bluuebunny/crossref_metadata_2025 huggingface dataset:
Impact of energetic particle precipitation on stratospheric polar constituents: an assessment using MIPAS data monitoring and assimilation
Authors: Robichaud A. , Ménard R. , Chabrillat S. , de Grandpré J. , Rochon Y. J. , Yang Y. , Charette C.
Published: April 2010

Abstract. In 2003, strong geomagnetic events occurred which produced massive amounts of energetic particles penetrating the top of the atmospheric polar region, significantly perturbing its chemical state down to the middle stratosphere. These events and their effects are generally left unaccounted for in current models of stratospheric chemistry and large differences between observations and models are then noted. In this study, we use a coupled 3-D stratospheric dynamical-chemical model and assimilation system to ingest MIPAS temperature and chemical observations. The goal is to gain further understanding and to evaluate the impacts of EPP (energetic particle precipitation) on stratospheric polar chemistry. Moreover, we investigate the feasibility of assimilating valid "outlier" observations associated with such events. We focus our analysis on OmF (Observation minus Forecast) residuals as they filter out phenomena well reproduced by the model (such as gas phase chemistry, transport, diurnal and seasonal cycles) thus revealing a clear trace of the EPP. Inspection of OmF statistics in both the passive (without chemical assimilation) and active (with chemical assimilation) cases altogether provides a powerful diagnostic tool to assess the model and assimilation system. We also show that passive OmF can permit a satisfactory evaluation of the ozone partial column loss due to EPP effects. Results suggest a small but significant loss of 5–6 DU (Dobson Units) during an EPP-IE (EPP indirect effects) event in the Antarctic winter of 2003, and about only 1 DU for the SPE (solar proton event) of October/November 2003. Despite large differences between the model and MIPAS chemical observations (NO2, HNO3, CH4 and O3), we demonstrate that a careful assimilation of these constituents with only gas phase chemistry included in the model (i.e. no provision for EPP impacts) and with relaxed quality control nearly eliminated the short-term bias and significantly reduced the standard deviation error below 1 hPa.
Nocturnal isoprene oxidation over the Northeast United States in summer and its impact on reactive nitrogen partitioning and secondary organic aerosol
Authors: Brown S. S. , deGouw J. A. , Warneke C. , Ryerson T. B. , Dubé W. P. , Atlas E. , Weber R. J. , Peltier R. E. , Neuman J. A. , Roberts J. M. , Swanson A. , Flocke F. , McKeen S. A. , Brioude J. , Sommariva R. , Trainer M. , Fehsenfeld F. C. , Ravishankara A. R.
Published: April 2010

Abstract. Isoprene is the largest single VOC emission to the atmosphere. Although it is primarily oxidized photochemically during daylight hours, late-day emissions that remain in the atmosphere at sunset undergo oxidation by NO3 in regionally polluted areas with large NOx levels. A recent aircraft study examined isoprene and its nocturnal oxidants in a series of night flights across the Northeast US, a region with large emissions of both isoprene and NOx. Substantial amounts of isoprene that were observed after dark were strongly anticorrelated with measured NO3 and were the most important factor determining the lifetime of this radical. The products of photochemical oxidation of isoprene, methyl vinyl ketone and methacrolein, were more uniformly distributed, and served as tracers for the presence of isoprene at sunset, prior to its oxidation by NO3. Comparison of a determination of the mass of isoprene oxidized in darkness by NO3 to a calculation of integrated isoprene emissions showed that large amounts (>20%) of emitted isoprene may undergo nocturnal oxidation in this region. Organic nitrates produced from the NO3+isoprene reaction, though not directly measured, were estimated to account for 2–9% of total reactive nitrogen and 7–31% of other long-lived organic nitrates such as PAN. The mass of isoprene oxidized by NO3 was comparable to and correlated with the organic aerosol loading for flights with relatively low organic aerosol background. The contribution of nocturnal isoprene oxidation to secondary organic aerosol was determined in the range 1–17%, and isoprene SOA mass derived from NO3 was calculated to exceed that due to OH by approximately 50%.
Ozone profile retrievals from the Ozone Monitoring Instrument
Authors: Liu X. , Bhartia P. K. , Chance K. , Spurr R. J. D. , Kurosu T. P.
Published: April 2010

Abstract. Ozone profiles from the surface to about 60 km are retrieved from Ozone Monitoring Instrument (OMI) ultraviolet radiances using the optimal estimation technique. OMI provides daily ozone profiles for the entire sunlit portion of the earth at a horizontal resolution of 13 km×48 km for the nadir position. The retrieved profiles have sufficient accuracy in the troposphere to see ozone perturbations caused by convection, biomass burning and anthropogenic pollution, and to track their spatiotemporal transport. However, to achieve such accuracy it has been necessary to calibrate OMI radiances carefully (using two days of Aura/Microwave Limb Sounder data taken in the tropics). The retrieved profiles contain ~6–7° of freedom for signal, with 5–7 in the stratosphere and 0–1.5 in the troposphere. Vertical resolution varies from 7–11 km in the stratosphere to 10–14 km in the troposphere. Retrieval precisions range from 1% in the middle stratosphere to 10% in the lower stratosphere and troposphere. Solution errors (i.e., root sum square of precisions and smoothing errors) vary from 1–6% in the middle stratosphere to 6–35% in the troposphere, and are dominated by smoothing errors. Total, stratospheric, and tropospheric ozone columns can be retrieved with solution errors typically in the few Dobson unit range at solar zenith angles less than 80°.
Size distributions of elemental carbon and its contribution to light extinction in urban and rural locations in the Pearl River Delta region, China
Authors: Yu H. , Wu C. , Wu D. , Yu J. Z.
Published: April 2010

