bluuebunny/crossref_metadata_2025 huggingface dataset:
Impact of dust on tropospheric photochemistry over polluted regions: a case study of the Beijing megacity
Authors: Zhu S. , Butler T. , Sander R. , Ma J. , Lawrence M. G.
Published: April 2010

Abstract. The box model MECCA (Module Efficiently Calculating the Chemistry of the Atmosphere) is updated by incorporating detailed heterogeneous chemistry occurring on mineral aerosol surfaces. The model is used to investigate the impact of dust on tropospheric photochemistry, when the dust is transported to a polluted region together with other trace gases. The impacts of dust via heterogeneous removal of gases are analyzed for different hypothetical transport rates which are described by four different exchange rate coefficients Kt in the model. Substantial impacts are found for many gases, including Ox (O3+O(3P), NOx (NO+NO2) and OH. The Ox daily average mixing ratio decreases due to heterogeneous reactions on dust. The change ranges from –2.5 to –18.4 nmol/mol, and is larger for faster mixing with upwind air masses (i.e. greater Kt). This translates into a large relative change in Ox, ranging from –44% to –55%, which changes slightly with increasing values of Kt. By assuming an artificial 50% decrease of all photolysis rates, the impacts of dust via photolysis perturbation are also estimated. Furthermore, the uncertainties in the results due to the uncertainties in the uptake coefficients are evaluated. It is found that for all gases which are heterogeneously removed, the self-removal results in the largest uncertainty (e.g. –49% for O3, –76% for NO2, –47% for HNO3, –92% for HCHO, –64% for CH3OH and –93% for SO2). The heterogeneous removal of NO2 is found to be particularly important, because it results in significant uncertainties not only in itself, but also in OH (340%) and HO2 (365%). Moreover, the heterogeneous removal of HCHO and O3 also has farther-reaching effects on the OH concentration (resulting in changes of –55% and 45%, respectively), and the heterogeneous removal of HCHO results in an uncertainty of –38% in the HO2 concentration.
A 15 year record of high-frequency, in situ measurements of hydrogen at Mace Head, Ireland
Authors: Grant A. , Witham C. S. , Simmonds P. G. , Manning A. J. , O'Doherty S.
Published: April 2010

Abstract. Continuous high-frequency measurements of atmospheric molecular hydrogen have been made at Mace Head atmospheric research station on the west coast of Ireland from March 1994 to December 2008. The presented data provides a wealth of information on long term trends and seasonal cycles of hydrogen in background northern hemispheric air. Individual measurements have been sorted using a Lagrangian dispersion model to separate clean background air from regionally polluted European air masses and those transported from southerly latitudes. No significant trend was observed in background northern hemispheric air over the 15 year record, elevations in yearly means were accounted for from large scale biomass burning events. Seasonal cycles show the expected pattern with maxima in spring and minima in late autumn. The mean hydrogen mole fraction in baseline northern hemispheric air was found to be 500.1 ppb. Air transported from southerly latitudes showed an elevation from baseline mean of 11.0 ppb, reflecting both the latitudinal gradient of hydrogen, with higher concentrations in the southern hemisphere, and the large photochemical source of hydrogen from southerly latitudes. European polluted air masses arriving at Mace Head showed mean elevation of 5.3 ppb from baseline air masses, reflecting hydrogen's source from primary emissions like fossil fuel combustion. Forward modelling of transport of hydrogen to Mace Head suggests that the ratio of hydrogen to carbon monoxide in primary emissions is considerably less in non-traffic sources than traffic sources.
Technical Note: A time-dependent calibration correction for solar occultation instruments
Authors: Burton S. P. , Thomason L. W. , Zawodny J. M.
Published: April 2010

Abstract. Solar occultation has proven to be a reliable technique for the measurement of atmospheric constituents in the stratosphere. NASA's Stratospheric Aerosol and Gas Experiments (SAGE, SAGE II, and SAGE III) together have provided over 25 years of quality solar occultation data, a data record which has been an important resource for the scientific exploration of atmospheric composition and climate change. Herein, we describe an improvement to the processing of SAGE data that corrects for a previously uncorrected short-term time-dependence in the calibration function. The variability relates to the apparent rotation of the scanning track with respect to the face of the sun due to the motion of the satellite. Correcting for this effect results in a decrease in the measurement noise in the Level 1 line-of-sight optical depth measurements of approximately 40% in the middle and upper stratospheric SAGE II and III observations where it has been applied. The technique is potentially useful for any scanning solar occultation instrument, and suggests further improvement for future occultation measurements if a full disk imaging system can be included.
Composition of cirrus-forming aerosols at the tropical tropopause
Authors: Froyd K. D. , Murphy D. M. , Lawson P. , Baumgardner D. , Herman R. L.
Published: April 2010

