bluuebunny/crossref_metadata_2025 huggingface dataset:
Winter and summer time size distributions and densities of traffic-related aerosol particles at a busy highway in Helsinki
Authors: Virtanen A. , Rönkkö T. , Kannosto J. , Ristimäki J. , Mäkelä J. M. , Keskinen J. , Pakkanen T. , Hillamo R. , Pirjola L. , Hämeri K.
Published: April 2010

Abstract. Number concentrations and size distributions of traffic related aerosol particles were measured at a roadside in Helsinki during two winter campaigns (10–26 February 2003, 28 January–12 February 2004) and two summer campaigns (12–27 August 2003, 6–20 August 2004). The measurements were performed simultaneously at distances of 9 m and 65 m from the highway. Total number concentrations were measured by a condensation particle counter (CPC) and particle size distributions by a scanning mobility particle sizer (SMPS) and an electrical low pressure impactor (ELPI). This study concentrates on data that were measured when the wind direction was from the road to the measurement site. The total concentrations in the wintertime were 2–3 times higher than in the summertime and the concentrations were dominated by nucleation mode particles. The particles smaller than 63 nm (aerodynamic diameter) constituted ~90% of all particles in the wintertime and ~80% of particles in the summer time. The particle total concentration increased with increasing traffic rate. The effect of traffic rate on particles smaller than 63 nm was stronger than on the larger particles. The particle distributions at the roadside consisted of two distinguishable modes. The geometric mean diameter (GMD) of nucleation mode (Mode 1) was 20.3 nm in summer and 18.9 nm in winter. The GMD of the larger mode consisting mostly of traffic related soot particles (Mode 2) was 72.0 nm in summer and 75.1 nm in winter. The GMD values of the modes did not depend on the traffic rate. The average particle density for each mode was determined by a parallel density fitting method based on the size distribution measurement made by ELPI and SMPS. The average density value for Mode 1 particles was 1.0±0.13 g/cm3 and 1.0±0.07 g/cm3 both in summer and winter respectively, while the average density value for Mode 2 was 1.5±0.1 g/cm3 and 1.8±0.3 g/cm3 for summer and winter, respectively.
Surprisingly small HONO emissions from snow surfaces at Browning Pass, Antarctica
Authors: Beine H. J. , Amoroso A. , Dominé F. , King M. D. , Nardino M. , Ianniello A. , France J. L.
Published: April 2010

Abstract. Measured Fluxes of nitrous acid at Browning Pass, Antarctica were very low, despite conditions that are generally understood as favorable for HONO emissions, including: acidic snow surfaces, an abundance of NO3- anions in the snow surface, and abundant UV light for NO3- photolysis. Photochemical modeling suggests noon time HONO fluxes of 5–10 nmol m-2 h-1; the measured fluxes, however, were close to zero throughout the campaign. The location and state of NO3- in snow is crucial to its reactivity. The analysis of soluble mineral ions in snow reveals that the NO3- ion is probably present in aged snows as NaNO3. This is peculiar to our study site, and we suggest that this may affect the photochemical reactivity of NO3-, by preventing the release of products, or providing a reactive medium for newly formed HONO. In fresh snow, the NO3- ion is probably present as dissolved or adsorbed HNO3 and yet, no HONO emissions were observed. We speculate that HONO formation from NO3- photolysis may involve electron transfer reactions of NO2 from photosensitized organics and that fresh snows at our site had insufficient concentrations of adequate organic compounds to favor this reaction.
20th century trends and budget implications of chloroform and related tri-and dihalomethanes inferred from firn air
Authors: Worton D. R. , Sturges W. T. , Schwander J. , Mulvaney R. , Barnola J.-M. , Chappellaz J.
Published: April 2010

Abstract. Four trihalomethane (THM; CHCl3, CHBrCl2, CHBr2Cl and CHBr3) and two dihalomethane (DHM; CH2BrCl and CH2Br2) trace gases have been measured in air extracted from polar firn collected at the North Greenland Icecore Project (NGRIP) site. CHCl3 was also measured in firn air from Devon Island (DI), Canada, Dronning Maud Land (DML), Antarctica and Dome Concordia (Dome C), Antarctica. All of these species are believed to be almost entirely of natural origin except for CHCl3 where anthropogenic sources have been reported to contribute ~10% to the global burden. A 2-D atmospheric model was run for CHCl3 using reported emission estimates to produce historical atmospheric trends at the firn sites, which were then input into a firn diffusion model to produce concentration depth profiles that were compared against the measurements. The anthropogenic emissions were modified in order to give the best model fit to the firn data at NGRIP, Dome C and DML. As a result, the contribution of CHCl3 from anthropogenic sources, mainly from pulp and paper manufacture, to the total chloroform budget appears to have been considerably underestimated and was likely to have been close to ~50% at the maximum in atmospheric CHCl3 concentrations around 1990, declining to ~29% at the beginning of the 21st century. We also show that the atmospheric burden of the brominated THM's in the Northern Hemisphere have increased over the 20th century while CH2Br2 has remained constant over time implying that it is entirely of natural origin.
Ice nucleation in sulfuric acid/organic aerosols: implications for cirrus cloud formation
Authors: Beaver M. R. , Elrod M. J. , Garland R. M. , Tolbert M. A.
Published: April 2010

