coefficient and atmospheric stability (&part;&theta;/&part;<i>z</i>) was investigated in the first 3000 m a.g.l., aiming to study how the unstable, stable or neutral atmospheric conditions of the atmosphere alter the distribution of aerosol backscatter with height over Barcelona. A positive correlation between unstable conditions and enhanced backscatter and vice versa was found.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/4963/2013/<b>Evaluating and constraining ice cloud parameterizations in CAM5 using aircraft measurements from the SPARTICUS campaign</b><br /><br />Atmospheric Chemistry and Physics, 13, 4963-4982, 2013<br /><br />Author(s): K. Zhang, X. Liu, M. Wang, J. M. Comstock, D. L. Mitchell, S. Mishra, and G. G. Mace<br /><br />This study uses aircraft measurements of relative humidity and ice crystal size distribution collected during the SPARTICUS (Small PARTicles In CirrUS) field campaign to evaluate and constrain ice cloud parameterizations in the Community Atmosphere Model version 5. About 200 h of data were collected during the campaign between January and June 2010, providing the longest aircraft measurements available so far for cirrus clouds in the midlatitudes. The probability density function (PDF) of ice crystal number concentration (<i>N</i>_i) derived from the high-frequency (1 Hz) measurements features a strong dependence on ambient temperature. As temperature decreases from −35 °C to −62 °C, the peak in the PDF shifts from 10–20 L⁻¹ to 200–1000 L⁻¹, while <i>N</i>_i shows a factor of 6–7 increase. <br><br> Model simulations are performed with two different ice nucleation schemes for pure ice-phase clouds. One of the schemes can reproduce a clear increase of <i>N</i>_i with decreasing temperature by using either an observation-based ice nuclei spectrum or a classical-theory-based spectrum with a relatively low (5–10%) maximum freezing ratio for dust aerosols. The simulation with the other scheme, which assumes a high maximum freezing ratio (100%), shows much weaker temperature dependence of <i>N</i>_i. Simulations are also performed to test empirical parameters related to water vapor deposition and the autoconversion of ice crystals to snow. Results show that a value between 0.05 and 0.1 for the water vapor deposition coefficient, and 250 μm for the critical diameter that distinguishes ice crystals from snow, can produce good agreement between model simulation and the SPARTICUS measurements in terms of <i>N</i>_i and effective radius. The climate impact of perturbing these parameters is also discussed.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/4941/2013/<b>CO₂ dispersion modelling over Paris region within the CO₂-MEGAPARIS project</b><br /><br />Atmospheric Chemistry and Physics, 13, 4941-4961, 2013<br /><br />Author(s): C. Lac, R. P. Donnelly, V. Masson, S. Pal, S. Riette, S. Donier, S. Queguiner, G. Tanguy, L. Ammoura, and I. Xueref-Remy<br /><br />Accurate simulation of the spatial and temporal variability of tracer mixing ratios over urban areas is a challenging and interesting task needed to be performed in order to utilise CO₂ measurements in an atmospheric inverse framework and to better estimate regional CO₂ fluxes. This study investigates the ability of a high-resolution model to simulate meteorological and CO₂ fields around Paris agglomeration during the March field campaign of the CO₂-MEGAPARIS project. The mesoscale atmospheric model Meso-NH, running at 2 km horizontal resolution, is coupled with the Town Energy Balance (TEB) urban canopy scheme and with the Interactions between Soil, Biosphere and Atmosphere CO₂-reactive (ISBA-A-gs) surface scheme, allowing a full interaction of CO₂ modelling between the surface and the atmosphere. Statistical scores show a good representation of the urban heat island (UHI) with stronger urban–rural contrasts on temperature at night than during the day by up to 7 °C. Boundary layer heights (BLH) have been evaluated on urban, suburban and rural sites during the campaign, and also on a suburban site over 1 yr. The diurnal cycles of the BLH are well captured, especially the onset time of the BLH increase and its growth rate in the morning, which are essential for tall tower CO₂ observatories. The main discrepancy is a small negative bias over urban and suburban sites during nighttime (respectively 45 m and 5 m), leading to a few overestimations of nocturnal CO₂ mixing ratios at suburban sites and a bias of +5 ppm. The diurnal CO₂ cycle is generally well captured for all the sites. At the Eiffel tower, the observed spikes of CO₂ maxima occur every morning exactly at the time at which the atmospheric boundary layer (ABL) growth reaches the measurement height. At suburban ground stations, CO₂ measurements exhibit maxima at the beginning and at the end of each night, when the ABL is fully contracted, with a strong