Abstract. The composition of residual particles from evaporated cirrus ice crystals near the tropical tropopause as well as unfrozen aerosols were measured with a single particle mass spectrometer. Subvisible cirrus residuals were predominantly composed of internal mixtures of neutralized sulfate with organic material and were chemically indistinguishable from unfrozen sulfate-organic aerosols. Ice residuals were also similar in size to unfrozen aerosol. Heterogeneous ice nuclei such as mineral dust were not enhanced in these subvisible cirrus residuals. Biomass burning particles were depleted in the residuals. Cloud probe measurements showing low cirrus ice crystal number concentrations were inconsistent with conventional homogeneous freezing. Recent laboratory studies provide heterogeneous nucleation scenarios that may explain tropopause level subvisible cirrus formation.
Summertime NOx measurements during the CHABLIS campaign: can source and sink estimates unravel observed diurnal cycles?
Authors: Bauguitte S. J.-B. , Bloss W. J. , Evans M. J. , Salmon R. A. , Anderson P. S. , Jones A. E. , Lee J. D. , Saiz-Lopez A. , Roscoe H. K. , Wolff E. W. , Plane J. M. C.
Published: April 2010

Abstract. NOx measurements were conducted at the Halley Research Station, Antarctica, during the austral summer period 1 January–10 February 2005. A clear NOx diurnal cycle was observed with minimum concentrations close to instrumental detection limit (5 pptv) measured between 04:00–05:00 GMT. NOx concentrations peaked (24 pptv) between 19:00–20:00 GMT, approximately 5 h after local solar noon. An optimised box model of NOx concentrations based on production from in-snow nitrate photolysis and chemical loss derives a mean noon emission rate of 3.48×108 molecules cm−2 s−1, assuming a 100 m boundary layer mixing height, and a relatively short NOx lifetime of ~6.4 h. This emission rate compares to directly measured values ranging from 1.7 to 3.4×108 molecules cm−2 s−1 made on 3 days at the end of the study period. Calculations of the maximum rate of NO2 loss via a variety of conventional HOx and halogen oxidation processes show that the lifetime of NOx is predominantly controlled by halogen processing, namely BrNO3 and INO3 gas-phase formation and their subsequent heterogeneous uptake, with a potential smaller contribution from HNO4 formation and uptake. Furthermore the presence of halogen oxides is shown to significantly perturb NOx concentrations by decreasing the NO/NO2 ratio. We conclude that in coastal Antarctica, the potential ozone production efficiency of NOx emitted from the snowpack is mitigated by the more rapid NOx loss due to halogen nitrate hydrolysis. These results suggest that the role of halogen oxides need to be considered when interpreting the isotopic signature of nitrate impurities held within snow and ice.
Satellite observations of long range transport of a large BrO cloud in the Arctic
Authors: Begoin M. , Richter A. , Kaleschke L. , Tian-Kunze X. , Stohl A. , Burrows J. P.
Published: April 2010

Abstract. Ozone Depletion Events (ODE) during polar springtime are a well known phenomenon in the Arctic and Antarctic boundary layer. They are caused by the catalytic destruction of ozone by halogens producing reactive halogen oxides like bromine monoxide (BrO). The key halogen bromine can be rapidly transferred into the gas phase in an autocatalytic process – the so called "Bromine Explosion". However, the exact mechanism, which leads to an initial bromine release as well as the influence of transport and chemical processes on BrO, is still not clearly understood. In this study, BrO measurements from the satellite instrument GOME-2 are used together with model calculations with the dispersion model FLEXPART and Potential Frost Flowers (PFF) maps to study a special arctic BrO event in March/April 2007, which could be tracked over many days and large areas. Full BrO activation was observed within one day east of Siberia with subsequent transport to the Hudson Bay. The event was linked to a cyclone with very high surface wind speeds which could have been involved in the production and the sustaining of aerosols providing the surface for BrO recycling within the plume. The evolution of the BrO plume could be well reproduced by FLEXPART calculations for a passive tracer indicating that the activated air mass was transported all the way from Siberia to the Hudson Bay without further activation at the surface. No direct link could be made to frost flower occurrence and BrO activation but enhanced PFF were observed a few days before the event in the source regions.
Three-dimensional numerical simulations of the evolution of a contrail and its transition into a contrail-cirrus
Authors: Paugam R. , Paoli R. , Cariolle D.
Published: April 2010

Abstract. This study describes high-resolution numerical simulations of the evolution of an aircraft contrail and its transition into a contrail-cirrus. The contrail is modeled as a vortex pair descending in a stratified atmosphere where turbulent fluctuations are sustained in the late regime of diffusion. The focus of this study is laid on the three-dimensional vortex dynamics – including vortex instability and turbulence – and on the interaction between dynamics and microphysics and its role in determining the growth and the distribution of ice crystals. The results show the feasibility of three-dimensional simulations of the entire lifetime of a contrail until its transformation into a contrail-cirrus in supersaturated conditions. In particular, it is shown that the contrail-cirrus optical depth is maintained as the cloud expands, in agreement with available observations.
Simultaneous coastal measurements of ozone deposition fluxes and iodine-mediated particle emission fluxes with subsequent CCN formation
Authors: Whitehead J. D. , McFiggans G. , Gallagher M. W. , Flynn M. J.
Published: April 2010