Abstract. Elemental carbon (EC) in size-segregated aerosol samples were determined at five urban, one suburban, and one rural locations in the Pearl River Delta region in South China during 2006–2008 period. The size modal characteristics of EC were different at the urban and suburban/rural locations. The urban EC had a dominant condensation mode with a mass median aerodynamic diameter (MMAD) in the 0.36–0.43 μm range and a slightly less abundant mode in the droplet mode size (MMAD: 0.8–1.1 μm), while the suburban/rural EC had a prominent mode in the droplet mode size (MMAD: 0.7–1.1 μm) and a minor condensation mode (MMAD: 0.22–0.33 μm). Calculations using Mie theory and the measured size distributions of EC, organic carbon, and major inorganic ions indicate that EC-containing particles contributed 76% of the observed light extinction at the urban sites. Among the EC-containing particles, EC mass alone contributed 21% of the observed light extinction while non-EC materials on the EC particles (i.e. organic matter, ammonia sulfate, and water) contributed 55%. At the suburban/rural locations, EC-containing particles contributed 37–41% of the measured light extinction, with EC mass contributing 4–8% and non-EC coating materials contributing the remaining light extinction. Our results suggest that EC-containing particles were important to the overall light extinction in the urban atmospheres due to their more abundant presence from vehicular emissions. The EC-containing particles in the suburban/rural locations made a reduced but still significant contribution to light extinction budget.
Technical Note: A new coupled system for global-to-regional downscaling of CO2 concentration estimation
Authors: Trusilova K. , Rödenbeck C. , Gerbig C. , Heimann M.
Published: April 2010

Abstract. We introduce a global-to-regional nesting scheme for atmospheric transport models that are used for simulating concentrations of green house gases from globally distributed surface fluxes. The coupled system of the regional Stochastic Time-Inverted Lagrangian Transport (STILT) model and the global atmospheric transport model (TM3) is designed to resolve atmospheric trace gas concentrations at high temporal and spatial resolutions in a specified domain e.g. for regional inverse applications. The nesting technique used for the coupling is based on a decomposition of the atmospheric concentration signal into a far-field and a near-field contribution that allows global and regional models being of different type, i.e. Eulerian (grid) and Lagrangian (trajectory). For illustrating the performance of the coupled TM3-STILT system we compare simulated mixing ratios of carbon dioxide with available observations at 10 sites in Europe. For all chosen sites the TM3-STILT provided higher correlations between the modelled and the measured time series than the TM3 global model. The autocorrelation analysis showed that in contrast to the global model TM3-STILT model is capable to represent the variability of the measured tracer concentrations.
Winter- and summertime continental influences on tropospheric O3 and CO observed by TES over the western North Atlantic Ocean
Authors: Hegarty J. , Mao H. , Talbot R.
Published: April 2010

Abstract. The distribution of tropospheric O3 and CO and the regulating factors over the western North Atlantic Ocean during winter (December, January, and February, DJF) and summer (June, July, August, JJA) were investigated using retrievals from the Tropospheric Emission Spectrometer (TES) for 2004–2006. Seasonal composites of TES retrievals, reprocessed to remove the artificial geographic and seasonal structure added from the a priori, exhibited strong seasonal differences. At the 681 hPa level during winter composite O3 levels were uniformly low (~45 ppbv), but continental export was evident in a channel of enhanced CO (100–110 ppbv) flowing eastward from the US coast. In summer O3 levels were variable (45–65 ppbv) and generally higher due to increased photochemical production. The main export pathway featured a channel of enhanced CO (95–105 ppbv) flowing northeastward around an anticyclone and exiting the continent over the Canadian Maritimes around 50° N. Offshore O3-CO slopes were generally 0.15–0.20 mol mol−1 in JJA, indicative of photochemical O3 production. Composites for 4 predominant circulation patterns or map types in DJF revealed that export to the lower free troposphere (681 hPa level) was enhanced by the warm conveyor belt (WCB) airstream of cyclones while stratospheric intrusions increased TES O3 levels at 316 hPa. A major finding in the DJF data was that offshore 681 hPa CO levels behind cold fronts could be enhanced up to >150 ppbv likely by lofting from the surface via shallow convection resulting from rapid destabilization of cold air flowing over much warmer ocean waters. In JJA composites for 5 map types showed that the main export pattern of seasonal composites contained the Bermuda High as the dominate feature. However, weak cyclones and frontal troughs could enhance offshore 681 hPa CO levels to greater than 110 ppbv with O3-CO slopes >0.50 mol mol−1 south of 45° N. Intense cyclones, which were not as common in the summer, enhanced export by lofting of boundary layer pollutants from over the US and also provided a possible mechanism for transporting pollutants from boreal fire outflow southward to the US east coast.
An aerosol chamber investigation of the heterogeneous ice nucleating potential of refractory nanoparticles
Authors: Saunders R. W. , Möhler O. , Schnaiter M. , Benz S. , Wagner R. , Saathoff H. , Connolly P. J. , Burgess R. , Gallagher M. , Wills R. , Murray B. J. , Plane J. M. C.
Published: April 2010

