-
KCl-coated denuders have become a standard method for measurement of gaseous oxidized mercury, but their performance has not been exhaustively evaluated, especially in field conditions. In this study, KCl-coated and uncoated quartz denuders loaded with HgCl<sub>2</sub> and HgBr<sub>2</sub> lost 29–55% of these compounds, apparently as elemental mercury, when exposed to ozone (range of 6–100 ppb tested). This effect was also observed for denuders loaded with gaseous oxidized mercury at a field site in Nevada (3–37% of oxidized mercury lost). In addition, collection efficiency decreased by 12–30% for denuders exposed to 50 ppb ozone during collection of HgCl<sub>2</sub>. While data presented were obtained from laboratory tests and as such do not exactly simulate field sampling conditions, these results indicate that the KCl denuder oxidized mercury collection method may not be as robust as previously thought. This work highlights needs for further testing of this method, clear identification of gaseous oxidized mercury compounds in the atmosphere, and development of field calibration methods for these compounds.
-
The aim of this paper is to study the impacts of overshooting convection at a local scale on the water distribution in the tropical UTLS. Overshooting convection is assumed to be one of the processes controlling the entry of water vapour mixing ratio in the stratosphere by injecting ice crystals above the tropopause which later sublimate and hydrate the lower stratosphere. For this purpose, we quantify the individual impact of two cases of overshooting convection in Africa observed during SCOUT-AMMA: the case of 4 August 2006 over Southern Chad which is likely to have influenced the water vapour measurements by micro-SDLA and FLASH-B from Niamey on 5 August, and the case of a mesoscale convective system over Aïr on 5 August 2006. We make use of high resolution (down to 1 km horizontally) nested grid simulations with the three-dimensional regional atmospheric model BRAMS (Brazilian Regional Atmospheric Modelling System). In both cases, BRAMS succeeds in simulating the main features of the convective activity, as well as overshooting convection, though the exact position and time of the overshoots indicated by MSG brightness temperature difference is not fully reproduced (typically 1° displacement in latitude compared with the overshoots indicated by brightness temperature difference from satellite observations for both cases, and several hours shift for the Aïr case on 5 August 2006). Total water budgets associated with these two events show a significant injection of ice particles above the tropopause with maximum values of about 3.7 ton s<sup>−1</sup> for the Chad case (4 August) and 1.4 ton s<sup>−1</sup> for the Aïr case (5 August), and a total upward cross tropopause transport of about 3300 ton h<sup>−1</sup> for the Chad case and 2400 ton h<sup>−1</sup> for the Aïr case in the third domain of simulation. The order of magnitude of these modelled fluxes is lower but comparable with similar studies in other tropical areas based on models. These two estimations exhibit significant differences and highlight variability among the cases of the impact of overshooting convection in hydrating the lower stratosphere. We show that the regional enhancement of water above the tropopause is between 0.21 to 0.67 ppmv between 380 and 400 K, generally in the range of other model estimations. The amount of water which remains in the stratosphere after the overshoot is estimated for both cases. A range of 330 to 507 tons is found for the Chad case and an upper limit of 200 tons is found for the Aïr case. Finally we emphasize that the hydrated area in the LS by overshooting convection can be advected relatively far away from the overshoot initial location, with locally mixing ratios of more than 3 ppmv higher than the background level, which is compatible with the balloon borne measurements performed above Niamey in the same air mass, 30 h after the overshoot.
