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We investigated the iron (Fe) dissolution kinetics of African (Tibesti) and Asian (Beijing) dust samples at acidic pH with the aim of reproducing the low pH conditions in atmospheric aerosols. The Beijing dust and three size fractions of the Tibesti dust (<20 μm: PM<sub>20</sub>; <10 μm: PM<sub>10</sub>; and <2.5 μm: PM<sub>2.5</sub>) were dissolved at pH 1, 2 and/or 3 for up to 1000 h. In the first 10 min, all dust samples underwent an extremely fast Fe solubilisation. Subsequently, the Fe dissolution proceeded at a much slower rate before reaching a stable dissolution plateau. The time-dependant Fe dissolution datasets were best described by a model comprising three acid-extractable Fe pools each dissolving according to first-order kinetics. The dissolution rate constant <i>k</i> (h<sup>−1</sup>) of each pool was independent of the source (Saharan or Asian) and the size (PM<sub>20</sub>, PM<sub>10</sub> or PM<sub>2.5</sub>) of the dust but highly dependent on pH. The "fast" Fe pool had a <i>k</i> (25 h<sup>−1</sup> at pH = 1) of a similar magnitude to "dry" ferrihydrite nanoparticles and/or poorly crystalline Fe(III) oxyhydroxide, while the "intermediate" and "slow" Fe pools had <i>k</i> values respectively 50–60 times and 3000–4000 times smaller than the "fast" pool. The "slow" Fe pool was likely to consist of both crystalline Fe oxide phases (i.e., goethite and/or hematite) and Fe contained in the clay minerals. The initial mass of the "fast", "intermediate" and "slow" Fe pools represented respectively about 0.5–2%, 1–3% and 15–40% of the total Fe in the dust samples. Furthermore, we showed that in systems with low dust/liquid ratios, Fe can be dissolved from all three pools, whereas at high dust/liquid ratios (e.g., in aerosols), sufficient Fe may be solubilised from the "fast" phase to dominate the Fe dissolved and to suppress the dissolution of Fe from the other Fe pools. These data demonstrated that dust/liquid ratio and pH are fundamental parameters controlling Fe dissolution kinetics in the dust. In order to reduce errors in atmospheric and climate models, these fundamental controlling factors need to be included.
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Using the ECMWF/NCEP reanalysis data, satellite observations from AURA MLS and UARS HALOE, and Oceanic Niño Index (ONI) data, the effects of El Niño and La Niña events on the stratospheric water vapor changes are investigated. Overall, El Niño events tend to moisten the lower stratosphere but dry the middle stratosphere. La Niña events are likely to dry the lower stratosphere over a narrow band of tropics (5° S–5° N) but have a moistening effect on the whole stratosphere when averaged over a broader region of tropics between 25° S–25° N. The moistening effect of La Niña events mainly occurs in lower stratosphere in the Southern Hemisphere tropics where a significant 20% increase in the tropical upwelling is caused by La Niña events. El Niño events have a more significant effect on the tropical upwelling in the Northern Hemisphere extratropics than in Southern Hemisphere extratropics. The net effect of ENSO activities on the lower stratospheric water vapor is stronger in the Southern Hemisphere tropics than in the Northern Hemisphere tropics.
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Isoprene is the most abundant non-methane biogenic volatile organic compound (BVOC), but the processes governing secondary organic aerosol (SOA) formation from isoprene oxidation are only beginning to become understood and selective quantification of the atmospheric particulate burden remains difficult. Organic aerosol above a tropical rainforest located in Danum Valley, Borneo, Malaysia, a high isoprene emission region, was studied during Summer 2008 using Aerosol Mass Spectrometry and offline detailed characterisation using comprehensive two dimensional gas chromatography. Observations indicate that a substantial fraction (up to 15% by mass) of atmospheric sub-micron organic aerosol was observed as methylfuran (MF) after thermal desorption. This observation was associated with the simultaneous measurements of established gas-phase isoprene oxidation products methylvinylketone (MVK) and methacrolein (MACR). Observations of MF were also made during experimental chamber oxidation of isoprene. Positive matrix factorisation of the AMS organic mass spectral time series produced a robust factor which accounts for an average of 23% (0.18 μg m<sup>−3</sup>), reaching as much as 53% (0.50 μg m<sup>−3</sup>) of the total oraganic loading, identified by (and highly correlated with) a strong MF signal. Assuming that this factor is generally representative of isoprene SOA, isoprene derived aerosol plays a significant role in the region. Comparisons with measurements from other studies suggest this type of isoprene SOA plays a role in other isoprene dominated environments, albeit with varying significance.
