bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurements of aerosol absorption and scattering in the Mexico City Metropolitan Area during the MILAGRO field campaign: a comparison of results from the T0 and T1 sites
Authors: Marley N. A. , Gaffney J. S. , Castro T. , Salcido A. , Frederick J.
Published: April 2010

Abstract. Measurements of aerosol absorption and scattering were obtained in Mexico City during the MILAGRO (Megacity Initiative: Local and Global Research Observations) field campaign in March 2006. A comparison of aerosol absorption and scattering was obtained in Mexico City at site T0 located in the northern part of Mexico City at the Instituto Mexicano del Petróleo Laboratories and at site T1 located at the Universidad Tecnológica de Tecamac, 18 miles northwest of T0. Hourly averages of aerosol absorption were similar at both sites, ranging from 6–93 Mm−1 with an average of 31 Mm−1 at T0; and from 2–104 Mm−1 with an average of 19 Mm−1 at T1. Aerosol scattering at T0 ranged from 16–344 Mm−1 with an average of 105 Mm−1; while the scattering values at T1 were lower than T0 ranging from 2–136 with an average of 53 Mm−1. Aerosol single scattering albedos (SSAs) were determined at both sites using these data. SSAs at T1 ranged from 0.44–0.90 with an average 0.75 as compared to hose at T0, range 0.51–0.93 with an average of 0.77. Broadband UV-B intensity was found to be higher at site T0, with an average of 64 μW/cm2 at solar noon, than at site T1, which had an average of 54 μW/cm2 at solar noon. Comparisons of clear-sky modeled UV-B intensities with the simultaneous UV-B measurements obtained at site T0 and at site T1 for cloudless days indicate a larger diffuse radiation field at site T0 than at site T1. The determination of aerosol scattering Ångstrom coefficient at T0 suggests the larger diffuse radiation is due to the predominance of submicron aerosols at T0 with aerosol scattering of UV-B radiation peaked in the forward direction, leading to the enhancement observed at ground level.
Classifying previously undefined days from eleven years of aerosol-particle-size distribution data from the SMEAR II station, Hyytiälä, Finland
Authors: Buenrostro Mazon S. , Riipinen I. , Schultz D. M. , Valtanen M. , Dal Maso M. , Sogacheva L. , Junninen H. , Nieminen T. , Kerminen V.-M. , Kulmala M.
Published: April 2010

Abstract. Studies of secondary aerosol-particle formation depend on identifying days in which new particle formation occurs and, by comparing them to days with no signs of particle formation, identifying the conditions favourable for formation. Continuous aerosol size distribution data has been collected at the SMEAR II station in a boreal forest in Hyytiälä, Finland, since 1996, making it the longest time series of aerosol size distributions available worldwide. In previous studies, the data have been classified as particle-formation event, nonevent, and undefined days, with almost 40% of the dataset classified as undefined. In the present study, eleven years (1996–2006) of undefined days (1628 days) were reanalyzed and subdivided into three new classes: failed events (37% of all previously undefined days), ultrafine-mode concentration peaks (34%), and pollution-related concentration peaks (19%). Unclassified days (10%) comprised the rest of the previously undefined days. The failed events were further subdivided into tail events (21%), where a tail of a formation event presumed to be advected to Hyytiälä from elsewhere, and quasi events (16%) where new particles appeared at sizes 3–10 nm, but showed unclear growth, the mode persisted for less than an hour, or both. The ultrafine concentration peaks days were further subdivided into nucleation-mode peaks (24%) and Aitken-mode peaks (10%), depending on the size range where the particles occurred. The mean annual distribution of the failed events has a maximum during summer, whereas the two peak classes have maxima during winter. The summer minimum previously found in the seasonal distribution of event days partially offsets a summer maximum in failed-event days. Daily-mean relative humidity and condensation sink values are useful in discriminating the new classes from each other. Specifically, event days had low values of relative humidity and condensation sink relative to nonevent days. Failed-event days possessed intermediate condensation sink and relative humidity values, whereas both ultrafine-mode peaks and, to a greater extent, pollution-related peaks had high values of both, similar to nonevent days. Using 96-h back trajectories, particle-size concentrations were plotted as a function of time the trajectory spent over land. Increases in particle size and number concentration during failed-event days were similar to that during the later stages of event days, whereas the particle size and number concentration for both nonevent and peaks classes did not increase as fast as for event and failed events days.
Secondary organic aerosol formation from primary aliphatic amines with NO3 radical
Authors: Malloy Q. G. J. ,   Li Qi, Warren B. , Cocker D. R. , Erupe M. E. , Silva P. J.
Published: April 2010

Abstract. Primary aliphatic amines are an important class of nitrogen containing compounds found to be emitted from automobiles, waste treatment facilities and agricultural animal operations. A series of experiments conducted at the UC-Riverside/CE-CERT Environmental Chamber is presented in which oxidation of methylamine, ethylamine, propylamine, and butylamine with NO3 has been investigated. Very little aerosol formation is observed in the presence of O3 only. However, after addition of NO, and by extension NO3, large yields of aerosol mass loadings (~44% for butylamine) are seen. Aerosol generated was determined to be organic in nature due to the small fraction of NO and NO2 in the total signal (<17% for all amines tested) as detected by an aerosol mass spectrometer (AMS). We propose a reaction mechanism between carbonyl containing species and the parent amine leading to formation of particulate imine products. These findings can have significant impacts on rural communities and lead to elevated nighttime PM loadings, when significant levels on NO3 exist.
Monthly-averaged anthropogenic aerosol direct radiative forcing over the Mediterranean from AERONET derived aerosol properties
Authors: Bergamo A. , Tafuro A. M. , Kinne S. , De Tomasi F. , Perrone M. R.
Published: April 2010