Abstract. Using an aerosol flow tube apparatus, we have studied the effects of aliphatic aldehydes (C3 to C10) and ketones (C3 and C9) on ice nucleation in sulfuric acid aerosols. Mixed aerosols were prepared by combining an organic vapor flow with a flow of sulfuric acid aerosols over a small mixing time (~60 s) at room temperature. No acid-catalyzed reactions were observed under these conditions, and physical uptake was responsible for the organic content of the sulfuric acid aerosols. In these experiments, aerosol organic content, determined by a Mie scattering analysis, was found to vary with the partial pressure of organic, the flow tube temperature, and the identity of the organic compound. The physical properties of the organic compounds (primarily the solubility and melting point) were found to play a dominant role in determining the inferred mode of nucleation (homogenous or heterogeneous) and the specific freezing temperatures observed. Overall, very soluble, low-melting organics, such as acetone and propanal, caused a decrease in aerosol ice nucleation temperatures when compared with aqueous sulfuric acid aerosol. In contrast, sulfuric acid particles exposed to organic compounds of eight carbons and greater, of much lower solubility and higher melting temperatures, nucleate ice at temperatures above aqueous sulfuric acid aerosols. Organic compounds of intermediate carbon chain length, C4-C7, (of intermediate solubility and melting temperatures) nucleated ice at the same temperature as aqueous sulfuric acid aerosols. Interpretations and implications of these results for cirrus cloud formation are discussed.
First space-borne measurements of methanol inside aged southern tropical to mid-latitude biomass burning plumes using the ACE-FTS instrument
Authors: Dufour G. , Boone C. D. , Rinsland C. P. , Bernath P. F.
Published: April 2010

Abstract. First measurements from space of upper tropospheric and lower stratospheric methanol profiles within aged fire plumes are reported. Elevated levels of methanol at 0–45° S from 30 September to 3 November 2004 have been measured by the high resolution infrared spectrometer ACE-FTS onboard the SCISAT satellite. Methanol volume mixing ratios higher than 4000 pptv are detected and are strongly correlated with other fire products such as CO, C2H6, and HCN. A sensitivity study of the methanol retrieval, accounting for random and systematic contributions, shows that the retrieved methanol profile for a single occultation exceeds 100% error above 16.5 km, with an accuracy of about 20% for measurements inside polluted air masses. The upper tropospheric enhancement ratio of methanol with respect to CO is estimated from the correlation plot between methanol and CO for aged tropical biomass burning plumes. This ratio is in good agreement with the ratio measured in the free troposphere (up to 12 km) by recent aircraft studies and does not suggest any secondary production of methanol by oxidation in aged biomass burning plumes.
A Lagrangian analysis of the impact of transport and transformation on the ozone stratification observed in the free troposphere during the ESCOMPTE campaign
Authors: Colette A. , Ancellet G. , Menut L. , Arnold S. R.
Published: April 2010

Abstract. The ozone variability observed by tropospheric ozone lidars during the ESCOMPTE campaign is analyzed by means of a hybrid-Lagrangian modeling study. Transport processes responsible for the formation of ozone-rich layers are identified using a semi-Lagrangian analysis of mesoscale simulations to identify the planetary boundary layer (PBL) footprint in the free troposphere. High ozone concentrations are related to polluted air masses exported from the Iberian PBL. The chemical composition of air masses coming from the PBL and transported in the free troposphere is evaluated using a Lagrangian chemistry model. The initial concentrations are provided by a model of chemistry and transport. Different scenarios are tested for the initial conditions and for the impact of mixing with background air in order to perform a quantitative comparison with the lidar observations. For this meteorological situation, the characteristic mixing time is of the order of 2 to 6 days depending on the initial conditions. Ozone is produced in the free troposphere within most air masses exported from the Iberian PBL at an average rate of 0.2 ppbv h−1, with a maximum ozone production of 0.4 ppbv h−1. Transport processes from the PBL are responsible for an increase of 13.3 ppbv of ozone concentrations in the free troposphere compared to background levels; about 45% of this increase is attributed to in situ production during the transport rather than direct export of ozone.
Technical note: Implementation of prescribed (OFFLEM), calculated (ONLEM), and pseudo-emissions (TNUDGE) of chemical species in the Modular Earth Submodel System (MESSy)
Authors: Kerkweg A. , Sander R. , Tost H. , Jöckel P.
Published: April 2010

Abstract. We present the submodels OFFLEM, ONLEM, and TNUDGE for the Modular Earth Submodel System (MESSy). Prescribed emissions from input files are handled by OFFLEM. ONLEM deals with online-calculated emissions, i.e., emissions that are calculated during the simulation. The submodel TNUDGE uses the "tracer nudging" technique for pseudo-sources and -sinks. For species with highly uncertain emission fluxes and/or with sufficiently long lifetimes, e.g., CH4, it is common to create such pseudo-fluxes by prescribing the observed mixing ratio of the species at a given boundary (e.g., the mixing ratio of methane at the surface, or the ozone mixing ratio at the tropopause). All three submodels substantially simplify the inclusion of emissions into a model. Specific emissions can easily be switched on or off. New prescribed emissions can be included without rewriting any code. New online emissions only require one additional subroutine containing the new parameterization. A major advantage is that input fields at arbitrary resolution can be used. The problem of incompatible grids between emission data and model is overcome by utilizing the MESSy data import interface. To further simplify the creation of new offline emission data, the preprocessing program EDGAR2NC is provided. EDGAR2NC transforms files from the EDGAR format into the netCDF format which is required by OFFLEM. The presented routines are a part of the community modeling project MESSy and can be made available for use to the atmospheric modeling community.
Liquid particle composition and heterogeneous reactions in a mountain wave Polar Stratospheric Cloud
Authors: Lowe D. , MacKenzie A. R. , Schlager H. , Voigt C. , Dörnbrack A. , Mahoney M. J. , Cairo F.
Published: April 2010