Abstract. Here we present the first observations of simultaneous ozone deposition fluxes and ultrafine particle emission fluxes over an extensive infra-littoral zone. Fluxes were measured by the eddy covariance technique at the Station Biologique de Roscoff, on the coast of Brittany, north-west France. This site overlooks a very wide (3 km) littoral zone controlled by very deep tides (9.6 m) exposing extensive macroalgae beds available for significant iodine mediated photochemical production of ultrafine particles. The aspect at the Station Biologique de Roscoff provides an extensive and relatively flat, uniform fetch within which micrometeorological techniques may be utilized to study links between ozone deposition to macroalgae (and sea water) and ultrafine particle production. Ozone deposition to seawater at high tide was significantly slower (vd[O3]=0.302±0.095 mm s−1) than low tidal deposition. A statistically significant difference in the deposition velocities to macroalgae at low tide was observed between night time (vd[O3]=1.00±0.10 mm s−1) and daytime (vd[O3]=2.05±0.16s−1) when ultrafine particle formation results in apparent particle emission. Very high emission fluxes of ultrafine particles were observed during daytime periods at low tides ranging from 50 000 particles cm−2 s−1 to greater than 200 000 particles cm−2 s−1 during some of the lowest tides. These emission fluxes exhibited a significant relationship with particle number concentrations comparable with previous observations at another location. Apparent particle growth rates were estimated to be in the range 17–150 nm h−1 for particles in the size range 3–10 nm. Under certain conditions, particle growth may be inferred to continue to greater than 120 nm over tens of hours; sizes at which they may readily behave as cloud condensation nuclei (CCN) under reasonable supersaturations that may be expected to pertain at the top of the marine boundary layer. These results link direct depositional loss and photochemical destruction of ozone to the formation of particles and hence CCN from macroalgal emissions at a coastal location.
Introducing the bromide/alkalinity ratio for a follow-up discussion on "Precipitation of salts in freezing seawater and ozone depletion events: a status report", by Morin et al., published in Atmos. Chem. Phys., 8, 7317–7324, 2008
Authors: Sander R. , Morin S.
Published: April 2010

Abstract. Sander et al. (2006) proposed that CaCO3 precipitation can be an important factor in triggering tropospheric ozone depletion events. Recently, Morin et al. (2008b) presented calculations with the FREZCHEM model and concluded that their results and interpretation cast doubt on the validity of this hypothesis. In this joint publication, we have re-analyzed the implications of the FREZCHEM results and show how they can be reconciled with the proposal of Sander et al. (2006). The chemical predictions of both approaches are consistent. Although an interpretation solely based on the alkalinity change in the brine does not support the conclusion of Sander et al. (2006), we show that the bromide/alkalinity ratio (which increases during the cooling of the brine) can be used as an indicator of the potential for triggering bromine explosions.
Hydroxyl radical reactivity at the air-ice interface
Authors: Kahan T. F. , Zhao R. , Donaldson D. J.
Published: April 2010

Abstract. Hydroxyl radicals are important oxidants in the atmosphere and in natural waters. They are also expected to be important in snow and ice, but their reactivity has not been widely studied in frozen aqueous solution. We have developed a spectroscopic probe to monitor the formation and reactions of hydroxyl radicals in situ. Hydroxyl radicals are produced in aqueous solution via the photolysis of nitrite, nitrate, and hydrogen peroxide, and react rapidly with benzene to form phenol. Similar phenol formation rates were observed in aqueous solution and bulk ice. However, no reaction was observed at the air-ice interface, or when bulk ice samples were crushed prior to photolysis to increase their surface area. We also monitored the heterogeneous reaction between benzene present at air-water and air-ice interfaces with gas-phase OH produced from HONO photolysis. Rapid phenol formation was observed on water surfaces, but no reaction was observed at the surface of ice. Under the same conditions, we observed rapid loss of the polycyclic aromatic hydrocarbon (PAH) anthracene at the air-water interface, but no loss was observed at the air-ice interface. Our results suggest that the reactivity of hydroxyl radicals toward aromatic organics is similar in bulk ice samples and in aqueous solution, but is significantly suppressed in the quasi-liquid layer (QLL) that exists at the air-ice interface.
Circumpolar measurements of speciated mercury, ozone and carbon monoxide in the boundary layer of the Arctic Ocean
Authors: Sommar J. , Andersson M. E. , Jacobi H.-W.
Published: April 2010

Abstract. Using the Swedish icebreaker Oden as a platform, continuous measurements of airborne mercury (gaseous elemental mercury (Hg0), divalent mercury HgII(g) (acronym RGM) and mercury attached to particles (PHg)) and some long-lived trace gases (carbon monoxide CO and ozone O3) were performed over the North Atlantic and the Arctic Ocean. The measurements were performed for nearly three months (July–September, 2005) during the Beringia 2005 expedition (from Göteborg, Sweden via the proper Northwest Passage to the Beringia region Alaska – Chukchi Penninsula – Wrangel Island and in-turn via a north-polar transect to Longyearbyen, Spitsbergen). The Beringia 2005 expedition was the first time that these species have been measured during summer over the Arctic Ocean going from 60° to 90° N. During the North Atlantic transect, concentration levels of Hg0, CO and O3 were measured comparable to typical levels for the ambient mid-hemispheric average. However, a rapid increase of Hg0 in air and surface water was observed when entering the ice-covered waters of the Canadian Arctic archipelago. Large parts of the measured waters were supersaturated with respect to Hg0, reflecting a strong disequilibrium. Heading through the sea ice of the Arctic Ocean, a fraction of the strong Hg0} pulse in the water was spilled with some time-delay into the air samples collected ~20 m a.s.l. Several episodes of elevated Hg0(g) were encountered along the sea ice route with higher mean concentration (1.81±0.43 ng m−3) compared to the marine boundary layer over ice-free oceanic waters (1.55±0.21 ng m−3). In addition, an overall majority of the variance in the temporal series of Hg0 concentrations was observed during July. Atmospheric boundary layer {O3} mixing ratios decreased when initially sailing northward. In the Arctic, an O3 minimum around 15–20 ppbv was observed during summer (July–August). Alongside the polar transect during the beginning of autumn, a steady trend of increasing O3 mixing ratios was measured returning to initial levels of the expedition (>30 ppbv). Ambient CO was fairly stable (84±12 ppbv) during the expedition. However, from the Beaufort Sea and moving onwards steadily increasing CO mixing ratios were observed (0.3 ppbv day−1). On a comparison with coeval archived CO and O3 data from the Arctic coastal strip monitoring sites Barrow and Alert, the observations from Oden indicate these species to be homogeneously distributed over the Arctic Ocean. Neither correlated low ozone and GEM events nor elevated concentrations of RGM and PHg were at any extent sampled, suggesting that atmospheric mercury deposition to the Arctic basin is low during the Polar summer and autumn. Elevated levels of Hg0 and CO were episodically observed in air along the Chukchi Peninsula indicating transport of regional pollution.
Total gaseous mercury depletion events observed at Cape Point during 2007–2008
Authors: Brunke E.-G. , Labuschagne C. , Ebinghaus R. , Kock H. H. , Slemr F.
Published: April 2010