-
A large number of activities have been carried out to characterise the levels of mercury (Hg) species in ambient air and precipitation, in order to understand how they vary over time and how they depend on meteorological conditions. Following the discovery of atmospheric Hg depletion events (AMDEs) in Polar Regions, a significant research effort was made to assess the chemical-physical mechanisms behind the rapid conversion of atmospheric gaseous Hg (Hg<sup>0</sup>) into reactive and water-soluble forms which are potentially bioavailable. The understanding of the way in which Hg is released into the atmosphere, transformed, deposited and eventually incorporated into biota is of crucial importance not only for the polar regions but also for the marine environment in general. The oceans and seas are both sources and sinks of Hg and play a major role in the Hg cycle. In this work, the available Hg concentration datasets from a number of terrestrial sites (industrial, rural and remote) in both the Northern and Southern Hemispheres as well as over oceans and seas have been investigated. The higher Hg species concentration and variability observed in the Northern Hemisphere suggest that the majority of emissions and re-emissions occur there. The inter-hemispherical gradient with higher total gaseous mercury (TGM) concentrations in the Northern Hemisphere has remained nearly constant over the years for which data are available. The analysis of Hg concentration patterns indicates the differences in regional source/sink characteristics, with increasing variability toward areas strongly influenced by anthropogenic sources. The large increase in Hg emissions in rapidly developing countries (i.e., China, India) over the last decade, due primarily to a sharp increase in energy production from coal combustion, are not currently reflected in the long-term measurements of TGM in ambient air and precipitation at continuous monitoring sites in either Northern Europe or North America. The discrepancy between observed gaseous Hg concentrations (steady or decreasing) and global Hg emission inventories (increasing) has not yet been explained, though the potential oxidation of the atmosphere during the last decade is increasing. Currently, however, a coordinated observational network for Hg does not exist.
-
The understanding of present atmospheric transport processes from Southern Hemisphere (SH) landmasses to Antarctica can improve the interpretation of stratigraphic data in Antarctic ice cores. In addition, long range transport can deliver key nutrients normally not available to marine ecosystems in the Southern Ocean and may trigger or enhance primary productivity. However, there is a dearth of observational based studies of dust transport in the SH. <br><br> This work aims to improve current understanding of dust transport in the SH by showing a characterization of two dust events originating in the Patagonia desert (south end of South America). The approach is based on a combined and complementary use of satellite retrievals (detectors MISR, MODIS, GLAS, POLDER, OMI), transport model simulation (HYSPLIT) and surface observations near the sources and aerosol measurements in Antarctica (Neumayer and Concordia sites). <br><br> Satellite imagery and visibility observations confirm dust emission in a stretch of dry lakes along the coast of the Tierra del Fuego (TdF) island (~54° S) and from the shores of the Colihue Huapi lake in Central Patagonia (~46° S) in February 2005. Model simulations initialized by these observations reproduce the timing of an observed increase in dust concentration at the Concordia Station and some of the observed increases in atmospheric aerosol absorption (here used as a dust proxy) in the Neumayer station. The TdF sources were the largest contributors of dust at both sites. The transit times from TdF to the Neumayer and Concordia sites are 6–7 and 9–10 days respectively. Lidar observations and model outputs coincide in placing most of the dust cloud in the boundary layer and suggest significant deposition over the ocean immediately downwind. Boundary layer dust was detected as far as 1800 km from the source and ~800 km north of the South Georgia Island over the central sub-Antarctic Atlantic Ocean. Although the analysis suggests the presence of dust at ~1500 km SW of South Africa five days after, the limited capabilities of existing satellite platforms to differentiate between aerosol types do not permit a definitive conclusion. In addition, the model simulations show dust lifting to the free troposphere as it travels south but it could not be confirmed by the satellite observations due to cloudiness. <br><br> This work demonstrates that complementary information from existing transport models, satellite and surface data can yield a consistent picture of the dust transport from the Patagonia desert to Antarctica. It also illustrates the limitation of using any of these approaches individually to characterize the transport of dust in a heavily cloudy area.
-
Satellite-based precipitation, Aerosol Optical Depth (AOD), Cloud Optical Depth (COD), and Aerosol Index (AI) data were used to characterize the linkages among landform and the intra-annual variability of aerosols, cloudiness and rainfall in the Himalayas using empirical orthogonal function (EOF) analysis. The first modes of AOD and AI show the presence of two branches of dust aerosol: over the Indus river basin and the Thar desert with a sharp west-east gradient parallel to the southern slopes of the Himalayas – the Southern Branch; and the second against the slopes of the Tian Shan and over the Takla Makan desert in the Tibetan Plateau-the Northern branch. The third EOF mode of AOD accounts for about 7% of overall variance of AOD. It is attached to the foothills of the Himalayas east of the Aravalli range peaking in April-May-June followed by a sharp decrease in July during the first active phase of the monsoon. The first and second EOF modes of COD and precipitation show consistent patterns against the central and eastern Himalayas and along the ocean-land boundaries in western India and the Bay of Bengal. The break in cloudiness and rainfall between the winter and the monsoon seasons is captured well by the second EOF mode of COD and rainfall concurrent with the aerosol build up mode (April–May) over the region depicted by the third mode of AOD. The results show that the Aravalli range separates the two different modes of aerosol variability over northern India with dust aerosols to the west and polluted mixed aerosols to the east consistent with its role in regional circulation and precipitations patterns as per Barros et al. (2004) and Chiao and Barros (2007). SVD analysis between rainfall, COD and AOD showed a pattern of aerosol loading (resembling EOF3 of MODIS AOD) extending from 80° E~90° E that peaks during the winter and pre-monsoon seasons and decays abruptly during the monsoon: the regions of aerosol buildup during the pre-monsoon season and the areas of high rainfall/cloudiness during the monsoon are collocated and have opposite signs suggesting aerosol-cloud-rainfall interaction. It is proposed that the third EOF of AOD maps the area where aerosol-cloud-rainfall interactions play an important role in the regional hydro-climatology.