Abstract. Mountain wave polar stratospheric clouds (PSCs) were detected on 8 February 2003 above the Scandinavian Mountains by in-situ instruments onboard the M55 Geophysica aircraft. PSC particle composition, backscatter and chlorine activation for this case are studied with a recently developed non-equilibrium microphysical box model for liquid aerosol. Results from the microphysical model, run on quasi-lagrangian trajectories, show that the PSC observed was composed of supercooled ternary (H2O/HNO3/H2SO4) solution (STS) particles, which are out of equilibrium with the gas phase. The measured condensed nitric acid and aerosol backscatter of the PSC can well be simulated with the model. Up to 0.15 ppbv Cl2 can be released by the PSC within 2 h, showing the propensity of these small-scale clouds for chlorine activation. Equilibrium calculations – of the sort commonly used in large scale chemistry transport models – overestimate the measured condensed nitrate by up to a factor of 3, and overestimates chlorine activation by 10%, in this mountain wave cloud.
Characterization of on-road vehicle emissions in the Mexico City Metropolitan Area using a mobile laboratory in chase and fleet average measurement modes during the MCMA-2003 field campaign
Authors: Zavala M. , Herndon S. C. , Slott R. S. , Dunlea E. J. , Marr L. C. , Shorter J. H. , Zahniser M. , Knighton W. B. , Rogers T. M. , Kolb C. E. , Molina L. T. , Molina M. J.
Published: April 2010

Abstract. A mobile laboratory was used to measure on-road vehicle emission ratios during the MCMA-2003 field campaign held during the spring of 2003 in the Mexico City Metropolitan Area (MCMA). The measured emission ratios represent a sample of emissions of in-use vehicles under real world driving conditions for the MCMA. From the relative amounts of NOx and selected VOC's sampled, the results indicate that the technique is capable of differentiating among vehicle categories and fuel type in real world driving conditions. Emission ratios for NOx, NOy, NH3, H2CO, CH3CHO, and other selected volatile organic compounds (VOCs) are presented for chase sampled vehicles in the form of frequency distributions as well as estimates for the fleet averaged emissions. Our measurements of emission ratios for both CNG and gasoline powered "colectivos" (public transportation buses that are intensively used in the MCMA) indicate that – in a mole per mole basis – have significantly larger NOx and aldehydes emissions ratios as compared to other sampled vehicles in the MCMA. Similarly, ratios of selected VOCs and NOy showed a strong dependence on traffic mode. These results are compared with the vehicle emissions inventory for the MCMA, other vehicle emissions measurements in the MCMA, and measurements of on-road emissions in U.S. cities. We estimate NOx emissions as 100 600±29 200 metric tons per year for light duty gasoline vehicles in the MCMA for 2003. According to these results, annual NOx emissions estimated in the emissions inventory for this category are within the range of our estimated NOx annual emissions. Our estimates for motor vehicle emissions of benzene, toluene, formaldehyde, and acetaldehyde in the MCMA indicate these species are present in concentrations higher than previously reported. The high motor vehicle aldehyde emissions may have an impact on the photochemistry of urban areas.
Free radical modelling studies during the UK TORCH Campaign in Summer 2003
Authors: Emmerson K. M. , Carslaw N. , Carslaw D. C. , Lee J. D. , McFiggans G. , Bloss W. J. , Gravestock T. , Heard D. E. , Hopkins J. , Ingham T. , Pilling M. J. , Smith S. C. , Jacob M. , Monks P. S.
Published: April 2010