Abstract. Gaseous mercury in the marine boundary layer has been measured with a 15 min temporal resolution at the Global Atmosphere Watch station Cape Point since March 2007. The most prominent features of the data until July 2008 are the frequent occurrences of pollution (PEs) and depletion events (DEs). Both types of events originate mostly within a short transport distance (up to about 100 km), which are embedded in air masses ranging from marine background to continental. The Hg/CO emission ratios observed during the PEs are within the range reported for biomass burning and industrial/urban emissions. The depletion of gaseous mercury during the DEs is almost quantitative in many cases and suggests a lifetime of elemental mercury as short as a few dozens of hours, which is in contrast to the commonly used estimate of approximately 1 year. The characteristics of the DE occurrence at Cape Point is neither similar to the halogen driven atmospheric mercury depletion events (AMDEs) observed in Polar Regions nor to the DEs reported for plumes of urban air. Additional measurements are necessary to reveal the chemical mechanism of the observed DEs and to assess its importance on larger scales.
What caused extreme ozone concentrations over Cotonou in December 2005?
Authors: Minga A. , Thouret V. , Saunois M. , Delon C. , Serça D. , Mari C. , Sauvage B. , Mariscal A. , Leriche M. , Cros B.
Published: April 2010

Abstract. This paper reports the first record of extreme ozone measurement in Africa. As part of the AMMA program, the ozone vertical profile recorded on 20 December over Cotonou presents exceptionally high ozone concentrations with up to 295 ppbv at 1 km altitude. Retroplumes from the Flexpart model show that the air masses sampled at 1 km over Cotonou on this day come from the burning area situated north-east of Cotonou and pass over Lagos, Nigeria, which is highly impacted by urban pollution. We used the Master Mechanism box model to simulate the chemical composition of the plume during its transit. We find that neither the biomass burning emissions of ozone precursors nor additional urban emissions from Lagos are high enough to simulate more than 120–150 ppbv of ozone. The only way to reach almost 300 ppb of ozone within a few hours is to feed the air mass with large amounts of reactive VOCs as those recorded in the vicinity of petrochemical area. Sensitivity tests show that 250–600 ppbv of VOCs combined with 35–80 ppb of NOx allow the ozone concentrations to be higher than 250 ppb. Nigeria is the first African country with gas extraction and petrochemical industries, and petrochemical explosions frequently happen in the vicinity of Lagos. The hypothesis of a petrochemical explosion in this area is the most likely scenario which explains the 295 ppbv ozone maximum measured over Cotonou, downwind of Lagos.
Arctic sea-ice extent and its effect on the absorbed (net) solar flux at the surface, based on ISCCP-D2 cloud data for 1983–2007
Authors: Matsoukas C. , Hatzianastassiou N. , Fotiadi A. , Pavlakis K. G. , Vardavas I.
Published: April 2010

Abstract. We estimate the effect of the Arctic sea ice on the absorbed (net) solar flux using a radiation transfer model. Ice and cloud input data to the model come from satellite observations, processed by the International Satellite Cloud Climatology Project (ISCCP) and span the period July 1983–June 2007. The sea-ice effect on the solar radiation fluctuates seasonally with the solar flux and decreases interannually in synchronisation with the decreasing sea-ice extent. A disappearance of the Arctic ice cap during the sunlit period of the year would radically reduce the local albedo and cause a 19.7 W m−2 increase in absorbed solar flux at the Arctic Ocean surface, or equivalently a 0.55 W m−2 increase on the planetary scale. In the clear-sky scenario these numbers increase to 34.9 and 0.97 W m−2, respectively. A meltdown only in September, with all other months unaffected, increases the Arctic annually averaged solar absorption by 0.32 W m−2. We examined the net solar flux trends for the Arctic Ocean and found that the areas absorbing the solar flux more rapidly are the North Chukchi and Kara Seas, Buffin and Hudson Bays, and Davis Strait. The sensitivity of the Arctic absorbed solar flux on sea-ice extent and cloud amount was assessed. Although sea ice and cloud affect jointly the solar flux, we found little evidence of strong non-linearities.
Saharan dust infrared optical depth and altitude retrieved from AIRS: a focus over North Atlantic – comparison to MODIS and CALIPSO
Authors: Peyridieu S. , Chédin A. , Tanré D. , Capelle V. , Pierangelo C. , Lamquin N. , Armante R.
Published: April 2010