-
The optical properties of externally mixed light absorbing carbon (LAC) aggregates are computed over the spectral range from 200 nm–12.2 μm by use of the numerically exact superposition T-matrix method. The spectral computations are tailored to the 14-band radiation model employed in the Integrated Forecasting System operated at the European Centre for Medium Range Weather Forecast. The size- and wavelength dependence of the optical properties obtained with the fractal aggregate model differs significantly from corresponding results based on the homogeneous sphere approximation, which is still commonly employed in climate models. The computational results are integrated into the chemical transport model MATCH (Multiple-scale Atmospheric Transport and CHemistry modelling system) to compute 3-D fields of size-averaged aerosol optical properties. Computational results obtained with MATCH are coupled to a radiative transfer model to compute the shortwave radiative impact of LAC. It is found that the fractal aggregate model gives a shortwave forcing estimate that is twice as high as that obtained with the homogeneous sphere approximation. Thus previous estimates based on the homogeneous sphere model may have substantially underestimated the shortwave radiative impact of freshly emitted LAC.
-
This paper investigates the latest version 1.07 SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) tropical ozone from the 1.27 μm as well as from the 9.6 μm retrieval and temperature data with respect to day time variations in the upper mesosphere. The processes involved are compared to day time variations of the three-dimensional general circulation and chemistry model HAMMONIA (Hamburg Model of the Neutral and Ionized Atmosphere). The results show a good qualitative agreement for ozone. The amplitude of daytime variations is in both cases approximately 60% of the daytime mean. During equinox the daytime maximum ozone abundance is for both, the observations and the model, higher than during solstice, especially above 0.01 hPa (approx. 80 km). The influence of tidal signatures either directly in ozone or indirectly via a temperature response above 0.01 hPa can not be fully eliminated. Below 0.01 hPa (photo-)chemistry is the main driver for variations. We also use the HAMMONIA output of daytime variation patterns of several other different trace gas species, e.g., water vapor and atomic oxygen, to discuss the daytime pattern in ozone. In contrast to ozone, temperature data show little daytime variations between 65 and 90 km and their amplitudes are on the order of less than 1.5%. In addition, SABER and HAMMONIA temperatures show significant differences above 80 km.
-
Quantification of exposure to traffic-related air pollutants near highways is hampered by incomplete knowledge of the scales of temporal variation of pollutant gradients. The goal of this study was to characterize short-term temporal variation of vehicular pollutant gradients within 200–400 m of a major highway (>150 000 vehicles/d). Monitoring was done near Interstate 93 in Somerville (Massachusetts) from 06:00 to 11:00 on 16 January 2008 using a mobile monitoring platform equipped with instruments that measured ultrafine and fine particles (6–1000 nm, particle number concentration (PNC)); particle-phase (>30 nm) NO<sub>3</sub><sup>−</sup>, SO<sub>4</sub><sup>2−</sup>, and organic compounds; volatile organic compounds (VOCs); and CO<sub>2</sub>, NO, NO<sub>2</sub>, and O<sub>3</sub>. We observed rapid changes in pollutant gradients due to variations in highway traffic flow rate, wind speed, and surface boundary layer height. Before sunrise and peak traffic flow rates, downwind concentrations of particles, CO<sub>2</sub>, NO, and NO<sub>2</sub> were highest within 100–250 m of the highway. After sunrise pollutant levels declined sharply (e.g., PNC and NO were more than halved) and the gradients became less pronounced as wind speed increased and the surface boundary layer rose allowing mixing with cleaner air aloft. The levels of aromatic VOCs and NO<sub>3</sub><sup>−</sup>, SO<sub>4</sub><sup>2−</sup> and organic aerosols were generally low throughout the morning, and their spatial and temporal variations were less pronounced compared to PNC and NO. O<sub>3</sub> levels increased throughout the morning due to mixing with O<sub>3</sub>-enriched air aloft and were generally lowest near the highway reflecting reaction with NO. There was little if any evolution in the size distribution of 6–225 nm particles with distance from the highway. These results suggest that to improve the accuracy of exposure estimates to near-highway pollutants, short-term (e.g., hourly) temporal variations in pollutant gradients must be measured to reflect changes in traffic patterns and local meteorology.