Abstract. The Tropospheric ORganic CHemistry experiment (TORCH) took place during the heatwave of summer 2003 at Writtle College, a site 2 miles west of Chelmsford in Essex and 25 miles north east of London. The experiment was one of the most highly instrumented to date. A combination of a large number of days of simultaneous, collocated measurements, a consequent wealth of model constraints and a highly detailed chemical mechanism, allowed the atmospheric chemistry of this site to be studied in detail. Between 25 July and 31 August, the concentrations of the hydroxyl radical and the hydroperoxy radical were measured using laser-induced fluorescence at low pressure and the sum of peroxy radicals was measured using the peroxy radical chemical amplifier technique. The concentrations of the radical species were predicted using a zero-dimensional box model based on the Master Chemical Mechanism version 3.1, which was constrained with the observed concentrations of relatively long-lived species. The model included a detailed parameterisation to account for heterogeneous loss of hydroperoxy radicals onto aerosol particles. Quantile-quantile plots were used to assess the model performance in respect of the measured radical concentrations. On average, measured hydroxyl radical concentrations were over-predicted by 24%. Modelled and measured hydroperoxy radical concentrations agreed very well, with the model over-predicting on average by only 7%. The sum of peroxy radicals was under-predicted when compared with the respective measurements by 22%. Initiation via OH was dominated by the reactions of excited oxygen atoms with water, nitrous acid photolysis and the ozone reaction with alkene species. Photolysis of aldehyde species was the main route for initiation via HO2 and RO2. Termination, under all conditions, primarily involved reactions with NOx for OH and heterogeneous chemistry on aerosol surfaces for HO2. The OH chain length varied between 2 and 8 cycles, the longer chain lengths occurring before and after the most polluted part of the campaign. Peak local ozone production of 17 ppb hr−1 occurred on 3 and 5 August, signifying the importance of local chemical processes to ozone production on these days. On the whole, agreement between model and measured radicals is good, giving confidence that our understanding of atmospheres influenced by nearby urban sources is adequate.
The effect of harmonized emissions on aerosol properties in global models – an AeroCom experiment
Authors: Textor C. , Schulz M. , Guibert S. , Kinne S. , Balkanski Y. , Bauer S. , Berntsen T. , Berglen T. , Boucher O. , Chin M. , Dentener F. , Diehl T. , Feichter J. , Fillmore D. , Ginoux P. , Gong S. , Grini A. , Hendricks J. , Horowitz L. , Huang P. , Isaksen I. S. A. , Iversen T. , Kloster S. , Koch D. , Kirkevåg A. , Kristjansson J. E. , Krol M. , Lauer A. , Lamarque J. F. , Liu X. , Montanaro V. , Myhre G. , Penner J. E. , Pitari G. , Reddy M. S. , Seland Ø. , Stier P. , Takemura T. , Tie X.
Published: April 2010

Abstract. The effects of unified aerosol sources on global aerosol fields simulated by different models are examined in this paper. We compare results from two AeroCom experiments, one with different (ExpA) and one with unified emissions, injection heights, and particle sizes at the source (ExpB). Surprisingly, harmonization of aerosol sources has only a small impact on the simulated inter-model diversity of the global aerosol burden, and consequently global optical properties, as the results are largely controlled by model-specific transport, removal, chemistry (leading to the formation of secondary aerosols) and parameterizations of aerosol microphysics (e.g., the split between deposition pathways) and to a lesser extent by the spatial and temporal distributions of the (precursor) emissions. The burdens of black carbon and especially sea salt become more coherent in ExpB only, because the large ExpA diversities for these two species were caused by a few outliers. The experiment also showed that despite prescribing emission fluxes and size distributions, ambiguities in the implementation in individual models can lead to substantial differences. These results indicate the need for a better understanding of aerosol life cycles at process level (including spatial dispersal and interaction with meteorological parameters) in order to obtain more reliable results from global aerosol simulations. This is particularly important as such model results are used to assess the consequences of specific air pollution abatement strategies.
Seasonal variation of peroxyacetylnitrate (PAN) in coastal Antarctica measured with a new instrument for the detection of sub-part per trillion mixing ratios of PAN
Authors: Mills G. P. , Sturges W. T. , Salmon R. A. , Bauguitte S. J.-B. , Read K. A. , Bandy B. J.
Published: April 2010

Abstract. An automated gas chromatograph with sample pre-concentration for the measurement of peroxyacetylnitrate (PAN) was constructed with a minimum detection limit below 1 pptv. This instrument was deployed at the British Antarctic Survey's Halley Research Station, Antarctica (75.6° S, 26.6° W) as part of the CHABLIS (Chemistry of the Antarctic Boundary Layer and the Interface with Snow) campaign. Hourly measurements were carried out between July 2004 and February 2005 with observed maximum and minimum mixing ratios of 52.3 and <0.6 pptv, respectively with a mean PAN mixing ratio for the measurement period of 9.2 pptv (standard deviation: 6.2 pptv). The changes in PAN mixing ratios typically occurred over periods of several days to a week and showed a strong similarity to the variation in alkenes. The mixing ratio of PAN at Halley has a possible seasonal cycle with a winter maximum and summer minimum, though the cycle is incomplete and the data are very variable. Calculations indicate that gross local PAN production is approximately 1 pptv d−1 in spring and 0.6 pptv d−1 in summer. Net loss of PAN transported to Halley in the summer is a small gas-phase source of NOx and net production of PAN in the spring is a very small NOx sink.
The influence of cloud top variability from radar measurements on 3-D radiative transfer
Authors: Richter F. , Barfus K. , Berger F. H. , Görsdorf U.
Published: April 2010

Abstract. In radiative transfer simulations the simplification of cloud top structure by homogeneous assumptions can cause mistakes in comparison to realistic heterogeneous cloud top structures. This paper examines the influence of cloud top heterogeneity on the radiation at the top of the atmosphere. The use of cloud top measurements with a high temporal resolution allows the analysis of small spatial cloud top heterogeneities by using the frozen turbulence assumption for the time – space conversion. Radiative observations are often based on satellite measurements, whereas small spatial structures are not considered in such treatments. A spectral analysis of the cloud top measurements showed slopes of power spectra between –1.8 and –2.0, these values are larger than the spectra of –5/3 which is often applied to generate cloud field variability. The comparison of 3-D radiative transfer results from cloud fields with homogeneous and heterogeneous tops has been done for a single wavelength of 0.6 μm. The radiative transfer calculations result in lower albedos for heterogeneous cloud tops. The differences of albedos between heterogeneous and homogeneous cloud top decrease with increasing solar zenith angle. The influence of cloud top variability on radiances is shown. The reflectances for heterogeneous tops are explicitly larger in forward direction, in backward direction lower. The largest difference of the mean reflectances (mean over cloud field) between homogeneous and heterogeneous cloud top is approximately 0.3, which is 30% of illumination.
Retrieval of stratospheric and tropospheric BrO columns from multi-axis DOAS measurements at Reunion Island (21° S, 56° E)
Authors: Theys N. , Van Roozendael M. , Hendrick F. , Fayt C. , Hermans C. , Baray J.-L. , Goutail F. , Pommereau J.-P. , De Mazière M.
Published: April 2010