Abstract. The all-sky direct radiative effect by anthropogenic aerosol (DREa) is calculated in the solar (0.3–4 μm) and infrared (4–200 μm) spectral ranges for six Mediterranean sites. The sites are differently affected by pollution and together reflect typical aerosol impacts that are expected over land sites of the central Mediterranean basin. Central to the simulations are aerosol optical properties from AERONET sun-/sky-photometer statistics for the year 2003. A discussion on the variability of the overall (natural+anthropogenic) aerosol properties with site location is provided. Supplementary data include MODIS satellite sensor based solar surface albedos, ISCCP products for high- mid- and low cloud cover and estimates for the anthropogenic aerosol fraction from global modelling. Since anthropogenic aerosol particles are considered to be smaller than 1 μm in size, mainly the solar radiation transfer is affected with impacts only during sun-light hours. At all sites the (daily average) solar DREa is negative all year round at the top of the atmosphere (ToA). Hence, anthropogenic particles produce over land sites of the central Mediterranean a significant cooling effect. Monthly DREa values vary from site to site and are seasonal dependent as a consequence of the seasonal dependence of available sun-light and microphysical aerosol properties. At the ToA the monthly average DREa is −(4±1) W m−2 during spring-summer (SS, April–September) and −(2±1) W m−2 during autumn-winter (AW, October–March) at the polluted sites. In contrast, it varies between −(3±1) W m−2 and −(1±1) W m−2 on SS and AW, respectively at the less polluted site. Due to atmospheric absorption the DREa at the surface is larger than at the ToA. At the surface the monthly average DREa varies between the most and the least polluted site between −(7±1) W m−2 and −(4±1) W m−2 during SS, and between −(4±3) W m−2 and −(1±1) W m−2 during AW. The DREa at infrared wavelengths is positive but negligible, especially at the ToA (<0.3 W m−2. DREa monthly-means referring to all sites have been averaged to evaluate the yearly-mean value of the DREa. The ToA- and sfc-DREa yearly-mean value is −(3±2) and −(5±3) W m−2, respectively at solar wavelengths. Last data further more reveal that the radiative energy-balance of the Central Mediterranean land sites is quite affected by anthropogenic particles.
Radiative forcing of the direct aerosol effect using a multi-observation approach
Authors: Myhre G. , Berglen T. F. , Johnsrud M. , Hoyle C. R. , Berntsen T. K. , Christopher S. A. , Fahey D. W. , Isaksen I. S. A. , Jones T. A. , Kahn R. A. , Loeb N. , Quinn P. , Remer L. , Schwarz J. P. , Yttri K. E.
Published: April 2010

Abstract. A high-resolution global aerosol model (Oslo CTM2) driven by meteorological data and allowing a comparison with a variety of aerosol observations is used to simulate radiative forcing (RF) of the direct aerosol effect. The model simulates all main aerosol components, including several secondary components such as nitrate and secondary organic carbon. The model reproduces the main chemical composition and size features observed during large aerosol campaigns. Although the chemical composition compares best with ground-based measurement over land for modelled sulphate, no systematic differences are found for other compounds. The modelled aerosol optical depth (AOD) is compared to remote sensed data from AERONET ground and MODIS and MISR satellite retrievals. To gain confidence in the aerosol modelling, we have tested its ability to reproduce daily variability in the aerosol content, and this is performing well in many regions; however, we also identified some locations where model improvements are needed. The annual mean regional pattern of AOD from the aerosol model is broadly similar to the AERONET and the satellite retrievals (mostly within 10–20%). We notice a significant improvement from MODIS Collection 4 to Collection 5 compared to AERONET data. Satellite derived estimates of aerosol radiative effect over ocean for clear sky conditions differs significantly on regional scales (almost up to a factor two), but also in the global mean. The Oslo CTM2 has an aerosol radiative effect close to the mean of the satellite derived estimates. We derive a radiative forcing (RF) of the direct aerosol effect of −0.35 Wm−2 in our base case. Implementation of a simple approach to consider internal black carbon (BC) mixture results in a total RF of −0.28 Wm−2. Our results highlight the importance of carbonaceous particles, producing stronger individual RF than considered in the recent IPCC estimate; however, net RF is less different. A significant RF from secondary organic aerosols (SOA) is estimated (close to −0.1 Wm−2). The SOA also contributes to a strong domination of secondary aerosol species for the aerosol composition over land. A combination of sensitivity simulations and model evaluation show that the RF is rather robust and unlikely to be much stronger than in our best estimate.
Assessing temporal clear-sky errors in assimilation of satellite CO2 retrievals using a global transport model
Authors: Corbin K. D. , Denning A. S. , Parazoo N. C.
Published: April 2010