Abstract. Nanoparticles of iron oxide (crystalline and amorphous), silicon oxide and magnesium oxide were investigated for their propensity to nucleate ice over the temperature range 180–250 K, using the AIDA chamber in Karlsruhe, Germany. All samples were observed to initiate ice formation via the deposition mode at threshold ice super-saturations (RHi thresh) ranging from 105% to 140% for temperatures below 220 K. Approximately 10% of amorphous Fe2O3 particles (modal diameter = 30 nm) generated in situ from a photochemical aerosol reactor, led to ice nucleation at RHi thresh = 140% at an initial chamber temperature of 182 K. Quantitative analysis using a singular hypothesis treatment provided a fitted function [ns (190 K) = 10(3.33×sice)+8.16] for the variation in ice-active surface site density (ns: m−2) with ice saturation (sice) for Fe2O3 nanoparticles. This was implemented in an aerosol-cloud model to determine a predicted deposition (mass accommodation) coefficient for water vapour on ice of 0.1 at temperatures appropriate for the upper atmosphere. Classical nucleation theory was used to determine representative contact angles (θ) for the different particle compositions. For the in situ generated Fe2O3 particles, a slight inverse temperature dependence was observed with θ = 10.5° at 182 K, decreasing to 9.0° at 200 K (compared with 10.2° and 11.4°, respectively for the SiO2 and MgO particle samples at the higher temperature). These observations indicate that such refractory nanoparticles are relatively efficient materials for the nucleation of ice under the conditions studied in the chamber which correspond to cirrus cloud formation in the upper troposphere. The results also show that Fe2O3 particles do not act as ice nuclei under conditions pertinent for tropospheric mixed phase clouds, which necessarily form above ~233 K. At the lower temperatures (<150 K) where noctilucent clouds form during summer months in the high latitude mesosphere, higher contact angles would be expected, which may reduce the effectiveness of these particles as ice nuclei in this part of the atmosphere.
Do biomass burning aerosols intensify drought in equatorial Asia during El Niño?
Authors: Tosca M. G. , Randerson J. T. , Zender C. S. , Flanner M. G. , Rasch P. J.
Published: April 2010

Abstract. During El Niño years, fires in tropical forests and peatlands in equatorial Asia create large regional smoke clouds. We characterized the sensitivity of these clouds to regional drought, and we investigated their effects on climate by using an atmospheric general circulation model. Satellite observations during 2000–2006 indicated that El Niño-induced regional drought led to increases in fire emissions and, consequently, increases in aerosol optical depths over Sumatra, Borneo and the surrounding ocean. Next, we used the Community Atmosphere Model (CAM) to investigate how climate responded to this forcing. We conducted two 30 year simulations in which monthly fire emissions were prescribed for either a high (El Niño, 1997) or low (La Niña, 2000) fire year using a satellite-derived time series of fire emissions. Our simulations included the direct and semi-direct effects of aerosols on the radiation budget within the model. Fire aerosols reduced net shortwave radiation at the surface during August–October by 19.1±12.9 W m−2 (10%) in a region that encompassed most of Sumatra and Borneo (90° E–120° E, 5° S–5° N). The reductions in net radiation cooled sea surface temperatures (SSTs) and land surface temperatures by 0.5±0.3 and 0.4±0.2° C during these months. Tropospheric heating from black carbon (BC) absorption averaged 20.5±9.3 W m−2 and was balanced by a reduction in latent heating. The combination of decreased SSTs and increased atmospheric heating reduced regional precipitation by 0.9±0.6 mm d−1 (10%). The vulnerability of ecosystems to fire was enhanced because the decreases in precipitation exceeded those for evapotranspiration. Together, the satellite and modeling results imply a possible positive feedback loop in which anthropogenic burning in the region intensifies drought stress during El Niño.
Temporal variability, sources, and sinks of C1-C5 alkyl nitrates in Coastal New England
Authors: Russo R. S. , Zhou Y. , Haase K. B. , Wingenter O. W. , Frinak E. K. , Mao H. , Talbot R. W. , Sive B. C.
Published: April 2010