Abstract. Monthly mean infrared (10 μm) dust layer aerosol optical depth (AOD) and mean altitude are simultaneously retrieved over the tropics (30° S–30° N) from six years of Atmospheric Infrared Sounder (AIRS) observations covering the period January 2003 to December 2008. The method developed relies on the construction of Look-up-Tables computed for a large selection of atmospheric situations and follows two main steps: first, determination of the observed atmospheric thermodynamic situation and, second, determination of the dust properties. A very good agreement is found between AIRS-retrieved AODs and visible optical depths from the Moderate resolution Imaging Spectroradiometer (MODIS/Aqua) during the main (summer) dust season, in particular for three regions of the tropical North Atlantic and one region of the north-western Indian Ocean. Outside this season, differences are mostly due to the sensitivity of MODIS to aerosol species other than dust and to the more specific sensitivity of AIRS to the dust coarse mode. AIRS-retrieved dust layer mean altitudes are compared to the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP/CALIPSO) aerosol mean layer altitude for the period June 2006 to December 2008. Results for a region of the north tropical Atlantic downwind of the Sahara show a good agreement between the two products (σ≈370 m). Differences observed in the peak-to-trough seasonal amplitude, smaller from AIRS, are principally attributed to the large difference in spatial sampling of the two instruments. They also come from the intrinsic limit in sensitivity of the passive infrared sounders at low altitudes. These results however demonstrate the capability of high resolution infrared sounders to measure not only dust aerosol AOD but also the mean dust layer altitude.
CCN predictions using simplified assumptions of organic aerosol composition and mixing state: a synthesis from six different locations
Authors: Ervens B. , Cubison M. J. , Andrews E. , Feingold G. , Ogren J. A. , Jimenez J. L. , Quinn P. K. , Bates T. S. , Wang J. , Zhang Q. , Coe H. , Flynn M. , Allan J. D.
Published: April 2010

Abstract. An accurate but simple quantification of the fraction of aerosol particles that can act as cloud condensation nuclei (CCN) is needed for implementation in large-scale models. Data on aerosol size distribution, chemical composition, and CCN concentration from six different locations have been analyzed to explore the extent to which simple assumptions of composition and mixing state of the organic fraction can reproduce measured CCN number concentrations. Fresher pollution aerosol as encountered in Riverside, CA, and the ship channel in Houston, TX, cannot be represented without knowledge of more complex (size-resolved) composition. For aerosol that has experienced processing (Mexico City, Holme Moss (UK), Point Reyes (CA), and Chebogue Point (Canada)), CCN can be predicted within a factor of two assuming either externally or internally mixed soluble organics although these simplified compositions/mixing states might not represent the actual properties of ambient aerosol populations. Under typical conditions, a factor of two uncertainty in CCN concentration translates to an uncertainty of ~15% in cloud drop concentration, which might be adequate for large-scale models given the much larger uncertainty in cloudiness.
The net climate impact of coal-fired power plant emissions
Authors: Shindell D. T. , Faluvegi G.
Published: April 2010

Abstract. Coal-fired power plants influence climate via both the emissions of long-lived carbon dioxide (CO2) and short-lived ozone and aerosol precursors. For steadily increasing emissions without substantial pollution controls, we find that the net global mean climate forcing ranges from near zero to a substantial negative value, depending on the magnitude of aerosol indirect effects, due to aerosol masking of the effects of CO2. Imposition of pollution controls on sulfur dioxide and nitrogen oxides leads to a rapid realization of the full positive forcing from CO2, however. The long-term forcing from stable (constant) emissions is positive regardless of pollution controls, with larger values in the case of pollutant controls. The results imply that historical emissions from coal-fired power plants until ~1970, including roughly 1/3 of total anthropogenic carbon dioxide emissions, likely contributed little net global mean climate forcing during that period. Those emissions likely led to weak cooling at Northern Hemisphere mid-latitudes and warming in the Southern Hemisphere, however. Subsequent imposition of pollution controls and the switch to low-sulfur coal in some areas kept global SO2 emissions roughly level from 1970 to 2000. Hence during that period, RF due to emissions during those decades and CO2 emitted previously was strongly positive and likely contributed to rapid global and regional warming. Most recently, construction of coal-fired power plants in China and India has been increasing rapidly with minimal application of pollution controls. Continuation of high-growth rates for another 30 years would lead to near zero to negative global mean climate forcing in the absence of expanded pollution controls, but severely degraded air quality. However, following the Western pattern of high coal usage followed by imposition of pollution controls could lead to accelerated global warming in the future.
The spatial distribution of the reactive iodine species IO from simultaneous active and passive DOAS observations
Authors: Seitz K. , Buxmann J. , Pöhler D. , Sommer T. , Tschritter J. , O'Dowd C. , Platt U.
Published: April 2010