spatio-temporal variability. A sensitivity test without urban parameterisation removes the UHI and underpredicts nighttime BLH over urban and suburban sites, leading to large overestimation of nocturnal CO₂ mixing ratio at the suburban sites (bias of +17 ppm). The agreement between observation and prediction for BLH and CO₂ concentrations and urban–rural increments, both day and night, demonstrates the potential of using the urban mesoscale system in the context of inverse modellingTue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/4917/2013/<b>The mass and number size distributions of black carbon aerosol over Europe</b><br /><br />Atmospheric Chemistry and Physics, 13, 4917-4939, 2013<br /><br />Author(s): C. L. Reddington, G. McMeeking, G. W. Mann, H. Coe, M. G. Frontoso, D. Liu, M. Flynn, D. V. Spracklen, and K. S. Carslaw<br /><br />Black carbon-containing aerosol particles play an important role in the direct and indirect radiative forcing of climate. However, the magnitude and sign of the net radiative effect is strongly dependent on the physical properties of the black carbon (BC) component of the particles, such as mass concentration, number size distribution and mixing state. Here we use a global aerosol model combined with aircraft measurements of BC particle number and size from the Single Particle Soot Photometer (SP2) to assess the realism with which these physical properties are predicted by global models. The comparison reveals a substantial mismatch between the measured and modelled BC size distribution over the size range of the SP2 instrument (90–400 nm BC diameter). The model predicts BC particle number concentrations a factor ~3.5–5.7 higher than measured and a mode diameter that is ~40–65 nm smaller than observed. More than ~90% of the model particles with dry diameters &gtrsim;260 nm contain BC, while the observations suggest only 14% on average. These model–observation biases in the BC properties are considerably greater than for the overall particle distribution, suggesting that the discrepancy is associated with model assumptions about the size and mixing state of the emitted carbonaceous particles. We expect the discrepancy in BC size distribution to be common among most global aerosol models, with implications for model estimates of absorption optical depth and direct radiative forcing.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/4907/2013/<b>Linkages between ozone-depleting substances, tropospheric oxidation and aerosols</b><br /><br />Atmospheric Chemistry and Physics, 13, 4907-4916, 2013<br /><br />Author(s): A. Voulgarakis, D. T. Shindell, and G. Faluvegi<br /><br />Coupling between the stratosphere and the troposphere allows changes in stratospheric ozone abundances to affect tropospheric chemistry. Large-scale effects from such changes on chemically produced tropospheric aerosols have not been systematically examined in past studies. We use a composition-climate model to investigate potential past and future impacts of changes in stratospheric ozone depleting substances (ODS) on tropospheric oxidants and sulfate aerosols. In most experiments, we find significant responses in tropospheric photolysis and oxidants, with small but significant effects on methane radiative forcing. The response of sulfate aerosols is sizeable when examining the effect of increasing future nitrous oxide (N₂O) emissions. We also find that without the regulation of chlorofluorocarbons (CFCs) through the Montreal Protocol, sulfate aerosols could have increased by 2050 by a comparable amount to the decreases predicted due to relatively stringent sulfur emissions controls. The individual historical radiative forcings of CFCs and N₂O through their indirect effects on methane (−22.6 mW m⁻² for CFCs and −6.7 mW m⁻² for N₂O) and sulfate aerosols (−3.0 mW m⁻² for CFCs and +6.5 mW m⁻² for N₂O when considering the direct aerosol effect) discussed here are non-negligible when compared to known historical ODS forcing. Our results stress the importance of accounting for stratosphere-troposphere, gas-aerosol and composition-climate interactions when investigating the effects of changing emissions on atmospheric composition and climate.