Abstract. Seven C1-C5 alkyl nitrates were measured both on the mainland and off the coast of New Hampshire using gas chromatographic techniques. Five separate data sets will be presented to characterize the seasonal and diurnal trends and the major sources and loss processes of these compounds. In situ measurements were conducted at the University of New Hampshire (UNH) Atmospheric Observing Station at Thompson Farm (TF) located in southeast NH during winter (January–February) and summer (June–August) 2002 and summer (July–August) 2004. The median (±standard deviation) total alkyl nitrate mixing ratio (ΣRONO2) was 25 (±7) in winter and 16 (±14) pptv in summer. Furthermore, daily canister samples collected at midday and later analyzed in the laboratory from January 2004–February 2008 gave median ΣRONO2 of 23 (±8) in winter and 14 (±10) pptv in summer. Alkyl nitrate mixing ratios increased throughout the morning and were highest in the afternoon reflecting mixing of remnant boundary layer air toward the surface and photochemical production during the day. During summers 2002 and 2004, MeONO2 decreased overnight and reached minimum hourly average mixing ratios in the early morning (05:00–07:00 LT). Comparison with wind speed and trace gas (i.e., hydrocarbons, ozone, carbon monoxide, total reactive nitrogen) trends suggested that dry deposition contributed to the early morning MeONO2 minimum which is a previously unaccounted for removal mechanism. The mean dry deposition rate and velocity of MeONO2 was estimated to be −0.5 nmol m−2 hr−1 and 0.13 cm s−1, respectively. Results from ambient air and surface seawater measurements made onboard the NOAA R/V Ronald H. Brown in the Gulf of Maine during the 2002 New England Air Quality Study and from ambient canister samples collected throughout the Great Bay estuary in August 2003 are also presented. Comparisons between the alkyl nitrate trends with anthropogenic and marine source fingerprints and tracers suggest that a marine source of alkyl nitrates is not significant in coastal New England. Given the apparent prominence of a secondary source, comparisons between observed and predicted alkyl nitrate/parent hydrocarbon ratios were made which demonstrated that background mixing ratios have a continuous and prevalent influence on the alkyl nitrate distribution.
Interpretation of Aura satellite observations of CO and aerosol index related to the December 2006 Australia fires
Authors: Luo M. , Boxe C. , Jiang J. , Nassar R. , Livesey N.
Published: April 2010

Abstract. Enhanced Carbon Monoxide (CO) in the upper troposphere (UT) is shown by collocated Tropospheric Emission Spectrometer (TES) and Microwave Limb Sounder (MLS) measurements near and down-wind from the known wildfire region of SE Australia from 12–19 December 2006. Enhanced UV aerosol index (AI) derived from Ozone Monitoring Instrument (OMI) measurements correlate with these high CO concentrations. HYSPLIT model back trajectories trace selected air parcels to the SE Australia fire region as their initial location, where TES observes enhanced CO in the upper and lower troposphere. Simultaneously, they show a lack of vertical advection along their tracks. TES retrieved CO vertical profiles in the higher and lower southern latitudes are examined together with the averaging kernels and show that TES CO retrievals are most sensitive at approximately 300–400 hPa. The enhanced CO observed by TES at the upper (215 hPa) and lower (681 hPa) troposphere are, therefore, influenced by mid-tropospheric CO. GEOS-Chem model simulations with an 8-day emission inventory, as the wildfire source over Australia, are sampled to the TES/MLS observation times and locations. These simulations only show CO enhancements in the lower troposphere near and down-wind from the wildfire region of SE Australia with drastic underestimates of UT CO. Although CloudSat along-track ice-water content curtains are examined to see whether possible vertical convection events can explain the high UT CO values, sparse observations of collocated Aura CO and CloudSat along-track ice-water content measurements for the single event precludes any conclusive correlation. Vertical convection that uplift fire-induced CO (i.e. most notably referred to as pyro-cumulonimbus, pyroCb) may provide an explanation for the incongruence between these simulations and the TES/MLS observations of enhanced CO in the UT. Future GEOS-Chem simulations are needed to validate this conjecture as the the PyroCb mechanism is currently not incorporated in GEOS-Chem.
Impact of solar radiation on aerosol-cloud interactions in thin stratocumulus clouds
Authors: Lee S. S. , Penner J. E.
Published: April 2010

Abstract. This study examines the role of solar radiation in the effect of aerosols on liquid-water path (LWP) in thin, marine stratocumulus clouds with LWP of ~50 g m−2 or less by performing four sets of simulations with different solar radiation. Each set is composed of a simulation with present-day (PD) aerosols and a simulation with preindustrial (PI) aerosols. As solar radiation increases, decoupling within the marine boundary layer (MBL) becomes stronger, leading to less condensation and less LWP and thus the absence of the surface precipitation. This enables the evaporation of rain to affect the cloud-base instability. As rain evaporation increases due to more conversion of cloud liquid to rain in the PI case, the cloud-base instability increases and thus updrafts increase which leads to larger LWP in the PI case than in the PD case. In the cases with no surface precipitation, when solar radiation decreases and thus decoupling becomes weaker, rain evaporation and cloud-base instability become larger, which increases the LWP more with PI aerosols than with PD aerosols. As solar radiation decreases further, condensation and, thus, the LWP increase, which leads to the presence of the surface precipitation. This stabilizes the entire MBL and thus prevents the interactions that cause the evaporation of rain to enhance the cloud-base instability. In cases with the surface precipitation, the in-cloud interactions among cloud droplet number concentration (CDNC), supersaturation, and updrafts play an important role in the effect of aerosols on the LWP; these in-cloud interactions produce larger LWP with the PD aerosols than with the PI aerosols. In a case with lower solar radiation and with surface precipitation, weaker decoupling induces stronger in-cloud interactions, which results in larger increases in LWP with PD aerosols compared to PI aerosols than that in a case with higher solar radiation. The results of this study demonstrate that solar radiation can act as an important environmental factor by inducing a large variation in the LWP and by changing the sign of aerosol effects on the LWP of thin stratocumulus clouds. Hence, the effect of solar radiation on decoupling and thus on the feedbacks between microphysics and dynamics needs to be included in climate models for a better prediction of the effect of aerosols on clouds and thus climate.
Applying an ensemble Kalman filter to the assimilation of AERONET observations in a global aerosol transport model
Authors: Schutgens N. A. J. , Miyoshi T. , Takemura T. , Nakajima T.
Published: April 2010