Abstract. The Orbiting Carbon Observatory (OCO) and the Greenhouse gases Observing SATellite (GOSAT) will make global observations of the total column dry-air mole fraction of atmospheric CO2 (XCO2) starting in 2008. Although satellites have global coverage, XCO2 retrieval will be made only a few times each month over a given location and will only be sampled in clear conditions. Modelers will use XCO2 in atmospheric inversions to estimate carbon sources and sinks; however, if satellite measurements are used to represent temporal averages, modelers may incur temporal sampling errors. We investigate these errors using a global transport model. Temporal sampling errors vary with time and location, exhibit spatially coherent patterns, and are greatest over land and during summer. These errors often exceed 1 ppm and must be addressed in a data assimilation system by correct simulation of synoptic CO2 variations associated with cloud systems.
Eclipse effects on field crops and marine zooplankton: the 29 March 2006 Total Solar Eclipse
Authors: Economou G. , Christou E. D. , Giannakourou A. , Gerasopoulos E. , Georgopoulos D. , Kotoulas V. , Lyra D. , Tsakalis N. , Tziortzou M. , Vahamidis P. , Papathanassiou E. , Karamanos A.
Published: April 2010

Abstract. The effects in the biosphere from the Total Solar Eclipse of 29 March 2006 were investigated in field crops and marine zooplankton. Taking into account the decisive role of light on the photoenergetic and photoregulatory plant processes, measurements of photosynthesis and stomatal behaviour were conducted on seven important field-grown cereal and leguminous crops. A drop in photosynthetic rates, by more than a factor of 5 in some cases, was observed, and the minimum values of photosynthetic rates ranged between 3.13 and 10.13 μmol CO2 m−2 s−1. However, since solar irradiance attenuation has not at the same time induced stomatal closure thus not blocking CO2 uptake by plants, it is probably other endogenous factors that has been responsible for the observed fall in photosynthetic rates. Field studies addressing the migratory responses of marine zooplankton (micro-zooplankton (ciliates), and meso-zooplankton) due to the rapid changes in underwater light intensity were also performed. The light intensity attenuation was simulated with the use of accurate underwater radiative transfer modeling techniques. Ciliates, responded to the rapid decrease in light intensity during the eclipse adopting night-time behaviour. From the meso-zooplankton assemblage, various vertical migratory behaviours were adopted by different species.
Traffic restrictions in Beijing during the Sino-African Summit 2006: aerosol size distribution and visibility compared to long-term in situ observations
Authors: Cheng Y. F. , Heintzenberg J. , Wehner B. , Wu Z. J. , Hu M. , Mao J. T.
Published: April 2010

Abstract. Based on the long-term in-situ observations of aerosol particle number size distributions and meteorological parameters, the traffic restriction measures during the Sino-African Summit (4–6 November 2006) in Beijing, China have been found to be remarkably efficient in reducing the number concentration of aerosol particles, in particular Aitken and accumulation mode particles, and in improving the visibility. The influence of traffic restriction in Beijing on the particle concentrations differed for different particle sizes. More significant effects on fine particles with diameters ranging from 40 to 800 nm have been found. Based on statistical analysis of long-term observation, under comparable weather conditions, the source strength of the particles in Aitken and accumulation modes seemingly was reduced by 40–60% when the traffic restrictions were in place. It may be mainly due to the reduction of secondary particle formation. Our size-dependent aerosol data also indicate that measures led to reductions in particulate air pollution in the optically most important diameter range, whereas further vehicle control measures may lead to an increase in ultrafine particle formation from the gas phase if the condensational sink further decreased. Assuming that there were no traffic restrictions and with normal levels of the vehicle emissions, the visibilities during the Summit would have been lower by about 50%. The importance of the restrictions is highest when the wind speed is lower than 3 m s−1. The fact that over 95% cases with visual range lower than 5 km during 2004 to 2007 occurred when the local wind speed was lower than 3 m s−1 may suggest that future traffic restrictions will lead to significant improvements of visibility in Beijing.
Clear sky UV simulations in the 21st century based on ozone and temperature projections from Chemistry-Climate Models
Authors: Tourpali K. , Bais A. F. , Kazantzidis A. , Zerefos C. S. , Akiyoshi H. , Austin J. , Brühl C. , Butchart N. , Chipperfield M. P. , Dameris M. , Deushi M. , Eyring V. , Giorgetta M. A. , Kinnison D. E. , Mancini E. , Marsh D. R. , Nagashima T. , Pitari G. , Plummer D. A. , Rozanov E. , Shibata K. , Tian W.
Published: April 2010