-
A dimensionless theory for new particle formation (NPF) was developed, using an aerosol population balance model incorporating recent developments in nucleation rates and measured particle growth rates. Based on this theoretical analysis, it was shown that a dimensionless parameter <i>L</i><sub>Γ</sub>, characterizing the ratio of the particle scavenging loss rate to the particle growth rate, exclusively determined whether or not NPF would occur on a particular day. This parameter determines the probability that a nucleated particle will grow to a detectable size before being lost by coagulation with the pre-existing aerosol. Cluster-cluster coagulation was shown to contribute negligibly to this survival probability under conditions pertinent to the atmosphere. Data acquired during intensive measurement campaigns in Tecamac (MILAGRO), Atlanta (ANARChE), Boulder, and Hyytiälä (QUEST II, QUEST IV, and EUCAARI) were used to test the validity of <i>L</i><sub>Γ</sub> as an NPF criterion. Measurements included aerosol size distributions down to 3 nm and gas-phase sulfuric acid concentrations. The model was applied to seventy-seven NPF events and nineteen non-events (characterized by growth of pre-existing aerosol without NPF) measured in diverse environments with broad ranges in sulfuric acid concentrations, ultrafine number concentrations, aerosol surface areas, and particle growth rates (nearly two orders of magnitude). Across this diverse data set, a nominal value of <i>L</i><sub>Γ</sub>=0.7 was found to determine the boundary for the occurrence of NPF, with NPF occurring when <i>L</i><sub>Γ</sub><0.7 and being suppressed when <i>L</i><sub>Γ</sub>>0.7. Moreover, nearly 45% of measured <i>L</i><sub>Γ</sub> values associated with NPF fell in the relatively narrow range of 0.1<<i>L</i><sub>Γ</sub><0.3.
-
Hazardous impact of air pollutant emissions from megacities on atmospheric composition on regional and global scales is currently an important issue in atmospheric research. However, the quantification of emissions and related effects is frequently a difficult task, especially in the case of developing countries, due to the lack of reliable data and information. This study examines possibilities to retrieve multi-annual NO<sub>x</sub> emissions changes in megacity regions from satellite measurements of nitrogen dioxide and to quantify them in terms of linear and nonlinear trends. By combining the retrievals of the GOME and SCIAMACHY satellite instrument data with simulations performed by the CHIMERE chemistry transport model, we obtain the time series of NO<sub>x</sub> emission estimates for the 12 largest urban agglomerations in Europe and the Middle East in the period from 1996 to 2008. We employ then a novel method allowing estimation of a nonlinear trend in a noisy time series of an observed variable. The method is based on the probabilistic approach and the use of artificial neural networks; it does not involve any quantitative a priori assumptions. As a result, statistically significant nonlinearities in the estimated NO<sub>x</sub> emission trends are detected in 5 megacities (Bagdad, Madrid, Milan, Moscow and Paris). Statistically significant upward linear trends are detected in Istanbul and Tehran, while downward linear trends are revealed in Berlin, London and the Ruhr agglomeration. The presence of nonlinearities in NO<sub>x</sub> emission changes in Milan, Paris and Madrid is confirmed by comparison of simulated NO<sub>x</sub> concentrations with independent air quality monitoring data. A good quantitative agreement between the linear trends in the simulated and measured near surface NO<sub>x</sub> concentrations is found in London.