Abstract. We present a global aerosol assimilation system based on an ensemble Kalman filter, which we believe leads to a significant improvement in aerosol fields. The ensemble allows realistic, spatially and temporally variable model covariances (unlike other assimilation schemes). As the analyzed variables are mixing ratios (prognostic variables of the aerosol transport model), there is no need for the extra assumptions required by previous assimilation schemes analyzing aerosol optical thickness (AOT). We describe the implementation of this assimilation system and in particular the construction of the ensemble. This ensemble should represent our estimate of current model uncertainties. Consequently, we construct the ensemble around randomly modified emission scenarios. The system is tested with AERONET observations of AOT and Angström exponent (AE). Particular care is taken in the prescribing the observational errors. The assimilated fields (AOT and AE) are validated through independent AERONET, SKYNET and MODIS Aqua observations. We show that, in general, assimilation of AOT observations leads to improved modelling of global AOT, while assimilation of AE only improves modelling when the AOT is high.
Homogeneous nucleation of sulfuric acid and water mixture: experimental setup and first results
Authors: Brus D. , Hyvärinen A.-P. , Viisanen Y. , Kulmala M. , Lihavainen H.
Published: April 2010

Abstract. In this study we introduce a new laminar flow tube suitable for binary and ternary homogeneous nucleation studies. The production of sulfuric acid and water vapor mixture, the experimental setup and the method of sulfuric acid concentration determination are discussed in details. Wall losses were estimated from the measured sulfuric acid concentration profile along the laminar flow tube and compared to a theoretical prediction. In this investigation the experimental evidence of new particle formation was observed at concentration of 109 molecules of sulfuric acid in cm3 and nucleation rates measured at three relative humidities (RH) 10, 30 and 50% cover six orders of magnitudes from 10−3 to 103 particles in cm3. Particle free air was used as a carrier gas. Our first results are compared to theoretical prediction of binary homogeneous nucleation, to results obtained by other investigators and to atmospheric nucleation.
The PreVOCA experiment: modeling the lower troposphere in the Southeast Pacific
Authors: Wyant M. C. , Wood R. , Bretherton C. S. , Mechoso C. R. , Bacmeister J. , Balmaseda M. A. , Barrett B. , Codron F. , Earnshaw P. , Fast J. , Hannay C. , Kaiser J. W. , Kitagawa H. , Klein S. A. , Köhler M. , Manganello J. , Pan H.-L. , Sun F. , Wang S. , Wang Y.
Published: April 2010

Abstract. The Preliminary VOCALS Model Assessment (PreVOCA) aims to assess contemporary atmospheric modeling of the subtropical South East Pacific, with a particular focus on the clouds and the marine boundary layer (MBL). Models results from fourteen modeling centers were collected including operational forecast models, regional models, and global climate models for the month of October 2006. Forecast models and global climate models produced daily forecasts, while most regional models were run continuously during the study period, initialized and forced at the boundaries with global model analyses. Results are compared in the region from 40° S to the equator and from 110° W to 70° W, corresponding to the Pacific coast of South America. Mean-monthly model surface winds agree well with QuikSCAT observed winds and models agree fairly well on mean weak large-scale subsidence in the region next to the coast. However they have greatly differing mean geographic patterns of cloud fraction with only a few models agreeing well with MODIS observations. Most models also underestimate the MBL depth by several hundred meters in the eastern part of the study region. The diurnal cycle of liquid water path is underestimated by most models at the 85° W 20° S stratus buoy site compared with satellite, consistent with previous modeling studies. The low cloud fraction is also underestimated during all parts of the diurnal cycle compared to surface-based climatologies. Most models qualitatively capture the MBL deepening around 15 October 2006 at the stratus buoy, associated with colder air at 700 hPa.
Formaldehyde and its relation to CO, PAN, and SO2 in the Houston-Galveston airshed
Authors: Rappenglück B. , Dasgupta P. K. , Leuchner M. , Li Q. , Luke W.
Published: April 2010

Abstract. The Houston-Galveston Airshed (HGA) is one of the major metropolitan areas in the US that is classified as a nonattainment area of Federal ozone standards. Formaldehyde (HCHO) is a key species in understanding ozone related air pollution; some of the highest HCHO concentrations in North America have been reported for the HGA. We report on HCHO measurements in the HGA from summer 2006. Among several sites, maximum HCHO mixing ratios were observed in the Houston Ship Channel (HSC), a region with a very high density of industrial/petrochemical operations. HCHO levels at the Moody Tower (MT) site close to downtown were dependent on the wind direction: southerly maritime winds brought in background levels (0.5–1 ppbv) while trajectories originating in the HSC resulted in high HCH (up to 31.5 ppbv). Based on the best multiparametric linear regression model fit, the HCHO levels at the MT site can be accounted for as follows: 38.5±12.3% from primary vehicular emissions (using CO as an index of vehicular emission), 24.1±17.7% formed photochemically (using peroxyacetic nitric anhydride (PAN) as an index of photochemical activity) and 8.9±11.2% from industrial emissions (using SO2 as an index of industrial emissions). The balance 28.5±12.7% constituted the residual which cannot be easily ascribed to the above categories and/or which is transported into the HGA. The CO related HCHO fraction is dominant during the morning rush hour (06:00–09:00 h, all times are given in CDT); on a carbon basis, HCHO emissions are up to 0.7% of the CO emissions. The SO2 related HCHO fraction is significant between 09:00–12:00 h. After 12:00 h HCHO is largely formed through secondary processes. The HCHO/PAN ratios are dependent on the SO2 levels. The SO2 related HCHO fraction at the downtown site originates in the ship channel. Aside from traffic-related primary HCHO emissions, HCHO of industrial origin serves as an appreciable source for OH in the morning.
Measurement and modelling of reactive halogen species over the tropical Atlantic Ocean
Authors: Mahajan A. S. , Plane J. M. C. , Oetjen H. , Mendes L. , Saunders R. W. , Saiz-Lopez A. , Jones C. E. , Carpenter L. J. , McFiggans G. B.
Published: April 2010