Abstract. Spectral measurements of BrO using zenith-sky and off-axis viewing geometries are combined in a linear multiple regression retrieval algorithm to provide stratospheric and tropospheric BrO vertical columns. One year of measurement data are investigated over Reunion-Island (20.9° S, 55.5° E), from August 2004 to June 2005. A comparison between the stratospheric columns retrieved at 45°, 80°, 85°, 87.5° and 92.5° solar zenith angles and photochemical simulations initialized by chemical fields from the 3-D-CTM SLIMCAT and further constrained by observed NO2 profiles shows a good agreement only by considering a contribution from the very short-lived organic bromine substances to the stratospheric inorganic bromine budget, of 6 to 8 pptv. Furthermore, stratospheric BrO profiles retrieved from late twilight zenith-sky observations are consistent with a total inorganic bromine (Bry) loading of approximately 23 pptv. This represents 6 to 7 pptv more than can be supplied by long-lived organic bromine sources, and therefore supports an added contribution from very short-lived organic bromine substances as recently suggested in several other studies. Moreover strong evidences are presented for the existence of a substantial amount of BrO in the tropical free-troposphere, around 6 km altitude, possibly supplied by the decomposition of short-lived biogenic bromine organic compounds. Tropospheric BrO vertical columns of 1.1±0.45×1013 molec/cm² are derived for the entire observation period. Comparisons between ground-based BrO vertical columns and total BrO columns derived from SCIAMACHY (onboard the ENVISAT satellite) nadir observations in a latitudinal band centered around 21° S present a good level of consistency, which further strengthens the conclusions of our study.
Comparison of the inversion algorithms applied to the ozone vertical profile retrieval from SCIAMACHY limb measurements
Authors: Rozanov A. , Eichmann K.-U. , von Savigny C. , Bovensmann H. , Burrows J. P. , von Bargen A. , Doicu A. , Hilgers S. , Godin-Beekmann S. , Leblanc T. , McDermid I. S.
Published: April 2010

Abstract. This paper is devoted to an intercomparison of ozone vertical profiles retrieved from the measurements of scattered solar radiation performed by the SCIAMACHY instrument in the limb viewing geometry. Three different inversion algorithms including the prototype of the operational Level 1 to 2 processor to be operated by the European Space Agency are considered. Unlike usual validation studies, this comparison removes the uncertainties arising when comparing measurements made by different instruments probing slightly different air masses and focuses on the uncertainties specific to the modeling-retrieval problem only. The intercomparison was performed for 5 selected orbits of SCIAMACHY showing a good overall agreement of the results in the middle stratosphere, whereas considerable discrepancies were identified in the lower stratosphere and upper troposphere altitude region. Additionally, comparisons with ground-based lidar measurements are shown for selected profiles demonstrating an overall correctness of the retrievals.
Contribution of mixing in the ABL to new particle formation based on observations
Authors: Lauros J. , Nilsson E. D. , Dal Maso M. , Kulmala M.
Published: April 2010

Abstract. The connection between new particle formation and micro- and mesoscale meteorology was studied based on measurements at SMEAR II station in Southern Finland. We analyzed turbulent conditions described by sodar measurements and utilized these combined with surface layer measurements and a simple model to estimate the upper boundary layer conditions. Turbulence was significantly stronger on particle formation days and the organic vapor saturation ratio increase due to large eddies was stronger on event than nonevent days. We examined which variables could be the best indicators of new particle formation and concluded that the formation probability depended on the condensation sink and temporal temperature change at the top of the atmospheric boundary layer. Humidity and heat flux may also be good indicators for particle formation.
Characterization of Polar Stratospheric Clouds with spaceborne lidar: CALIPSO and the 2006 Antarctic season
Authors: Pitts M. C. , Thomason L. W. , Poole L. R. , Winker D. M.
Published: April 2010

Abstract. The role of polar stratospheric clouds in polar ozone loss has been well documented. The CALIPSO satellite mission offers a new opportunity to characterize PSCs on spatial and temporal scales previously impossible. A PSC detection algorithm based on a single wavelength threshold approach has been developed for CALIPSO. The method appears to accurately detect PSCs of all opacities, including tenuous clouds, with a very low rate of false positives and few missed clouds. We applied the algorithm to CALIOP data acquired during the 2006 Antarctic winter season from 13 June through 31 October. The spatial and temporal distribution of CALIPSO PSC observations is illustrated with weekly maps of PSC occurrence. The evolution of the 2006 PSC season is depicted by time series of daily PSC frequency as a function of altitude. Comparisons with "virtual" solar occultation data indicate that CALIPSO provides a different view of the PSC season than attained with previous solar occultation satellites. Measurement-based time series of PSC areal coverage and vertically-integrated PSC volume are computed from the CALIOP data. The observed area covered with PSCs is significantly smaller than would be inferred from the commonly used temperature-based proxy TNAT but is similar in magnitude to that inferred from TSTS. The potential of CALIOP measurements for investigating PSC composition is illustrated using combinations of lidar backscatter and volume depolarization for two CALIPSO PSC scenes.
Modeling secondary organic aerosol formation through cloud processing of organic compounds
Authors: Chen J. , Griffin R. J. , Grini A. , Tulet P.
Published: April 2010