Abstract. We have used total ozone columns and vertical profiles of ozone and temperature from 11 coupled Chemistry-Climate Models (CCMs) to project future solar ultraviolet radiation levels at the surface in the 21st century. The CCM simulations are used as input to a radiative transfer model for the simulation of the corresponding future UV irradiance levels under cloud free conditions, presented here as time series of monthly erythemal irradiance received at the surface during local noon covering the period 1960 to 2100. Starting from the first decade of the 21st century, the surface erythemal irradiance decreases globally as a result of the projected ozone recovery, at rates which are larger in the first half of the 21st century, compared to the period up to 2100. The magnitude of these decreases varies with latitude and is more pronounced at areas where ozone has been depleted most considerably after 1980. Over midlatitudes surface erythemal irradiance decreases between 5 and 15% by 2100 relative to 2000, while at the southern high latitudes these changes are twice as much. Climate change may affect future cloudiness, surface reflectivity and tropospheric aerosol loading, the effects of which are not included in this study. Therefore, the actual changes in future UV radiation are likely to change accordingly in the areas affected.
Simulation of atmospheric mercury depletion events (AMDEs) during polar springtime using the MECCA box model
Authors: Xie Z.-Q. , Sander R. , Pöschl U. , Slemr F.
Published: April 2010

Abstract. Atmospheric mercury depletion events (AMDEs) during polar springtime are closely correlated with bromine-catalyzed tropospheric ozone depletion events (ODEs). To study gas- and aqueous-phase reaction kinetics and speciation of mercury during AMDEs, we have included mercury chemistry into the box model MECCA (Module Efficiently Calculating the Chemistry of the Atmosphere), which enables dynamic simulation of bromine activation and ODEs. We found that the reaction of Hg with Br atoms dominates the loss of gaseous elemental mercury (GEM). To explain the experimentally observed synchronous destruction of Hg and O3, the reaction rate of Hg+BrO has to be much lower than that of Hg+Br. The synchronicity is best reproduced with rate coefficients at the lower limit of the literature values for both reactions, i.e. kHg+Br≈3×10-13 and kHg+BrO≤1×10-15cm3 mol-1 s-1, respectively. Throughout the simulated AMDEs, BrHgOBr was the most abundant reactive mercury species, both in the gas phase and in the aqueous phase. The aqueous phase concentrations of BrHgOBr, HgBr2, and HgCl2 were several orders of magnitude larger than that of Hg(SO3)2-2. Considering chlorine chemistry outside depletion events (i.e. without bromine activation), the concentration of total divalent mercury in sea-salt aerosol particles (mostly HgCl2) was much higher than in dilute aqueous droplets (mostly Hg(SO3)2-2), and did not exhibit a diurnal cycle (no correlation with HO2 radicals).
Cosmic rays, CCN and clouds – a reassessment using MODIS data
Authors: Kristjánsson J. E. , Stjern C. W. , Stordal F. , Fjǽraa A. M. , Myhre G. , Jónasson K.
Published: April 2010

Abstract. The response of clouds to sudden decreases in the flux of galactic cosmic rays (Forbush decrease events) has been investigated using cloud products from the space-borne MODIS instrument, which has been in operation since 2000. By focusing on pristine Southern Hemisphere ocean regions we examine areas which are particularly susceptible to changes in cloud condensation nuclei (CCN) concentrations, and where a cosmic ray signal should be easier to detect than elsewhere. While previous studies on the subject have mainly considered cloud cover, the high spatial and spectral resolution of MODIS allows for a more thorough study of microphysical parameters such as cloud droplet size, cloud water content and cloud optical depth, in addition to cloud cover. Averaging the results from the 13 Forbush decrease events that were considered, no systematic correlation was found between any of the four cloud parameters and galactic cosmic radiation, with a seemingly random distribution of positive and negative correlations. When only the three Forbush decrease events with the largest amplitude are studied, the correlations fit the hypothesis better, with 8 out of 12 correlations having the expected sign. Splitting the area of study into several sub-regions, one sub-region in the Atlantic Ocean showed statistically significant correlations compatible with a cosmic ray-induced enhancement of CCN and cloud droplet number concentrations. However, the lack of correlation in any of the other 5 sub-regions suggests that this may be a statistical co-incidence. Introducing a time lag of a few days for clouds to respond to the cosmic ray signal did not change the overall results. Singling out low clouds of intermediate optical depth with large susceptibility did not lead to higher correlations. In conclusion, no response to variations in cosmic rays associated with Forbush decrease events was found in marine low clouds in remote regions using MODIS data.
Methane plume over South Asia during the monsoon season: satellite observation and model simulation
Authors: Xiong X. , Houweling S. , Wei J. , Maddy E. , Sun F. , Barnet C.
Published: April 2010

Abstract. Satellite observations of methane (CH4) using the Atmospheric Infrared Sounder (AIRS) on the EOS/Aqua platform from 2003–2007 demonstrate a strong, plume-like enhancement of CH4 in the middle to upper troposphere over the South Asia during July, August and September, and its maximum occurs in early September. Simulations using the global tracer model version 3 (TM3) also show similar seasonal enhancement of CH4 in the same region. The model results also suggest that this enhancement is associated with transport process and local surface emissions, thus the observations to tropospheric CH4during the monsoon season may be used to constrain the models for a better estimation of Asian CH4 sources. Further comparisons between AIRS observations and the model simulations indicate a possible overestimate of CH4 emissions from rice paddies in Southeast Asia. Moreover, the observed tropospheric CH4 enhancement from AIRS provides evidence for the strong transport of atmospheric pollutants from the lower to the upper troposphere in Asia during the monsoon season, and the observed rapid disappearance of local CH4 maximum in September may provide valuable information for studying the dissipation of the Tibetan anticyclone and the withdrawal of monsoon.
Emission of sunscreen salicylic esters from desert vegetation and their contribution to aerosol formation
Authors: Matsunaga S. N. , Guenther A. B. , Potosnak M. J. , Apel E. C.
Published: April 2010