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Base cations exert a large impact on various geochemical and geophysical processes both in the atmosphere and at the Earth surface. One of the essential roles of these compounds is impact on surface pH causing an increase in alkalinity and neutralizing the effects of acidity generated by sulphur and nitrogen deposition. During recent years anthropogenic emissions of base cations in the UK have decreased substantially, by about 70%, 78%, 75% and 48% for Na<sup>+</sup>, Mg<sup>2+</sup>, Ca<sup>2+</sup> and K<sup>+</sup>, respectively, over the period 1990–2006. For the island regions, such as the UK, the main source of base cation particles is the aerosol produced from the sea surface. Here, the sea salt aerosol (SSA) emissions are calculated with parameterisations proposed by Mårtensson et al. (2003) for ultra fine particles, Monahan et al. (1986) for fine particles and Smith and Harisson (1998) for coarse particles continuously with a 0.1 μm size step using WRF-modelled wind speed data at a 5 km × 5 km grid square resolution with a 3 h time step for two selected years 2003 and 2006. SSA production has been converted into base cation emissions, with the assumption that the chemical composition of the particle emitted from the sea surface is equal to the chemical composition of sea water, and used as input data in the Fine Resolution Atmospheric Multi-pollutant Exchange Model (FRAME). FRAME model annual mean concentrations and total wet deposition at a 5 km × 5 km grid resolution, are compared with concentrations in air and wet deposition from the National Monitoring Network and measurements based estimates of UK deposition budget. The correlation coefficient for wet deposition achieves high values (<i>R</i> = 0.8) for Na<sup>+</sup> and Mg<sup>2+</sup>, whereas for Ca<sup>2+</sup> the correlation is poor (<i>R</i> < 0.3). Base cation concentrations are also represented well, with some overestimations on the west coast and underestimations in the centre of the land.
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Since data from the Infrared Atmospheric Sounding Interferometer (IASI) became available in 2007, a number of papers have appeared in the literature which have reported relatively large discrepancies between IASI spectra and forward calculations in the centre of the CO<sub>2</sub> Q-branch at 667 cm<sup>−1</sup>. In this paper we show that these discrepancies are primarily due to errors in the temperature profiles used in the forward calculations. In particular, we have used forecasts of temperature profiles from the European Centre for Medium-Range Weather Forecasts (ECMWF) to demonstrate that, for the case study considered in this paper, these profiles are affected by systematic errors of the order of ≈10 K at the level of the stratopause. To derive the magnitude and the spatial location of the systematic errors in the temperature profile, we have carried out forward/inverse calculations for a number of clear-sky, daytime, IASI tropical soundings over the sea. The forward calculations have been performed using atmospheric state vectors which have been obtained either from the direct inversion of the IASI radiances or from space-time co-located profiles derived from radiosonde observations and from the ECMWF model. To rule out any effect due to the accuracy of the forward model, we have performed the forward calculations using two independent models. The sensitivity of the temperature biases to the variability of the CO<sub>2</sub> profile and to spectroscopy errors has also been studied.
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We use global models to explore the microphysical effects of carbonaceous aerosols on liquid clouds. Although absorption of solar radiation by soot warms the atmosphere, soot may cause climate cooling due to its contribution to cloud condensation nuclei (CCN) and therefore cloud brightness. Six global models conducted three soot experiments; four of the models had detailed aerosol microphysical schemes. The average cloud radiative response to biofuel soot (black and organic carbon), including both indirect and semi-direct effects, is −0.11 Wm<sup>−2</sup>, comparable in size but opposite in sign to the respective direct effect. In a more idealized fossil fuel black carbon experiment, some models calculated a positive cloud response because soot provides a deposition sink for sulfuric and nitric acids and secondary organics, decreasing nucleation and evolution of viable CCN. Biofuel soot particles were also typically assumed to be larger and more hygroscopic than for fossil fuel soot and therefore caused more negative forcing, as also found in previous studies. Diesel soot (black and organic carbon) experiments had relatively smaller cloud impacts with five of the models <±0.06 Wm<sup>−2</sup> from clouds. The results are subject to the caveats that variability among models, and regional and interrannual variability for each model, are large. This comparison together with previously published results stresses the need to further constrain aerosol microphysical schemes. The non-linearities resulting from the competition of opposing effects on the CCN population make it difficult to extrapolate from idealized experiments to likely impacts of realistic potential emission changes.