Abstract. Interest in the potential formation of secondary organic aerosol (SOA) through reactions of organic compounds in condensed aqueous phases is growing. In this study, the potential formation of SOA from irreversible aqueous-phase reactions of organic species in clouds was investigated. A new proposed aqueous-phase chemistry mechanism (AqChem) is coupled with the existing gas-phase Caltech Atmospheric Chemistry Mechanism (CACM) and the Model to Predict the Multiphase Partitioning of Organics (MPMPO) that simulate SOA formation. AqChem treats irreversible organic reactions that lead mainly to the formation of carboxylic acids, which are usually less volatile than the corresponding aldehydic compounds. Zero-dimensional model simulations were performed for tropospheric conditions with clouds present for three consecutive hours per day. Zero-dimensional model simulations show that 48-h average SOA formation is increased by 27% for a rural scenario with strong monoterpene emissions and 7% for an urban scenario with strong emissions of aromatic compounds, respectively, when irreversible organic reactions in clouds are considered. AqChem was also incorporated into the Community Multiscale Air Quality Model (CMAQ) version 4.4 with CACM/MPMPO and applied to a previously studied photochemical episode (3–4 August 2004) focusing on the eastern United States. The CMAQ study indicates that the maximum contribution of SOA formation from irreversible reactions of organics in clouds is 0.28 μg m−3 for 24-h average concentrations and 0.60 μg m−3 for one-hour average concentrations at certain locations. On average, domain-wide surface SOA predictions for the episode are increased by 9% when irreversible, in-cloud processing of organics is considered. Because aldehydes of carbon number greater than four are assumed to convert fully to the corresponding carboxylic acids upon reaction with OH in cloud droplets and this assumption may overestimate carboxylic acid formation from this reaction route, the present study provides an upper bound estimate of SOA formation via this pathway.
Medium-range mid-tropospheric transport of ozone and precursors over Africa: two numerical case studies in dry and wet seasons
Authors: Sauvage B. , Gheusi F. , Thouret V. , Cammas J.-P. , Duron J. , Escobar J. , Mari C. , Mascart P. , Pont V.
Published: April 2010

Abstract. A meso-scale model was used to understand and describe the dynamical processes driving high ozone concentrations observed during both dry and monsoon season in monthly climatologies profiles over Lagos (Nigeria, 6.6° N, 3.3° E), obtained with the MOZAIC airborne measurements (ozone and carbon monoxide). This study focuses on ozone enhancements observed in the upper-part of the lower troposphere, around 3000 m. Two individual cases have been selected in the MOZAIC dataset as being representative of the climatological ozone enhancements, to be simulated and analyzed with on-line Lagrangian backtracking of air masses. This study points out the role of baroclinic low-level circulations present in the Inter Tropical Front (ITF) area. Two low-level thermal cells around a zonal axis and below 2000 m, in mirror symmetry to each other with respect to equator, form near 20° E and around 5° N and 5° S during the (northern hemisphere) dry and wet seasons respectively. They are caused by surface gradients – the warm dry surface being located poleward of the ITF and the cooler wet surface equatorward of the ITF. A convergence line exists between the poleward low-level branch of each thermal cell and the equatorward low-level branch of the Hadley cell. Our main conclusion is to point out this line as a preferred location for fire products – among them ozone precursors – to be uplifted and injected into the lower free troposphere. The free tropospheric transport that occurs then depends on the hemisphere and season. In the NH dry season, the AEJ allows transport of ozone and precursors westward to Lagos. In the NH monsoon (wet) season, fire products are transported from the southern hemisphere to Lagos by the southeasterly trade that surmounts the monsoon layer. Additionally ozone precursors uplifted by wet convection in the ITCZ can also mix to the ones uplifted by the baroclinic cell and be advected up to Lagos by the trade flow.
Design of and initial results from a Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC)
Authors: Glowacki D. R. , Goddard A. , Hemavibool K. , Malkin T. L. , Commane R. , Anderson F. , Bloss W. J. , Heard D. E. , Ingham T. , Pilling M. J. , Seakins P. W.
Published: April 2010

Abstract. The design of a Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) is described and initial results obtained from HIRAC are presented. The ability of HIRAC to perform in-situ laser-induced fluorescence detection of OH and HO2 radicals with the Fluorescence Assay by Gas Expansion (FAGE) technique establishes it as internationally unique for a chamber of its size and pressure/temperature variable capabilities. In addition to the FAGE technique, HIRAC features a suite of analytical instrumentation, including: a multipass FTIR system; a conventional gas chromatography (GC) instrument and a GC instrument for formaldehyde detection; NO/NO2, CO, O3, and H2O vapour analysers. Ray tracing simulations and NO2 actinometry have been utilized to develop a detailed model of the radiation field within HIRAC. Comparisons between the analysers and the FTIR coupled to HIRAC have been performed, and HIRAC has also been used to investigate pressure dependent kinetics of the chlorine atom reaction with ethene and the reaction of O3 and t-2-butene. The results obtained are in good agreement with literature recommendations and Master Chemical Mechanism predictions. HIRAC thereby offers a highly instrumented platform with the potential for: (1) high precision kinetics investigations over a range of atmospheric conditions; (2) detailed mechanism development, significantly enhanced according to its capability for measuring radicals; and (3) field instrument intercomparison, calibration, development, and investigations of instrument response at a range of atmospheric conditions.
Towards a better representation of the solar cycle in general circulation models
Authors: Nissen K. M. , Matthes K. , Langematz U. , Mayer B.
Published: April 2010