Abstract. We present investigations of the reactive iodine species (RIS) IO, OIO and I2 in a coastal region from a field campaign simultaneously employing active long path differential optical absorption spectroscopy (LP-DOAS) as well as passive multi-axis differential optical absorption spectroscopy (MAX-DOAS). The campaign took place at the Martin Ryan Institute (MRI) in Carna, County Galway at the Irish West Coast about 6 km south-east of the atmospheric research station Mace Head in summer 2007. In order to study the horizontal distribution of the trace gases of interest, we established two almost parallel active LP-DOAS light paths, the shorter of 1034 m length just crossing the intertidal area, whereas the longer one of 3946 m length also crossed open water during periods of low tide. In addition we operated two passive Mini-MAX-DOAS instruments with the same viewing direction. While neither OIO nor I2 could be unambiguously identified with any of the instruments, IO could be detected with active as well as passive DOAS. The IO column densities seen at both active LP-DOAS light paths are almost the same. Thus it can be concluded that coastal IO is almost exclusively located in the intertidal area, where we detected mixing ratios of up to 35±7.7 ppt (equivalent to pmol/mol). Nucleation events with particle concentrations of 106 cm−3 particles were observed each day correlating with high IO mixing ratios. Therefore we feel that our detected IO concentrations confirm the results of model studies, which state that in order to explain such particle bursts, IO mixing ratios of 50 to 100 ppt in so called "hot-spots" are required.
OMI observations of the anomalous 2008 Southern Hemisphere biomass burning season
Authors: Torres O. , Chen Z. , Jethva H. , Ahn C. , Freitas S. R. , Bhartia P. K.
Published: April 2010

Abstract. The 2008 season of biomass burning in the Southern Hemisphere was marked by a significant reduction in the number of fires in South America and, therefore, a large drop of the atmospheric load of carbonaceous aerosols over the subcontinent, relative to previous years, was registered by the Ozone Monitoring Instrument onboard the Aura satellite. In contrast, the 2008 copious aerosol production by fires in Central and Southern Africa generated an unusually large, synoptic scale aerosol layer that blanketed most of the tropical Southern Atlantic Ocean (0°–25° S) in August and September. Satellite observations on fire statistics and precipitation were analyzed to understand the anomalous Southern Hemisphere fire season. The fire reduction in South America was confined to Brazil that experienced a 62% reduction in fire activity in relation to 2007, and it was the result of factors other than meteorological reasons. The large spatial extent of the South Atlantic smoke layer seem to have been the result of unusually high free troposphere easterly winds that efficiently mobilized particulate matter thousands of kilometers away from the African source areas.
Validation of the GRAPE single view aerosol retrieval for ATSR-2 and insights into the long term global AOD trend
Authors: Thomas G. E. , Poulsen C. A. , Siddans R. , Sayer A. M. , Carboni E. , Marsh S. H. , Dean S. M. , Grainger R. G. , Lawrence B. N.
Published: April 2010

Abstract. The Global Retrieval of ATSR Cloud Parameters and Evaluation (GRAPE) project has produced a global data-set of cloud and aerosol properties from the Along Track Scanning Radiometer-2 (ATSR-2) instrument, covering the time period 1995–2001. This paper presents the validation of aerosol optical depths (AODs) from this product against AERONET sun-photometer measurements, as well as a comparison to the Advanced Very High Resolution Radiometer (AVHRR) optical depth product produced by the Global Aerosol Climatology Project (GACP). The GRAPE AOD over ocean is found to be in good agreement with AERONET measurements, with a correlation of 0.79 and a best-fit slope of 1.0±0.1, but with a positive bias of 0.08±0.04. Although the GRAPE and GACP datasets show reasonable agreement, there are significant differences. These discrepancies are explored, and suggest that the downward trend in AOD reported by GACP may arise from changes in sampling due to the orbital drift of the AVHRR instruments.
On the seasonal dependence of tropical lower-stratospheric temperature trends
Authors: Fu Q. , Solomon S. , Lin P.
Published: April 2010

Abstract. This study examines the seasonality of tropical lower-stratospheric temperature trends using the Microwave Sounding Unit lower-stratospheric channel (T4) for 1979–2007. We present evidence that this seasonality is a response to changes in the Brewer–Dobson circulation (BDC) driven by extratropical wave forcing. We show how the tropical T4 trend can be used as an indicator of the change in the BDC, and find that the BDC is strengthening for 1979–2007 in June–November related to the Southern Hemisphere (SH) and in December–February to the Northern Hemisphere (NH). In marked contrast, we find that the BDC is weakening in March–May, apparently because of a weakening of its northern cell. The novel observational evidence on the seasonal dependence of the BDC trends presented in this study has important implications for the understanding of climate change in the stratosphere as well as testing climate model simulations.
Particle formation in the Arctic free troposphere during the ASTAR 2004 campaign: a case study on the influence of vertical motion on the binary homogeneous nucleation of H2SO4/H2O
Authors: Khosrawi F. , Ström J. , Minikin A. , Krejci R.
Published: April 2010