Abstract. Biogenic volatile organic compounds (BVOC) produced by plants are known to have an important role in atmospheric chemistry. However, our knowledge of the range of BVOCs produced by different plant processes is still expanding, and there remain poorly understood categories of BVOCs. In this study, emissions of a novel class of BVOC emissions were investigated in a desert region. Our study considered 8 species of common desert plants: blackbrush (Coleogyne ramosissima), desert willow (Chilopsis linearis), mesquite (Prosopis glandulosa), mondel pine (Pinus eldarica), pinyon pine (Pinus monophylla), cottonwood (Populus deltoides), saguaro cactus (Carnegiea gigantea) and yucca (Yucca baccata). The measurements focused on BVOCs with relatively high molecular weight (>C15) and/or an oxygenated functional group. Significantly high emission rates of two salicylic esters were found for blackbrush, desert willow and mesquite with emission rates of 1.4, 2.1 and 0.46 μgC dwg−1 h−1, respectively. The salicylic esters were identified as 2-ethylhexenyl salicylate (2-EHS) and 3,3,5-trimethylcyclohexenyl salicylate (homosalate) and are known as effective ultraviolet (UV) absorbers. We propose that the plants derive a protective benefit against UV radiation from the salicylic esters and that the emission process is driven by the physical evaporation of the salicylic esters due to the high ambient temperatures. In addition, the salicylic esters are predicted to be an effective precursor of secondary organic aerosol (SOA) because of their low vapor pressure due to a high number of carbon atoms (15 or 16) and the presence of three oxygen atoms. We estimated the contribution of the sunscreen esters themselves and their oxidation products on the SOA formation for the Las Vegas region using a BVOC emission model. The contribution was estimated to reach 90% of the biogenic SOA in the landscapes dominated by desert willow and mesquite and 25% in Las Vegas area.
The von Kármán constant retrieved from CASE-97 dataset using a variational method
Authors: Zhang Y. , Ma J. , Cao Z.
Published: April 2010

Abstract. A variational method is developed to retrieve the von Kármán constant κ from the CASES-97 dataset, collected near Wichita, Kansas, the United States from 6 April to 24 May 1997. In the variational method, a cost function is defined to measure the difference between observed and computed gradients of wind speed, air temperature and specific humidity. An optimal estimated von Kármán constant is obtained by minimizing the cost function through adjusting values of the von Kármán constant. Under neutral stratification, the variational analysis confirms the conventional value of κ(=0.40). For non-neutral stratification, however, κ varies with stability. The computational results show that the κ decreases monotonously from stable to unstable stratification. The variational calculated mean value of the von Kármán constant is 0.390 when the atmospheric stratification is taken into consideration. Relations between κ and surface momentum and heat flux are also examined.
The Tropical Tropopause Layer 1960–2100
Authors: Gettelman A. , Birner T. , Eyring V. , Akiyoshi H. , Plummer D. A. , Dameris M. , Bekki S. , Lefèvre F. , Lott F. , Brühl C. , Shibata K. , Rozanov E. , Mancini E. , Pitari G. , Struthers H. , Tian W. , Kinnison D. E.
Published: April 2010

Abstract. The representation of the Tropical Tropopause Layer in 13 different Chemistry Climate Models designed to represent the stratosphere is analyzed. Simulations for 1960–present and 1980–2100 are analyzed and compared to reanalysis model output. Results indicate that the models are able to reproduce the basic structure of the TTL. There is a large spread in cold point tropopause temperatures that may be linked to variation in TTL ozone values. The models are generally able to reproduce historical trends in tropopause pressure obtained from reanalysis products. Simulated historical trends in cold point tropopause temperatures and in the meridional extent of the TTL are not consistent across models. The pressure of both the tropical tropopause and the level of main convective outflow appear to be decreasing (increasing altitude) in historical runs. Similar trends are seen in the future. Models consistently predict decreasing tropopause and convective outflow pressure, by several hPa/decade. Tropical cold point temperatures increase by 0.2 K/decade. This indicates that tropospheric warming dominates stratospheric cooling at the tropical tropopause. Stratospheric water vapor at 100 hPa increases by up to 0.5–1 ppmv by 2100. This is less than implied directly by the temperature and methane increases, highlighting the correlation of tropopause temperatures with stratospheric water vapor, but also the complex nature of TTL transport.
Sensitivity of tracer transport to model resolution, forcing data and tracer lifetime in the general circulation model ECHAM5
Authors: Aghedo A. , Rast S. , Schultz M. G.
Published: April 2010