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/4895/2013/<b>Effect of atmospheric organic complexation on iron-bearing dust solubility</b><br /><br />Atmospheric Chemistry and Physics, 13, 4895-4905, 2013<br /><br />Author(s): R. Paris and K. V. Desboeufs<br /><br />Recent studies reported that the effect of organic complexation may be a potentially important process to be considered by models estimating atmospheric iron flux to the ocean. In this study, we investigated this process effect by a series of dissolution experiments on iron-bearing dust in the presence or the absence of various organic compounds (acetate, formate, oxalate, malonate, succinate, glutarate, glycolate, lactate, tartrate and humic acid as an analogue of humic like substances, HULIS) typically found in atmospheric waters. Only 4 of tested organic ligands (oxalate, malonate, tartrate and humic acid) caused an enhancement of iron solubility which was associated with an increase of dissolved Fe(II) concentrations. For all of these organic ligands, a positive linear dependence of iron solubility to organic concentrations was observed and showed that the extent of organic complexation on iron solubility decreased in the following order: oxalate >malonate = tartrate > humic acid. This was attributed to the ability of electron donors of organic ligands and implies a reductive ligand-promoted dissolution. This study confirms that among the known atmospheric organic binding ligands of Fe, oxalate is the most effective ligand promoting dust iron solubility and showed, for the first time, the potential effect of HULIS on iron dissolution under atmospheric conditions.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/4877/2013/<b>Overview of aerosol properties associated with air masses sampled by the ATR-42 during the EUCAARI campaign (2008)</b><br /><br />Atmospheric Chemistry and Physics, 13, 4877-4893, 2013<br /><br />Author(s): S. Crumeyrolle, A. Schwarzenboeck, J. C. Roger, K. Sellegri, J. F. Burkhart, A. Stohl, L. Gomes, B. Quennehen, G. Roberts, R. Weigel, P. Villani, J. M. Pichon, T. Bourrianne, and P. Laj<br /><br />Within the frame of the European Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) project, the Météo-France aircraft ATR-42 performed 22 research flights over central Europe and the North Sea during the intensive observation period in May 2008. For the campaign, the ATR-42 was equipped to study the aerosol physical, chemical, hygroscopic and optical properties, as well as cloud microphysics. For the 22 research flights, retroplume analyses along the flight tracks were performed with FLEXPART in order to classify air masses into five sectors of origin, allowing for a qualitative evaluation of emission influence on the respective air parcel. <br></br> This study shows that the extensive aerosol parameters (aerosol mass and number concentrations) show vertical decreasing gradients and in some air masses maximum mass concentrations (mainly organics) in an intermediate layer (1–3 km). The observed mass concentrations (in the boundary layer (BL): between 10 and 30 μg m⁻³; lower free troposphere (LFT): 0.8 and 14 μg m⁻³) are high especially in comparison with the 2015 European norms for PM_2.5 (25 μg m⁻³) and with previous airborne studies performed over England (Morgan et al., 2009; McMeeking et al., 2012). <br></br> Particle number size distributions show a larger fraction of particles in the accumulation size range in the LFT compared to BL. The chemical composition of submicron aerosol particles is dominated by organics in the BL, while ammonium sulphate dominates the submicron aerosols in the LFT, especially in the aerosol particles originated from north-eastern Europe (~ 80%), also experiencing nucleation events along the transport. As a consequence, first the particle CCN acting ability, shown by the CCN/CN ratio, and second the average values of the scattering cross sections of optically active particles (i.e. scattering coefficient divided by the optical active particle concentration) are increased in the LFT compared to BL.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/4865/2013/<b>Evidence and quantitation of aromatic organosulfates in ambient aerosols in Lahore, Pakistan</b><br /><br />Atmospheric Chemistry and Physics, 13, 4865-4875, 2013<br /><br />Author(s): S. Kundu, T. A. Quraishi, G. Yu, C. Suarez, F. N. Keutsch, and E. A. Stone<br /><br />Organosulfates are important components of atmospheric organic aerosols, yet their structures, abundances, sources and formation processes are not adequately understood. This study presents the identification and quantitation of benzyl sulfate in atmospheric aerosols, which is the first confirmed atmospheric organosulfate with aromatic carbon backbone. Benzyl sulfate was identified and quantified in fine particulate matter (PM_2.5) collected in Lahore, Pakistan, during 2007–2008. An authentic standard of benzyl sulfate was synthesized, standardized, and identified in atmospheric aerosols with quadrupole time-of-flight (Q-ToF) mass spectrometry (MS). Benzyl sulfate was quantified in aerosol samples using ultra performance liquid chromatography (UPLC) coupled to negative electrospray ionization triple quadrupole (TQ) MS. The highest benzyl sulfate concentrations were recorded in November and January 2007 (0.50 ± 0.11 ng m⁻³) whereas the lowest concentration was observed in July (0.05 ± 0.02 ng m⁻³). To evaluate matrix effects, benzyl sulfate concentrations were determined using external calibration and the method of