Abstract. Although reactive halogen chemistry is well studied in coastal and polar environments, the presence of halogens over the open ocean environment has not been widely reported. The impacts of halogens on the tropical open ocean marine boundary layer (MBL), in particular, are not well characterised. This paper describes observations of iodine monoxide (IO) and bromine oxide (BrO) over eight months in the tropical open ocean MBL, on the north-eastern side of São Vicente (Cape Verde Islands, 16.85° N, 24.87° W). The highest BrO mixing ratio observed was 5.6±1 ppt, while the maximum observed IO mixing ratio was 3.1±0.4 ppt. The average values seen between 09:00–17:00 GMT were ~2.8 ppt for BrO and ~1.5 ppt for IO; these averages showed little variability over the entire campaign from November 2006 to June 2007. A 1-dimensional chemistry and transport model is used to study the evolution of iodine species and quantify the combined impact of iodine and bromine chemistry on the oxidising capacity of the MBL. It appears that the measured fluxes of iodocarbons are insufficient to account for the observed levels of IO, and that an additional I atom source is required, possibly caused by the deposition of O3 onto the ocean surface in the presence of solar radiation. Modelling results also show that the total O3 depletion observed at Cape Verde cannot be explained in the absence of halogen chemistry, which contributes ~45% of the total O3 depletion at the height of measurements (10 m) during summer. The model also predicts that halogens decrease the hydroperoxy radical (HO2) concentration by ~14% and increase the hydroxyl radical (OH) concentration by ~13% near the ocean surface. The oxidation of dimethyl sulphide (DMS) by BrO takes place at a comparable rate to oxidation by OH in this environment. Finally, the potential of iodine chemistry to form new particles is explored and conditions under which particle formation could be important in the remote MBL are discussed.
Sources of uncertainties in modelling Black Carbon at the global scale
Authors: Vignati E. , Karl M. , Krol M. , Wilson J. , Stier P. , Cavalli F.
Published: April 2010

Abstract. Our understanding of the global black carbon cycle is essentially qualitative due to uncertainties in our knowledge of the properties of black carbon. This work investigates uncertainties related to modelling black carbon: due to the use of different schemes for BC ageing and its removal rate in the global Transport-Chemistry model TM5 and due to the uncertainties in the definition and quantification of observed black carbon, which propagate through to both the emission inventories, and the measurements used for the model evaluation. The schemes for the atmospheric processing of black carbon that have been tested with the model are (i) a simple approach considering black carbon as bulk aerosol and a simple treatment in the removal and (ii) a more complete description of microphysical aging within an aerosol dynamics model, where removal is coupled to the microphysical properties of the aerosol. In the first approach a fixed 70% of black carbon is scavenged in clouds and removed when rain is present. The second leads to a global average of 40% black carbon that is scavenged in clouds and subsequently removed by rain, thus resulting in a longer lifetime. This difference is reflected in comparisons between both sets of modelled results and the measurements. Close to the sources, both anthropogenic and vegetation fire source regions, the model results do not differ significantly, showing that the emissions are the prevailing mechanism determining the concentrations and the choice of the aerosol scheme does not influence the levels. In more remote areas such as oceanic and polar regions the differences can be orders of magnitude, due to the differences between the two schemes. The more complete description reproduces the seasonal trend of the black carbon observations in those areas, although not always the magnitude of the signal, while the more simplified approach underestimates black carbon concentrations by orders of magnitude. The sensitivity to wet scavenging has been tested varying in-cloud and below-cloud removals. BC lifetime increases by 10% when large scale and convective scale precipitation are reduced by 30%, while the variation is very small when below-cloud scavenging is zero. Since the emission inventories are representative of elemental carbon-like substance, the model output should be compared to elemental carbon measurements, and, if known, the ratio of black carbon to elemental carbon mass should be taken into account when the model is compared with black carbon observations.
Modelling the reversible uptake of chemical species in the gas phase by ice particles formed in a convective cloud
Authors: Marécal V. , Pirre M. , Rivière E. D. , Pouvesle N. , Crowley J. N. , Freitas S. R. , Longo K. M.
Published: April 2010