Abstract. The transport of tracers in the general circulation model ECHAM5 is analysed using 9 independent idealized tracers with constant lifetimes released in different altitude regions of the atmosphere. The source regions were split into the tropics, Northern and Southern Hemisphere. The dependency of tracer transport on model resolution is tested in the resolutions T21L19, T42L19, T42L31, T63L31 and T106L31, by employing tracers with a globally uniform lifetime of 5 months. Each of the experiments uses prescribed sea surface temperatures and sea ice fields of the 1990s. The influence of meteorology and tracer lifetimes were tested by performing additional experiments in the T63L31 resolution, by nudging ECHAM5 towards the European Centre for Medium Range Weather Forecast 40 years re-analysis data (ERA40), and by using tracer lifetimes of 0.5 and 50 months, respectively. The transport of tracers is faster in the finer resolution models and is mostly dependent on the number of vertical levels. We found a decrease in the inter-hemispheric transport of tracers with source region at the surface or the tropopause in the coarse resolution models due to increasing recirculation within the source region and vertical mixing. However, a coarse model resolution leads to enhanced inter-hemispheric transport in the stratosphere. The use of ERA40 data only slightly affects the inter-hemispheric transport of surface and tropopause tracers, whereas it increases the inter-hemispheric and vertical transport in the stratosphere by up to 100% and by a factor of 2.5, respectively. The inter-hemispheric transport time was deduced from simulations with tracers of infinite lifetime and source regions at the surface in the Northern and Southern Hemisphere. Again, the transport was found to be faster for models with higher vertical resolution. We find inter-hemispheric transport times of about 7 to 9 months which are lower than the values reported in the literature, based for example on 85Kr observations.
Characterization of the size-segregated water-soluble inorganic ions at eight Canadian rural sites
Authors: Zhang L. , Vet R. , Wiebe A. , Mihele C. , Sukloff B. , Chan E. , Moran M. D. , Iqbal S.
Published: April 2010

Abstract. Size-segregated water-soluble inorganic ions, including particulate sulphate (SO42-), nitrate (NO3-), ammonium (NH4+), chloride (Cl-) and base cations (K+, Na+, Mg2+, Ca2+), were measured using a Micro-Orifice Uniform Deposit Impactor (MOUDI) during fourteen short-term field campaigns at eight locations in both polluted and remote regions of eastern and central Canada. The size distributions of SO42- and NH4+ were unimodal, peaking at 0.3–0.6 μm in diameter, during most of the campaigns, although a bimodal distribution was found during one campaign and a trimodal distribution during another campaign made at a coastal site. The size distributions of NO3- were unimodal, peaking at 4.0–7.0 μm, during the warm-season campaigns and bimodal, with one peak at 0.3–0.6 μm and another at 4–7 μm, during the cold-season campaigns. A unimodal size distribution, peaking at 4–6 μm, was found for Cl-, Na+, Mg2+ and Ca2+ during approximately half of the campaigns and a bimodal distribution, with one peak at 2 μm and the other at 6 μm, was found during the rest of the campaigns. For K+, a bimodal distribution, with one peak at 0.3 μm and the other at 4 μm, was observed during most of the campaigns. The measured ion concentrations varied by one order of magnitude across the various sites. The air-mass origins and meteorological conditions both played important roles in formulating the observed geographical and seasonal patterns of these ion species concentration levels, size distributions and fine particle acidity.
Possible influence of anthropogenic aerosols on cirrus clouds and anthropogenic forcing
Authors: Penner J. E. , Chen Y. , Wang M. , Liu X.
Published: April 2010

Abstract. Cirrus clouds have a net warming effect on the atmosphere and cover about 30% of the Earth's area. Aerosol particles initiate ice formation in the upper troposphere through modes of action that include homogeneous freezing of solution droplets, heterogeneous nucleation on solid particles immersed in a solution, and deposition nucleation of vapor onto solid particles. Here, we examine the possible change in ice number concentration from anthropogenic soot originating from surface sources of fossil fuel and biomass burning, from anthropogenic sulfate aerosols, and from aircraft that deposit their aerosols directly in the upper troposphere. We find that fossil fuel and biomass burning soot aerosols exert a radiative forcing of −0.68 to 0.01 Wm−2 while anthropogenic sulfate aerosols exert a forcing of −0.01 to 0.18 Wm−2. Our calculations show that the sign of the forcing by aircraft soot depends on the model configuration and can be both positive or negative, ranging from −0.16 to 0.02 Wm−2. The magnitude of the forcing in cirrus clouds can be comparable to the forcing exerted by anthropogenic aerosols on warm clouds, but this forcing has not been included in past assessments of the total anthropogenic radiative forcing of climate.
Parameterization of middle atmospheric water vapor photochemistry for high-altitude NWP and data assimilation
Authors: McCormack J. P. , Hoppel K. W. , Siskind D. E.
Published: April 2010

Abstract. This report describes CHEM2D-H2O, a new parameterization of H2O photochemical production and loss based on the CHEM2D photochemical-transport model of the middle atmosphere. This parameterization accounts for the altitude, latitude, and seasonal variations in the photochemical sources and sinks of water vapor over the pressure region from 100–0.001 hPa (~16–90 km altitude). A series of free-running NOGAPS-ALPHA forecast model simulations offers a preliminary assessment of CHEM2D-H2O performance over the June 2007 period. Results indicate that the CHEM2D-H2O parameterization improves global 10-day forecasts of upper mesospheric water vapor compared to forecasts using an existing one-dimensional (altitude only) parameterization. Most of the improvement is seen at high winter latitudes where the one-dimensional parameterization specifies photolytic H2O loss year round despite the lack of sunlight in winter. The new CHEM2D-H2O parameterization should provide a better representation of the downwelling of dry mesospheric air into the stratospheric polar vortex in operational analyses that do not assimilate middle atmospheric H2O measurements.
Distribution of VOCs between air and snow at the Jungfraujoch high alpine research station, Switzerland, during CLACE 5 (winter 2006)
Authors: Starokozhev E. , Fries E. , Cycura A. , Püttmann W.
Published: April 2010