standard addition; comparable concentrations were detected by the two methods, which ruled out significant matrix effects in benzyl sulfate quantitation. Three additional organosulfates with <i>m/z</i> 187, 201 and 215 were qualitatively identified as aromatic organosulfates with additional methyl substituents by high-resolution mass measurements and tandem MS. The observed aromatic organosulfates form a homologous series analogous to toluene, xylene, and trimethylbenzene, which are abundant anthropogenic volatile organic compounds (VOC), suggesting that aromatic organosulfates may be formed by secondary reactions. However, stronger statistical correlations of benzyl sulfate with combustion tracers (EC and levoglucosan) than with secondary tracers (SO₄^2&minus; and &alpha;-pinene-derived nitrooxy organosulfates) suggest that aromatic organosulfates may be emitted from the combustion sources or their subsequent atmospheric processing. Further studies are needed to elucidate the sources and formation pathways of aromatic organosulfates in the atmosphere.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/4839/2013/<b>Evaluation of discrepancy between measured and modelled oxidized mercury species</b><br /><br />Atmospheric Chemistry and Physics, 13, 4839-4863, 2013<br /><br />Author(s): G. Kos, A. Ryzhkov, A. Dastoor, J. Narayan, A. Steffen, P. A. Ariya, and L. Zhang<br /><br />L. Zhang et al. (2012), in a recent report, compared model estimates with new observations of oxidized and particulate mercury species (Hg²⁺ and Hg_p) in the Great Lakes region and found that the sum of Hg²⁺ and Hg_p varied between a factor of 2 to 10 between measurements and model. They suggested too high emission inputs as Hg²⁺ and too fast oxidative conversion of Hg⁰ to Hg²⁺ and Hg_p as possible causes. This study quantitatively explores measurement uncertainties in detail. These include sampling efficiency, composition of sample, interfering species and calibration errors. Model (Global/Regional Atmospheric Heavy Metals Model – GRAHM) sensitivity experiments are used to examine the consistency between various Hg measurements and speciation of Hg near emission sources to better understand the discrepancies between modelled and measured concentrations of Hg²⁺ and Hg_p. We find that the ratio of Hg⁰, Hg²⁺ and Hg_p in the emission inventories, measurements of surface air concentrations of oxidized Hg and measurements of wet deposition are currently inconsistent with each other in the vicinity of emission sources. Current speciation of Hg emissions suggests higher concentrations of Hg²⁺ in air and in precipitation near emission sources; however, measured air concentrations of Hg²⁺ and measured concentrations of Hg in precipitation are not found to be significantly elevated near emission sources compared to the remote regions. The averaged unbiased root mean square error (RMSE) between simulated and observed concentrations of Hg²⁺ is found to be reduced by 42% and for Hg_p reduced by 40% for 21 North American sites investigated, when a ratio for Hg⁰ : Hg²⁺ : Hg_p in the emissions is changed from 50 : 40 : 10 (as specified in the original inventories) to 90 : 8 : 2. Unbiased RMSE reductions near emissions sources in the eastern United States and Canada are found to be reduced by up to 58% for Hg²⁺. Significant improvement in the model simulated spatial distribution of wet deposition of mercury in North America is noticed with the modified Hg emission speciation. Measurement-related uncertainties leading to lower estimation of Hg²⁺ concentrations are 86%. Uncertainties yielding either to higher or lower Hg²⁺ concentrations are found to be 36%. Finally, anthropogenic emission uncertainties are 106% for Hg²⁺. Thus it appears that the identified uncertainties for model estimates related to mercury speciation near sources, uncertainties in measurement methodology and uncertainties in emissions can close the gap between modelled and observed estimates of oxidized mercury found in L. Zhang et al. (2012). Model sensitivity simulations show that the measured concentrations of oxidized mercury, in general, are too low to be consistent with measured wet deposition fluxes in North America. Better emission inventories (with respect to speciation), better techniques for measurements of oxidized species and knowledge of mercury reduction reactions in different environments (including in-plume) in all phases are needed for improving the mercury models.