Abstract. We introduce the improved Freie Universität Berlin (FUB) high-resolution radiation scheme FUBRad and compare it to the 4-band standard ECHAM5 SW radiation scheme of Fouquart and Bonnel (FB). Both schemes are validated against the detailed radiative transfer model libRadtran. FUBRad produces realistic heating rate variations during the solar cycle. The SW heating rate response with the FB scheme is about 20 times smaller than with FUBRad and cannot produce the observed temperature signal. A reduction of the spectral resolution to 6 bands for solar irradiance and ozone absorption cross sections leads to a degradation (reduction) of the solar SW heating rate signal by about 20%. The simulated temperature response agrees qualitatively well with observations in the summer upper stratosphere and mesosphere where irradiance variations dominate the signal. Comparison of the total short-wave heating rates under solar minimum conditions shows good agreement between FUBRad, FB and libRadtran up to the middle mesosphere (60–70 km) indicating that both parameterizations are well suited for climate integrations that do not take solar variability into account. The FUBRad scheme has been implemented as a sub-submodel of the Modular Earth Submodel System (MESSy).
Analysis of Visible/SWIR surface reflectance ratios for aerosol retrievals from satellite in Mexico City urban area
Authors: de Almeida Castanho A. D. , Prinn R. , Martins V. , Herold M. , Ichoku C. , Molina L. T.
Published: April 2010

Abstract. The surface reflectance ratio between the visible (VIS) and shortwave infrared (SWIR) radiation is an important quantity for the retrieval of the aerosol optical depth (τa) from the MODIS sensor data. Based on empirically determined VIS/SWIR ratios, MODIS τa retrieval uses the surface reflectance in the SWIR band (2.1 µm), where the interaction between solar radiation and the aerosol layer is small, to predict the visible reflectances in the blue (0.47 µm) and red (0.66 µm) bands. Therefore, accurate knowledge of the VIS/SWIR ratio is essential for achieving accurate retrieval of aerosol optical depth from MODIS. We analyzed the surface reflectance over some distinct surface covers in and around the Mexico City metropolitan area (MCMA) using MODIS radiances at 0.66 µm and 2.1 µm. The analysis was performed at 1.5 km×1.5 km spatial resolution. Also, ground-based AERONET sun-photometer data acquired in Mexico City from 2002 to 2005 were analyzed for aerosol depth and other aerosol optical properties. In addition, a network of hand-held sun-photometers deployed in Mexico City, as part of the MCMA-2006 Study during the MILAGRO Campaign, provided an unprecedented measurement of τa in 5 different sites well distributed in the city. We found that the average RED/SWIR ratio representative of the urbanized sites analyzed is 0.73±0.06 for scattering angles <140° and goes up to 0.77±0.06 for higher ones. The average ratio for non-urban sites was significantly lower (approximately 0.55). In fact, this ratio strongly depends on differences in urbanization levels (i.e. relative urban to vegetation proportions and types of surface materials). The aerosol optical depth retrieved from MODIS radiances at a spatial resolution of 1.5 km×1.5 km and averaged within 10×10 km boxes were compared with collocated 1-h τa averaged from sun-photometer measurements. The use of the new RED/SWIR ratio of 0.73 in the MODIS retrieval over Mexico City led to a significant improvement in the agreement between the MODIS and sun-photometer AOD results; with the slope, offset, and the correlation coefficient of the linear regression changing from (τaMODIS=0.91τa sun-photometer+0.33, R2=0.66) to (τaMODIS=0.96 τa sun-photometer−0.006, R2=0.87). Indeed, an underestimation of this ratio in urban areas lead to a significant overestimation of the AOD retrieved from satellite. Therefore, we strongly encourage similar analyses in other urban areas to enhance the development of a parameterization of the surface ratios accounting for urban heterogeneities.
Bromine in the tropical troposphere and stratosphere as derived from balloon-borne BrO observations
Authors: Dorf M. , Butz A. , Camy-Peyret C. , Chipperfield M. P. , Kritten L. , Pfeilsticker K.
Published: April 2010

Abstract. The first tropospheric and stratospheric (4 to 33 km) BrO profile is presented for the inner tropics derived from balloon-borne DOAS (Differential Optical Absorption Spectroscopy) measurements. In combination with photochemical modelling, total stratospheric inorganic bromine (Bry) is deduced to be (21.5±2.5) ppt in 4.5-year-old air, probed in 2005. We derive a total contribution of (5.2±2.5) ppt from brominated very short-lived substances and inorganic product gases to stratospheric Bry Tropospheric BrO was found to be <1 ppt. Our results are compared to two 3-D CTM SLIMCAT model runs, which differ in the lifetime of the bromine source gases, affecting the vertical distribution of Bry in the lower stratosphere. Bromine source gas measurements performed 10 days earlier Laube et al., 2008, indicate a lower Bry of (17.5±0.4) ppt. Potential reasons for this discrepancy are discussed.
The effects of global changes upon regional ozone pollution in the United States
Authors: Chen J. , Avise J. , Lamb B. , Salathé E. , Mass C. , Guenther A. , Wiedinmyer C. , Lamarque J.-F. , O'Neill S. , McKenzie D. , Larkin N.
Published: April 2010