Abstract. The present paper is a preliminary study preparing the introduction of reversible trace gas uptake by ice particles into a 3-D cloud resolving model. For this a 3-D simulation of a tropical deep convection cloud was run with the BRAMS cloud resolving model using a two-moment bulk microphysical parameterization. Trajectories encountering the convective clouds were computed from these simulation outputs along which the variations of the pristine ice, snow and aggregate mixing ratios and size distributions were extracted. The reversible uptake of 11 trace gases by ice was examined assuming applicability of Langmuir isotherms using recently evaluated (IUPAC) laboratory data. The results show that ice uptake is only significant for HNO3, HCl, CH3COOH and HCOOH. For H2O2, using new results for the partition coefficient results in significant partitioning to the ice phase for this trace gas also. It was also shown that the uptake is largely dependent on the temperature for some species. The adsorption saturation at the ice surface for large gas concentrations is generally not a limiting factor except for HNO3 and HCl for gas concentration greater than 1 ppbv. For HNO3, results were also obtained using a trapping theory, resulting in a similar order of magnitude of uptake, although the two approaches are based on different assumptions. The results were compared to those obtained using a BRAMS cloud simulation based on a single-moment microphysical scheme instead of the two moment scheme. We found similar results with a slightly more important uptake when using the single-moment scheme which is related to slightly higher ice mixing ratios in this simulation. The way to introduce these results in the 3-D cloud model is discussed.
Mechanism of UV-light induced SO2 oxidation to H2SO4
Authors: Sorokin A.
Published: April 2010

Abstract. The electron-excited chemistry of sulfur dioxide oxidation induced by UV irradiation of air with trace O3 and SO2 is considered. The importance of this mechanism is evaluated based on recent laboratory experiments on SO2 oxidation in a laminar tube with air induced by UV irradiation. Results show that under respective conditions the route of SO2 oxidation involving electron excited oxygen molecules may be as efficient as a known OH-radical mechanism. Hence, an influence of UV irradiation on SO2 oxidation chemistry, if confirmed, may provide an additional external source affecting the oxidation and aerosol formation processes in atmosphere.
Turbulent exchange and segregation of HOx radicals and volatile organic compounds above a deciduous forest
Authors: Dlugi R. , Berger M. , Zelger M. , Hofzumahaus A. , Siese M. , Holland F. , Wisthaler A. , Grabmer W. , Hansel A. , Koppmann R. , Kramm G. , Möllmann-Coers M. , Knaps A.
Published: April 2010

Abstract. The eddy covariance method was applied for the first time to estimate fluxes of OH and HO2 together with fluxes of isoprene, the sum of methyl vinyl ketone (MVK) and methacrolein (MACR) and the sum of monoterpenes above a mixed deciduous forest. Highly sensitive measurements of OH and HO2 were performed by laser induced fluorescence (LIF), and biogenic volatile organic compounds (BVOCs) were measured by Proton-Transfer-Reaction Mass Spectrometry (PTR-MS) at a time resolution of 5 s, each. Wind speed was measured by a sonic anemometer at 10 Hz. The one-day feasibility study was conducted at a total height of 37 m, about 7 m above forest canopy, during the ECHO 2003 intensive field study in July 2003. The daytime measurements yielded statistically significant OH fluxes that indicate downward transport of OH into the direction of the canopy and HO2 fluxes mainly upward out of the canopy. This hints towards a significant chemical sink of OH by reaction with BVOCs and conversion of OH to HO2 at the canopy. In addition, the highly time-resolved trace gas measurements were used to calculate the intensity of segregation of OH and BVOCs, demonstrating that the effective reaction rate of isoprene and OH was slowed down as much as 15% due to inhomogeneous mixing of the reactants. The paper describes the applied methods and provides a detailed analysis of possible systematic errors of the covariance products.
Impact of mineral dust on nitrate, sulfate, and ozone in transpacific Asian pollution plumes
Authors: Fairlie T. D. , Jacob D. J. , Dibb J. E. , Alexander B. , Avery M. A. , van Donkelaar A. , Zhang L.
Published: April 2010

Abstract. We use a 3-d global chemical transport model (GEOS-Chem) to interpret aircraft observations of nitrate and sulfate partitioning in transpacific dust plumes during the INTEX-B campaign of April–May 2006. The model includes explicit transport of size-resolved mineral dust and its alkalinity, nitrate, and sulfate content. The observations show that particulate nitrate is primarily associated with dust, sulfate is primarily associated with ammonium, and Asian dust remains alkaline across the Pacific. This can be reproduced in the model by using a reactive uptake coefficient for HNO3 on dust (γ(HNO3)~10−3) much lower than commonly assumed in models and likely reflecting limitation of uptake by dust dissolution. The model overestimates gas-phase HNO3 by a factor of 2–3, typical of previous model studies; we show that this cannot be corrected by uptake on dust. We find that the fraction of aerosol nitrate on dust in the model increases from ~30% in fresh Asian outflow to 80–90% over the Northeast Pacific, reflecting in part the volatilization of ammonium nitrate and the resulting transfer of nitrate to the dust. Consumption of dust alkalinity by uptake of acid gases in the model is slow relative to the lifetime of dust against deposition, so that dust in general does not acidify. This argues against the hypothesis that dust iron released by acidification could become bio-available upon dust deposition. Observations in INTEX-B show no detectable ozone depletion in Asian dust plumes, consistent with the model. Uptake of HNO3 by dust, suppressing its recycling to NOx, reduces Asian pollution influence on US surface ozone in the model by 10–15% or up to 1 ppb.
Comprehensively accounting for the effect of giant CCN in cloud droplet activation parameterizations
Authors: Barahona D. , West R. E. L. , Stier P. , Romakkaniemi S. , Kokkola H. , Nenes A.
Published: April 2010