Abstract. Volatile organic compounds (VOCs) were analyzed in air and snow samples at the Jungfraujoch high alpine research station in Switzerland as part of CLACE 5 (CLoud and Aerosol Characterization Experiment) during February/March 2006. The fluxes of individual compounds in ambient air were calculated from gas phase concentrations and wind speed. The highest flux values were observed for the aromatic hydrocarbons benzene (14.3 μg m−2s−1), 1,3,5-trimethylbenzene (5.27 μg m−2s−1), toluene (4.40 μg m−2s−1), and the aliphatic hydrocarbons i-butane (7.87 μg m−2s−1), i-pentane (3.61 μg m−2s−1) and n-butane (3.23 μg m−2s−1). The fluxes were used to calculate the efficiency of removal of VOCs by snow, and the effect of temperature on removal efficiency. The removal efficiency was calculated at – 24°C (−13.7°C) and ranged from 37% (35%) for o-xylene to 93% (63%) for i-pentane. The distribution coefficients of VOCs between the air and snow phases were derived from published poly-parameter linear free energy relationship (pp-LFER) data, and compared with distribution coefficients obtained from the simultaneous measurements of VOC concentrations in air and snow at Jungfraujoch. The coefficients calculated from pp-LFER exceeded those values measured in the present study, which indicates more efficient snow scavenging of the VOCs investigated than suggested by theoretical predictions.
Diffusional and accretional growth of water drops in a rising adiabatic parcel: effects of the turbulent collision kernel
Authors: Grabowski W. W. , Wang L.-P.
Published: April 2010

Abstract. A large set of rising adiabatic parcel simulations is executed to investigate the combined diffusional and accretional growth of cloud droplets in maritime and continental conditions, and to assess the impact of enhanced droplet collisions due to small-scale cloud turbulence. The microphysical model applies the droplet number density function to represent spectral evolution of cloud and rain/drizzle drops, and various numbers of bins in the numerical implementation, ranging from 40 to 320. Simulations are performed applying two traditional gravitational collection kernels and two kernels representing collisions of cloud droplets in the turbulent environment, with turbulent kinetic energy dissipation rates of 100 and 400 cm2 s−3. The overall result is that the rain initiation time significantly depends on the number of bins used, with earlier initiation of rain when the number of bins is low. This is explained as a combination of the increase of the width of activated droplet spectrum and enhanced numerical spreading of the spectrum during diffusional and collisional growth when the number of model bins is low. Simulations applying around 300 bins seem to produce rain at times which no longer depend on the number of bins, but the activation spectra are unrealistically narrow. These results call for an improved representation of droplet activation in numerical models of the type used in this study. Despite the numerical effects that impact the rain initiation time in different simulations, the turbulent speedup factor, the ratio of the rain initiation time for the turbulent collection kernel and the corresponding time for the gravitational kernel, is approximately independent of aerosol characteristics, parcel vertical velocity, and the number of bins used in the numerical model. The turbulent speedup factor is in the range 0.75–0.85 and 0.60–0.75 for the turbulent kinetic energy dissipation rates of 100 and 400 cm2 s−3, respectively.
Coupling aerosol-cloud-radiative processes in the WRF-Chem model: investigating the radiative impact of elevated point sources
Authors: Chapman E. G. , Gustafson, W. I. , Easter R. C. , Barnard J. C. , Ghan S. J. , Pekour M. S. , Fast J. D.
Published: April 2010

Abstract. The local and regional influence of elevated point sources on summertime aerosol forcing and cloud-aerosol interactions in northeastern North America was investigated using the WRF-Chem community model. The direct effects of aerosols on incoming solar radiation were simulated using existing modules to relate aerosol sizes and chemical composition to aerosol optical properties. Indirect effects were simulated by adding a prognostic treatment of cloud droplet number and adding modules that activate aerosol particles to form cloud droplets, simulate aqueous-phase chemistry, and tie a two-moment treatment of cloud water (cloud water mass and cloud droplet number) to an existing radiation scheme. Fully interactive feedbacks thus were created within the modified model, with aerosols affecting cloud droplet number and cloud radiative properties, and clouds altering aerosol size and composition via aqueous processes, wet scavenging, and gas-phase-related photolytic processes. Comparisons of a baseline simulation with observations show that the model captured the general temporal cycle of aerosol optical depths (AODs) and produced clouds of comparable thickness to observations at approximately the proper times and places. The model overpredicted SO2 mixing ratios and PM2.5 mass, but reproduced the range of observed SO2 to sulfate aerosol ratios, suggesting that atmospheric oxidation processes leading to aerosol sulfate formation are captured in the model. The baseline simulation was compared to a sensitivity simulation in which all emissions at model levels above the surface layer were set to zero, thus removing stack emissions. Instantaneous, site-specific differences for aerosol and cloud related properties between the two simulations could be quite large, as removing above-surface emission sources influenced when and where clouds formed within the modeling domain. When summed spatially over the finest resolution model domain (the extent of which corresponds to the typical size of a single GCM grid cell) and temporally over a three day analysis period, total rainfall in the sensitivity simulation increased by 31% over that in the baseline simulation. Fewer optically thin clouds, arbitrarily defined as a cloud exhibiting an optical depth less than 1, formed in the sensitivity simulation. Domain-averaged AODs dropped from 0.46 in the baseline simulation to 0.38 in the sensitivity simulation. The overall net effect of additional aerosols attributable to primary particulates and aerosol precursors from point source emissions above the surface was a domain-averaged reduction of 5 W m−2 in mean daytime downwelling shortwave radiation.
Modeled and observed ozone sensitivity to mobile-source emissions in Mexico City
Authors: Zavala M. , Lei W. F. , Molina M. J. , Molina L. T.
Published: April 2010