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/4829/2013/<b>Insights into dissolved organic matter complexity in rainwater from continental and coastal storms by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry</b><br /><br />Atmospheric Chemistry and Physics, 13, 4829-4838, 2013<br /><br />Author(s): R. N. Mead, K. M. Mullaugh, G. Brooks Avery, R. J. Kieber, J. D. Willey, and D. C. Podgorski<br /><br />A series of seven rainwater samples were collected in Wilmington, North Carolina USA originating from both continental and coastal storms and analyzed by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). This data set is unique in that it represents a detailed comparison of the molecular level composition of DOM in rainwater collected from distinctly different air mass back trajectories by FT-ICR MS. Approximately 25% of the roughly 2000 assigned CHO molecular formulas are unique to a single storm classification indicating the importance of air mass back trajectory on the composition of rainwater dissolved organic matter (DOM). Analysis of the unique molecular formula assignments highlighted distinct groupings of various bio- and geo-molecule classes with coastal storms containing unique formulas representative of lignin and cellulose-like formulas while continental storms had lipid-like formulas. A series of 18 distinct methylene oligomers were identified in coastal storms and 13 unique methylene oligomers in continental storms, suggesting oligomer formation is ubiquitous in rainwater albeit different for each storm classification. Oligomers of small acids and C₃H₄O₂ were detected in both storm types indicating their processing may be similar in both back trajectories. Condensed aromatic hydrocarbons were detected in continental storms with phenol moieties that are not as oxidized as similar compounds detected in aquatic DOM.Tue, 14 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/4815/2013/<b>Continuous monitoring of summer surface water vapor isotopic composition above the Greenland Ice Sheet</b><br /><br />Atmospheric Chemistry and Physics, 13, 4815-4828, 2013<br /><br />Author(s): H. C. Steen-Larsen, S. J. Johnsen, V. Masson-Delmotte, B. Stenni, C. Risi, H. Sodemann, D. Balslev-Clausen, T. Blunier, D. Dahl-Jensen, M. D. Ellehøj, S. Falourd, A. Grindsted, V. Gkinis, J. Jouzel, T. Popp, S. Sheldon, S. B. Simonsen, J. Sjolte, J. P. Steffensen, P. Sperlich, A. E. Sveinbjörnsdóttir, B. M. Vinther, and J. W. C. White<br /><br />We present here surface water vapor isotopic measurements conducted from June to August 2010 at the NEEM (North Greenland Eemian Drilling Project) camp, NW Greenland (77.45° N, 51.05° W, 2484 m a.s.l.). Measurements were conducted at 9 different heights from 0.1 m to 13.5 m above the snow surface using two different types of cavity-enhanced near-infrared absorption spectroscopy analyzers. For each instrument specific protocols were developed for calibration and drift corrections. The inter-comparison of corrected results from different instruments reveals excellent reproducibility, stability, and precision with a standard deviations of ~ 0.23&permil; for &delta;¹⁸O and ~ 1.4&permil; for δD. Diurnal and intraseasonal variations show strong relationships between changes in local surface humidity and water vapor isotopic composition, and with local and synoptic weather conditions. This variability probably results from the interplay between local moisture fluxes, linked with firn–air exchanges, boundary layer dynamics, and large-scale moisture advection. Particularly remarkable are several episodes characterized by high (> 40&permil;) surface water vapor deuterium excess. Air mass back-trajectory calculations from atmospheric analyses and water tagging in the LMDZiso (Laboratory of Meteorology Dynamics Zoom-isotopic) atmospheric model reveal that these events are associated with predominant Arctic air mass origin. The analysis suggests that high deuterium excess levels are a result of strong kinetic fractionation during evaporation at the sea-ice margin.Mon, 13 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/4801/2013/<b>Aerosol size-resolved trace metal composition in remote northern tropical Atlantic marine environment: case study Cape Verde islands</b><br /><br />Atmospheric Chemistry and Physics, 13, 4801-4814, 2013<br /><br />Author(s): K. W. Fomba, K. Müller, D. van Pinxteren, and H. Herrmann<br /><br />Size-resolved trace metal concentrations of 15 elements in aerosol particles at the Cape Verde Atmospheric Observatory (CVAO) under remote background conditions were investigated through analysis of aerosol samples collected during intensive field studies from January 2007 to November 2011 using total reflection x-ray fluorescence (TXRF). The identification of the main air mass origin that influence remote marine aerosol in the northern tropical Atlantic has been investigated. In total, 317 samples were collected. The dataset was analyzed according to the main air mass inflow at the station. We found that remote conditions make up about 45% of the meteorological conditions in a year at CVAO and thus the northern tropical Atlantic. Surprisingly, air masses from North America are often responsible for higher trace metal concentrations in this