Abstract. During the ASTAR (Arctic Study of Tropospheric Aerosol and Radiation) campaign nucleation mode particles (4 to 13 nm) were quite frequently observed at altitudes below 4000 m. However, in the upper free troposphere, nucleation mode particles were only observed once, namely during the flight on 24 May 2004 (7000 m). To investigate if vertical motion are the reason for this difference that on one particular day nucleation mode particles were observed but not on the other days we employ a microphysical box model. The box model simulations were performed along air parcel trajectories calculated 6-d backwards based on European Center for Medium-Range Weather Forecasts (ECMWF) meteorological analyses using state parameters such as pressure and temperature in combination with additional parameters such as vertical stability. Box model simulations were performed for the 24 May where nucleation mode particles were observed (nucleation event) as well as for the day with measurements before and after (22 and 26 May) which are representative for no nucleation (none nucleation event). A nucleation burst was simulated along all trajectories, however, in the majority of the simulations the nucleation rate was either too low or too high so that no nucleation mode particles were left at the time were the measurements were performed. Further, the simulation results could be divided into three cases. Thereby, we found that for case 1 the temperature was the only driving mechanism while for case 2 and 3 vertical motion have influenced the formation of new particles. The reason why nucleation mode particles were observed on 24 May, but not on the other day, can be explained by the conditions under which particle formation occurred. On 24 May the particle formation was caused by a slow updraft, while on the other two days the particle formation was caused by a fast updraft.
Influences of in-cloud aerosol scavenging parameterizations on aerosol concentrations and wet deposition in ECHAM5-HAM
Authors: Croft B. , Lohmann U. , Martin R. V. , Stier P. , Wurzler S. , Feichter J. , Hoose C. , Heikkilä U. , van Donkelaar A. , Ferrachat S.
Published: April 2010

Abstract. A diagnostic nucleation scavenging scheme, which determines stratiform cloud scavenging ratios for both aerosol mass and number distributions, based on cloud droplet, and ice crystal number concentrations, is introduced into the ECHAM5-HAM global climate model. This is coupled with a size-dependent in-cloud impaction scavenging parameterization for both cloud droplet-aerosol, and ice crystal-aerosol collisions. Sensitivity studies are presented, which compare aerosol concentrations, and deposition between a variety of in-cloud scavenging approaches, including prescribed fractions, several diagnostic schemes, and a prognostic aerosol cloud processing treatment that passes aerosol in-droplet and in-ice crystal concentrations between model time steps. For one sensitivity study, assuming 100% of the in-cloud aerosol is scavenged into the cloud droplets and ice crystals, the annual global mean accumulation mode number burden is decreased by 65%, relative to a simulation with prognostic aerosol cloud processing. Diagnosing separate nucleation scavenging ratios for aerosol number and mass distributions, as opposed to equating the aerosol mass scavenging to the number scavenging ratios, reduces the annual global mean sulfate burden by near to 10%. The annual global mean sea salt burden is 30% lower for the diagnostic approach, which does not carry aerosol in-droplet and in-crystal concentrations between model time-steps as compared to the prognostic scheme. Implementation of in-cloud impaction scavenging reduced the annual, global mean black carbon burden by 30% for the prognostic aerosol cloud processing scheme. Better agreement with observations of black carbon profiles from aircraft (changes near to one order of magnitude for mixed phase clouds), 210Pb surface layer concentrations and wet deposition, and the geographic distribution of aerosol optical depth are found for the new diagnostic scavenging as compared to prescribed ratio scavenging scheme of the standard ECHAM5-HAM.
Impact of open crop residual burning on air quality over Central Eastern China during the Mount Tai Experiment 2006 (MTX2006)
Authors: Yamaji K. , Li J. , Uno I. , Kanaya Y. , Komazaki Y. , Pochanart P. , Liu Y. , Takigawa M. , Ohara T. , Yan X. , Wang Z. , Akimoto H.
Published: April 2010

Abstract. The impact of open crop residual burning on O3, CO, Black Carbons (BC), and Organic Carbons (OC) concentrations over Central Eastern China (CEC) during the Mount Tai Experiment 2006 (MTX2006) was evaluated using the regional chemical transport model, the Models-3 Community Multiscale Air Quality Modeling System (CMAQ). To investigate these pollutants during the MTX2006 period in June, daily gridded emissions from open crop residual burning were developed based on a bottom-up methodology and using land cover and hotspot information from satellites. This model system which involves daily emissions from open biomass burning, captured monthly-averaged observed concentrations and day-to-day variations in the patterns of O3, CO, BC, and OC with good correlation coefficients between models and observations, ranging from 0.54 to 0.66. These results were significantly improved from those using annual emissions. For monthly-averaged O3, the simulated concentration of 81.5 ppbv was close to the observed concentration (82.5 ppbv). The period of MTX2006 was roughly divided into two parts: 1) polluted days with heavy open crop residual burning in the first half of June; 2) cleaner days with negligible field burning in the latter half of June. Additionally, the first half of June was defined by two high pollution episodes during 5–7 and 12–13 June, and a relatively cleaner episode during 8–10 June between these two high pollution episodes. In the first polluted episode, this model captured high O3, CO, BC, and OC concentrations at the summit of Mount Tai which were affected by open crop residual burning in the south of CEC and northward transport. For this episode, the impacts from open crop residual burning were 12% for O3, 35% for CO, 56% for BC, and 80% for OC over CEC. The daily emissions from open crop residual burning were an essential factor to evaluate the pollutants during the MTX2006. These emissions have a large impact not only on primary pollutants but also on secondly pollutions, such as O3, in the first half of June over northeastern Asia. On the other hand, this model did not capture the second polluted episode and underestimated observed CO and BC. Improvements of both anthropogenic and open burning emissions and CO inflow from model boundary are necessary to improve both anthropogenic and open burning emissions and CO inflow to evaluate the pollutants using this model.
Toward a real-time measurement of atmospheric mercury concentrations using cavity ring-down spectroscopy
Authors: Faïn X. , Moosmüller H. , Obrist D.
Published: April 2010