Abstract. The emission characteristics of mobile sources in the Mexico City Metropolitan Area (MCMA) have changed significantly over the past few decades in response to emission control policies, advancements in vehicle technologies and improvements in fuel quality, among others. Along with these changes, concurrent non-linear changes in photochemical levels and criteria pollutants have been observed, providing a unique opportunity to understand the effects of perturbations of mobile emission levels on the photochemistry in the region using observational and modeling approaches. The observed historical trends of ozone (O3), carbon monoxide (CO) and nitrogen oxides (NOx) suggest that ozone production in the MCMA has changed from a low to a high VOC-sensitive regime over a period of 20 years. Comparison of the historical emission trends of CO, NOx and hydrocarbons derived from mobile-source emission studies in the MCMA from 1991 to 2006 with the trends of the concentrations of CO, NOx, and the CO/NOx ratio during peak traffic hours also indicates that fuel-based fleet average emission factors have significantly decreased for CO and VOCs during this period whereas NOx emission factors do not show any strong trend, effectively reducing the ambient VOC/NOx ratio. This study presents the results of model analyses on the sensitivity of the observed ozone levels to the estimated historical changes in its precursors. The model sensitivity analyses used a well-validated base case simulation of a high pollution episode in the MCMA with the mathematical Decoupled Direct Method (DDM) and the standard Brute Force Method (BFM) in the 3-D CAMx chemical transport model. The model reproduces adequately the observed historical trends and current photochemical levels. Comparison of the BFM and the DDM sensitivity techniques indicates that the model yields ozone values that increase linearly with NOx emission reductions and decrease linearly with VOC emission reductions only up to 30% from the base case. We further performed emissions perturbations from the gasoline fleet, diesel fleet, all mobile (gasoline plus diesel) and all emission sources (anthropogenic plus biogenic). The results suggest that although large ozone reductions obtained in the past were from changes in emissions from gasoline vehicles, currently significant benefits could be achieved with additional emission control policies directed to regulation of VOC emissions from diesel and area sources that are high emitters of alkenes, aromatics and aldehydes.
Inverse modeling and mapping US air quality influences of inorganic PM2.5 precursor emissions using the adjoint of GEOS-Chem
Authors: Henze D. K. , Seinfeld J. H. , Shindell D. T.
Published: April 2010

Abstract. Influences of specific sources of inorganic PM2.5 on peak and ambient aerosol concentrations in the US are evaluated using a combination of inverse modeling and sensitivity analysis. First, sulfate and nitrate aerosol measurements from the IMPROVE network are assimilated using the four-dimensional variational (4D-Var) method into the GEOS-Chem chemical transport model in order to constrain emissions estimates in four separate month-long inversions (one per season). Of the precursor emissions, these observations primarily constrain ammonia (NH3). While the net result is a decrease in estimated US NH3 emissions relative to the original inventory, there is considerable variability in adjustments made to NH3 emissions in different locations, seasons and source sectors, such as focused decreases in the midwest during July, broad decreases throughout the US~in January, increases in eastern coastal areas in April, and an effective redistribution of emissions from natural to anthropogenic sources. Implementing these constrained emissions, the adjoint model is applied to quantify the influences of emissions on representative PM2.5 air quality metrics within the US. The resulting sensitivity maps display a wide range of spatial, sectoral and seasonal variability in the susceptibility of the air quality metrics to absolute emissions changes and the effectiveness of incremental emissions controls of specific source sectors. NH3 emissions near sources of sulfur oxides (SOx) are estimated to most influence peak inorganic PM2.5 levels in the East; thus, the most effective controls of NH3 emissions are often disjoint from locations of peak NH3 emissions. Controls of emissions from industrial sectors of SOx and NOx are estimated to be more effective than surface emissions, and changes to NH3 emissions in regions dominated by natural sources are disproportionately more effective than regions dominated by anthropogenic sources. NOx controls are most effective in northern states in October; in January, SOx controls may be counterproductive. When considering ambient inorganic PM2.5 concentrations, intercontinental influences are small, though transboundary influences within North America are significant, with SOx emissions from surface sources in Mexico contributing almost a fourth of the total influence from this sector.