region. <br></br> Elements such as Zn, Pb, Cu, Cr, Ni, and V were mostly found in the submicron size fractions, while elements with dominant crustal or oceanic origin such as Fe, Ti, Mn, Sr, and Rb were found in the coarse fractions (>1 μm). The highest metal concentrations, especially for Zn (3.23 ng m⁻³), Cu (0.81 ng m⁻³), Sr (2.63 ng m⁻³), and Cr (0.53 ng m⁻³), were observed in air masses originating from North America and the concentrations were within the same concentration range to those reported previously in the literature for remote marine aerosols. Fe (12.26 ng m⁻³), Ti (0.91 ng m⁻³), and Mn (0.35 ng m⁻³) showed higher concentrations when air mass came from Europe and the Canary Islands. Pb concentration was low (<0.20 ng m⁻³) and did not vary significantly with air mass direction. The low Pb concentration is indicative of the complete phase-out of leaded gasoline even in African countries. Crustal enrichment factor values decreased from fine to coarse-mode particles with low values (<4) observed for Fe, Mn, and Rb, and high values (>20) for Zn, Cu, Ni, Cr, Pb, and Se. The observed enrichment of the elements was attributed to crustal, marine, anthropogenic, and biogenic sources, as well as long-range transport and resuspension. Zn, Cu and Pb were indicators of anthropogenic activities, while Ti and Sr were indicators of crustal and marine origin, respectively. Oceanic and biogenic emissions might have contributed to most of the Se observed. This work provides the first long-term size-resolved trace metals study for remote tropical northern Atlantic marine aerosols and the dataset could serve as good initiation of yearly flux estimates.Wed, 08 May 2013 00:00:00 +0200http://www.atmos-chem-phys.net/13/4783/2013/<b>Comparison between summertime and wintertime Arctic Ocean primary marine aerosol properties</b><br /><br />Atmospheric Chemistry and Physics, 13, 4783-4799, 2013<br /><br />Author(s): J. Zábori, R. Krejci, J. Ström, P. Vaattovaara, A. M. L. Ekman, M. E. Salter, E. M. Mårtensson, and E. D. Nilsson<br /><br />Primary marine aerosols (PMAs) are an important source of cloud condensation nuclei, and one of the key elements of the remote marine radiative budget. Changes occurring in the rapidly warming Arctic, most importantly the decreasing sea ice extent, will alter PMA production and hence the Arctic climate through a set of feedback processes. In light of this, laboratory experiments with Arctic Ocean water during both Arctic winter and summer were conducted and focused on PMA emissions as a function of season and water properties. Total particle number concentrations and particle number size distributions were used to characterize the PMA population. A comprehensive data set from the Arctic summer and winter showed a decrease in PMA concentrations for the covered water temperature (<i>T</i>_w) range between &minus;1&deg;C and 15&deg;C. A sharp decrease in PMA emissions for a <i>T</i>_w increase from &minus;1&deg;C to 4&deg;C was followed by a lower rate of change in PMA emissions for <i>T</i>_w up to about 6&deg;C. Near constant number concentrations for water temperatures between 6&deg;C to 10&deg;C and higher were recorded. Even though the total particle number concentration changes for overlapping <i>T</i>_w ranges were consistent between the summer and winter measurements, the distribution of particle number concentrations among the different sizes varied between the seasons. Median particle number concentrations for a dry diameter (<i>D</i>_<i>p</i>< 0.125&mu;m measured during winter conditions were similar (deviation of up to 3%), or lower (up to 70%) than the ones measured during summer conditions (for the same water temperature range). For <i>D</i>_<i>p</i> > 0.125&mu;m, the particle number concentrations during winter were mostly higher than in summer (up to 50%). The normalized particle number size distribution as a function of water temperature was examined for both winter and summer measurements. An increase in <i>T</i>_w from &minus;1&deg;C to 10&deg;C during winter measurements showed a decrease in the peak of relative particle number concentration at about a <i>D</i>_<i>p</i> of 0.180&mu;m, while an increase was observed for particles with <i>D</i>_<i>p</i> > 1&mu;m. Summer measurements exhibited a relative shift to smaller particle sizes for an increase of <i>T</i>_w in the range 7–11&deg;C. The differences in the shape of the number size distributions between winter and summer may be caused by different production of organic material in water, different local processes modifying the water masses within the fjord (for example sea ice production in winter and increased glacial meltwater inflow during summer) and different origin of the dominant sea water mass. Further research is needed regarding the contribution of these factors to the PMA production.Wed, 08 May 2013 00:00:00 +0200