Abstract. Cloud droplet activation parameterizations used in aerosol indirect effect assessments often assume that droplet growth after activation is much greater than their equilibrium size close to cloud base. This assumption does not hold for large CCN which may experience limited growth. If a large fraction of the aerosol is composed of such particles (such as regions with large fractions of dust particles and seasalt), neglecting such kinetic limitations in cloud droplet activation parameterizations leads to an underestimation of droplet surface area during cloud formation, hence overestimation of maximum supersaturation and cloud droplet number. Here we present a simple approach to address this problem and that can easily be incorporated into cloud droplet activation parameterizations. A demonstration of this method is done for activation parameterizations based on the ''population splitting'' concept of Nenes and Seinfeld (2003).
Estimations of climate sensitivity based on top-of-atmosphere radiation imbalance
Authors: Lin B. , Chambers L. , Stackhouse Jr. P. , Wielicki B. , Hu Y. , Minnis P. , Loeb N. , Sun W. , Potter G. , Min Q. , Schuster G. , Fan T.-F.
Published: April 2010

Abstract. Large climate feedback uncertainties limit the accuracy in predicting the response of the Earth's climate to the increase of CO2 concentration within the atmosphere. This study explores a potential to reduce uncertainties in climate sensitivity estimations using energy balance analysis, especially top-of-atmosphere (TOA) radiation imbalance. The time-scales studied generally cover from decade to century, that is, middle-range climate sensitivity is considered, which is directly related to the climate issue caused by atmospheric CO2 change. The significant difference between current analysis and previous energy balance models is that the current study targets at the boundary condition problem instead of solving the initial condition problem. Additionally, climate system memory and deep ocean heat transport are considered. The climate feedbacks are obtained based on the constraints of the TOA radiation imbalance and surface temperature measurements of the present climate. Currently, there is a lack of high accuracy measurements of TOA radiation imbalance. Available estimations indicate that TOA net radiative heating to the climate system is about 0.85 W/m2. Based on this value, a positive climate feedback with a feedback coefficient ranging from −1.3 to −1.0 W/m2/K is found. The range of feedback coefficient is determined by climate system memory. The longer the memory, the stronger the positive feedback. The estimated time constant of the climate is large (70~120 years) mainly owing to the deep ocean heat transport, implying that the system may be not in an equilibrium state under the external forcing during the industrial era. For the doubled-CO2 climate (or 3.7 W/m2 forcing), the estimated global warming would be 3.1 K if the current estimate of 0.85 W/m2 TOA net radiative heating could be confirmed. With accurate long-term measurements of TOA radiation, the analysis method suggested by this study provides a great potential in the estimations of middle-range climate sensitivity.
Identification of potential regional sources of atmospheric total gaseous mercury in Windsor, Ontario, Canada using hybrid receptor modeling
Authors: Xu X. , Akhtar U. S.
Published: April 2010

Abstract. Windsor (Ontario) – the automotive capital of Canada does not have any significant mercury (Hg) sources. However, Windsor experiences trans-boundary air pollution as it is located immediately downwind of industrialized regions of the United States of America. A study was conducted in 2007 aimed to identify the potential regional sources of total gaseous mercury (TGM) and investigate the effects of regional sources and other factors on seasonal variability of TGM concentrations in Windsor. TGM concentration was measured at the University of Windsor campus using a Tekran® 2537A Hg vapour analyzer. An annual mean of 2.02±1.63 ng/m3 was observed in 2007. The average TGM concentration was high in the summer (2.48 ng/m3) and winter (2.17 ng/m3), compared to spring (1.88 ng/m3) and fall (1.76 ng/m3). Hybrid receptor modeling potential source contribution function (PSCF) was used by incorporating 72-h backward trajectories and measurements of TGM in Windsor. The results of PSCF were analyzed in conjunction with the Hg emissions inventory of North America (by state/province) to identify regions affecting Windsor. In addition to annual modeling, seasonal PSCF modeling was also conducted. The potential source region was identified between 24–61° N and 51–143° W. Annual PSCF modeling identified major sources southwest of Windsor, stretching from Ohio to Texas. The emissions inventory also supported the findings, as Hg emissions were high in those regions. Results of seasonal PSCF modeling were analyzed to find the combined effects of regional sources, meteorological conditions, and surface reemissions, on intra-annual variability of Hg concentrations. It was found that the summer and winter highs of atmospheric Hg can be attributed to areas where large numbers of coal fired power plants are located in the USA. Weak atmospheric dispersion due to low winds and high reemission from surfaces due to higher temperatures contributed to high concentrations in the summer. In the winter, the atmospheric removal of Hg was slow, but strong winds led to more dispersion, resulting in lower concentrations than the summer. Future studies could use smaller grid sizes and refined emission inventories, for more accurate analysis of source-receptor relationship of atmospheric Hg. Abbreviations of states/provinces: Alabama (AL), Arkansas (AR), British Columbia (BC), Georgia (GA), Iowa (IA), Illinois (IL), Indiana (IN), Kentucky (KY), Louisiana (LA), Manitoba (MB), Michigan (MI), Minnesota (MN), Mississippi (MS), Missouri (MO), Newfoundland and Labrador (NL), Ohio (OH), Ontario (ON), Oregon (OR), Pennsylvania (PA), Tennessee (TN), Texas (TX), West Virginia (WV), Wisconsin (WI).