Abstract. A comprehensive numerical modeling framework was developed to estimate the effects of collective global changes upon ozone pollution in the US in 2050. The framework consists of the global climate and chemistry models, PCM (Parallel Climate Model) and MOZART-2 (Model for Ozone and Related Chemical Tracers v.2), coupled with regional meteorology and chemistry models, MM5 (Mesoscale Meteorological model) and CMAQ (Community Multi-scale Air Quality model). The modeling system was applied for two 10-year simulations: 1990–1999 as a present-day base case and 2045–2054 as a future case. For the current decade, the daily maximum 8-h moving average (DM8H) ozone mixing ratio distributions for spring, summer and fall showed good agreement with observations. The future case simulation followed the Intergovernmental Panel on Climate Change (IPCC) A2 scenario together with business-as-usual US emission projections and projected alterations in land use, land cover (LULC) due to urban expansion and changes in vegetation. For these projections, US anthropogenic NOx (NO+NO2) and VOC (volatile organic carbon) emissions increased by approximately 6% and 50%, respectively, while biogenic VOC emissions decreased, in spite of warmer temperatures, due to decreases in forested lands and expansion of croplands, grasslands and urban areas. A stochastic model for wildfire emissions was applied that projected 25% higher VOC emissions in the future. For the global and US emission projection used here, regional ozone pollution becomes worse in the 2045–2054 period for all months. Annually, the mean DM8H ozone was projected to increase by 9.6 ppbv (22%). The changes were higher in the spring and winter (25%) and smaller in the summer (17%). The area affected by elevated ozone within the US continent was projected to increase; areas with levels exceeding the 75 ppbv ozone standard at least once a year increased by 38%. In addition, the length of the ozone season was projected to increase with more pollution episodes in the spring and fall. For selected urban areas, the system projected a higher number of pollution events per year and these events had more consecutive days when DM8H ozone exceed 75 ppbv.
Technical Note: Novel method for water vapour monitoring using wireless communication networks measurements
Authors: David N. , Alpert P. , Messer H.
Published: April 2010

Abstract. We propose a new technique that overcomes the obstacles of the existing methods for monitoring near-surface water vapour, by estimating humidity from data collected through existing wireless communication networks. Weather conditions and atmospheric phenomena affect the electromagnetic channel, causing attenuations to the radio signals. Thus, wireless communication networks are in effect built-in environmental monitoring facilities. The wireless microwave links, used in these networks, are widely deployed by cellular providers for backhaul communication between base stations, a few tens of meters above ground level. As a result, if all available measurements are used, the proposed method can provide moisture observations with high spatial resolution and potentially high temporal resolution. Further, the implementation cost is minimal, since the data used are already collected and saved by the cellular operators. In addition – many of these links are installed in areas where access is difficult such as orographic terrain and complex topography. As such, our method enables measurements in places that have been hard to measure in the past, or have never been measured before. The technique is restricted to weather conditions which exclude rain, fog or clouds along the propagation path. Strong winds that may cause movement of the link transmitter or receiver (or both) may also interfere with the ability to conduct accurate measurements. We present results from real-data measurements taken from two microwave links used in a backhaul cellular network that show convincing correlation to surface station humidity measurements. The measurements were taken daily in two sites, one in northern Israel (28 measurements), the other in central Israel (29 measurements). The correlation between the microwave link measurements and the humidity gauges were 0.9 and 0.82 for the north and central sites, respectively. The Root Mean Square Differences (RMSD) were 1.8 g/m3 and 3.4 g/m3 for the northern and central site measurements, respectively.
Performance evaluation of a high-resolution parallel-plate differential mobility analyzer
Authors: Santos J. P. , Hontañón E. , Ramiro E. , Alonso M.
Published: April 2010

Abstract. A high-resolution differential mobility analyzer (DMA), specially designed for (i) the measurement of ion mobility spectra, and (ii) the generation of a continuous stream of monomobile ions, has been developed and tested. The apparatus consists of two parallel-plate electrodes between which an electric field is applied. The test ion stream flows into the instrument through a narrow rectangular slit made in one of the electrodes, and migrates toward the other electrode driven by the applied field, while being transported by a stream of clean air which flows parallel to the plates at Reynolds number between 2×104 and 9×104 in laminar flow conditions. The collector electrode contains also a narrow slit through which ions of the desired mobility are withdrawn out of DMA. The classified ion current is measured with a high-sensitivity electrometer having a noise level around 0.1 fA. The theory behind the DMA operation is first discussed, focusing on the special case of parallel-plate geometry. Some generic results showing the stability and repeatability of the measurements and the resolving power of the instrument are presented next. The last part of the paper deals with the application of the apparatus to the study of the effect of humidity and aging time on the mobility spectra of air ions generated by a low-activity 241Am source.