Abstract. A new sensor based on cavity ring-down spectroscopy (CRDS) has been developed for the measurement of gaseous elemental mercury (Hg0) mass concentration with sub-ng m−3 detection limit and high temporal resolution. Cavity ring-down spectroscopy is a direct absorption technique that utilizes path lengths of up to multiple kilometers in a compact absorption cell and has a significantly higher sensitivity than conventional absorption spectroscopy. Our prototype uses a frequency-doubled, tuneable dye laser emitting pulses at ~253.65 nm with a pulse repetition frequency of 50 Hz. The dye laser incorporates a unique piezo element attached to its tuning grating allowing it to tune the laser on and off the Hg0 absorption line on a pulse to pulse basis to facilitate differential absorption measurements. Hg0 absorption measurements with this CRDS laboratory prototype are highly linearly related to Hg0 concentrations determined by a Tekran 2537B analyzer over a Hg0 concentration range of four orders of magnitude, from 0.2 ng m−3 to 573 ng m−3 implying excellent linearity of both instruments. The current CRDS instrument has a~sensitivity of 0.10 ng m−3 at 10 s time resolution. This tool opens new prospects for the study of Hg0 because of its high temporal resolution and reduced limited sample volume requirements (<0.5 l of sample air). Future applications may include ambient Hg0 flux measurements with eddy covariance techniques, which require measurements of Hg0 concentrations with sub-ng m−3 sensitivity and sub-second time resolution.
Sulfur cycle and sulfate radiative forcing simulated from a coupled global climate-chemistry model
Authors: Tsai I.-C. , Chen J.-P. , Lin P.-Y. , Wang W.-C. , Isaksen I. S. A.
Published: April 2010

Abstract. The sulfur cycle and radiative effects of sulfate aerosol on climate are studied with a Global tropospheric Climate-Chemistry Model in which chemistry, radiation and dynamics are fully coupled. Production and removal mechanisms of sulfate are analyzed for the conditions of natural and anthropogenic sulfur emissions. Results show that the 1985 anthropogenic emission doubled the global SO2 and sulfate loadings from its natural value of 0.15 and 0.27 Tg S, respectively. Under natural conditions, the fraction of sulfate produced in-cloud is 87%, and the lifetime of SO2 and sulfate are 1.8 and 4.0 days, respectively; whereas with anthropogenic emissions, changes in in-cloud sulfate production are small, while SO2 and sulfate lifetimes are significant reduced (1.0 and 2.4 days, respectively). The doubling of sulfate results in a direct radiative forcing of −0.32 and −0.14 W m−2 under clear-sky and all-sky conditions, respectively, and a significant first indirect forcing of −1.69 W m−2. The first indirect forcing is sensitive to the relationship between aerosol concentration and cloud droplet number concentration. Two aspects of chemistry-climate interaction are addressed. Firstly, the coupling effects lead to 10% and 2% decreases in sulfate loading, respectively, for the cases of natural and anthropogenic added sulfur emissions. Secondly, only the indirect effect of sulfate aerosols yields significantly stronger signals in changes of near surface temperature and sulfate loading than changes due to intrinsic climate variability, while other responses to the indirect effect and all responses to the direct effect are weak.
Regional-scale geostatistical inverse modeling of North American CO2 fluxes: a synthetic data study
Authors: Gourdji S. M. , Hirsch A. I. , Mueller K. L. , Andrews A. E. , Michalak A. M.
Published: April 2010

Abstract. Using synthetic continuous CO2 measurements from the nine sampling locations operational across North America in 2004, this paper investigates the optimal setup for, and constraint on fluxes achieved by, a regional geostatistical atmospheric CO2 inversion over the continent. The geostatistical framework does not require explicit prior flux estimates, nor any other process-based information, and is therefore particularly well suited for investigating the information content of the atmospheric CO2 measurements from a limited network. The atmospheric data are first used with the Restricted Maximum Likelihood (RML) algorithm to infer the model-data mismatch and a priori spatial covariance parameters applied in the inversion. The implemented RML algorithm is found to infer robust spatial covariance parameters from the atmospheric data, as compared to the "true" solution, for cases where the flux and measurement timescales match, while model-data mismatch variances are inferred correctly across all examined cases. A series of analyses is also performed investigating the impact of the temporal scale of concentration measurements and fluxes on inversion results. Inversions using measurement data at sub-daily resolution are found to yield fluxes with a lower Root Mean Square Error (RMSE) relative to inversions using coarser-scale observations, whereas the flux resolution appears to have a lesser impact on the inversion quality. In addition, night-time data for the tall and marine boundary layer towers are found to help constrain fluxes across the continent, although they can potentially bias near-field fluxes. These general conclusions are likely to also be applicable to inversions using a synthesis Bayesian inversion approach. Overall, despite the relatively sparse and unevenly distributed network of nine towers across the North American continent, a geostatistical inversion using an optimal setup and relying solely on the atmospheric data constraint is found to estimate the North American sink for June 2004 to within approximately 10%.