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The widely used two-product secondary organic aerosol (SOA) formation model has been extended in this study to consider the volatility changes of secondary organic gases (SOG) arising from the aging process as well as the kinetic condensation of low volatile SOG (LV-SOG). In addition to semi-volatile SOG (SV-SOG) with saturation vapor pressure at 290 K (<i>C</i><sup>*</sup><sub>290</sub>) in the range of ~3 ppt–3 ppb and medium-volatile SOG (MV-SOG) with <i>C</i><sup>*</sup><sub>290</sub> in the range of ~0.3–300 ppb, we add a third component representing LV-SOG with <i>C</i><sup>*</sup><sub>290</sub> below ~3 ppt and design a scheme to transfer MV-SOG to SV-SOG and SV-SOG to LV-SOG associated with oxidation aging. This extended SOA formation model has been implemented in a global aerosol model (GEOS-Chem) and the co-condensation of H<sub>2</sub>SO<sub>4</sub> and LV-SOG on pre-existing particles is explicitly simulated. We show that, over many parts of the continents, LV-SOG concentrations are generally a factor of ~2–20 higher than those of H<sub>2</sub>SO<sub>4</sub> and the kinetic condensation of LV-SOG significantly enhances particle growth rates. Comparisons of the simulated and observed evolution of particle size distributions at a boreal forest site (Hyytiälä, Finland) clearly show that LV-SOG condensation is critical in order to bring the simulations closer to the observations. With the new SOA formation scheme, annual mean SOA mass increases by a factor of 2–10 in many parts of the boundary layer and reaches above 0.5 μg m<sup>−3</sup> in most parts of the main continents, improving the agreement with aerosol mass spectrometer (AMS) SOA measurements. While the new scheme generally decreases the concentration of condensation nuclei larger than 10 nm by 3–30% in the lower boundary layer as a result of enhanced surface area and reduced nucleation rates, it substantially increases the concentration of cloud condensation nuclei at a water supersaturation ratio of 0.2%, ranging from ~5–20% over a large fraction of oceans and high latitude continents to more than 50% over some parts of South America, Australia, and Indonesia. Our study highlights the importance for global aerosol models to explicitly account for the oxidation aging of SOGs and their contribution to particle growth.
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From end of February until March 2009 and July 2009 the experimental campaign named DAURE took place in northeastern Spain in both an urban and rural sites (Barcelona city and Montseny Natural Park) with the main objective of studying the formation and transport processes of particulate matter in the region. Several groups collaborated in an extensive measurement campaign with aerosol monitoring, meteorological measurements, atmospheric vertical structure retrievals from LIDAR and supported by numerical simulations of the meteorological and air quality conditions over the region. In this article, we present a description of the main meteorological conditions that affected the Barcelona geographical area during the campaign. The main synoptic conditions are identified and discussed by means of meteorological observations and numerical weather prediction models. Furthermore, a detailed analysis of the local meteorological conditions during the campaign is also presented. The characteristic surface wind field and the vertical structure of the main flows affecting Barcelona and the Montseny rural site are discussed using high-resolution mesoscale meteorological simulations, vertical profiles of LIDAR measurements, radiosoundings, and analysis of backward dispersion simulations with a Lagrangian model. The analysis permits the identification of three main meteorological regimes for the winter campaign (February and March 2009): a first regime dominated by high-pressure conditions over Barcelona and western Mediterranean Basin, high insolation, and the development of thermally-driven wind flows. A second regime is characterized by a strong northwestern advection that produced a cleansing action over the atmosphere. And a third identified regime is dominated by strong stagnant conditions produced by thermal inversions that decouple the low troposphere of plain and coastal areas from mountainous terrains. On the other hand, the main meteorological regimes identified for the summer campaign (July 2009) are: a typical summer recirculation pattern, with low pressure gradients over the western Mediterranean basin, high insolation and development of mesoscale features, a second pattern characterized by the advection of southern air masses enriched with mineral dust from North Africa, and a third zonal advective scenario associated with frontal systems that inhibits the mesoscale circulations and leads to some precipitating events over northeastern Spain. Finally, a classification of the main dispersion patterns deduced from backward Lagrangian simulations is presented and discussed. The results help identify the main flow patterns that lead to the formation and accumulation of particulate matter within the boundary layer, and may produce high pollutant concentrations at Montseny or Barcelona sites. During the DAURE winter and summer campaigns, typical meteorological conditions that lead to air pollution episodes of particulate matter within the Western Mediterranean Basin were observed.
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This paper presents the aerosol budget over Europe in 2006 calculated with the global transport model TM5 coupled to the size-resolved aerosol module M7. Comparison with ground observations indicates that the model reproduces the observed concentrations quite well with an expected slight underestimation of PM<sub>10</sub> due to missing emissions (e.g. resuspension). We model that a little less than half of the anthropogenic aerosols emitted in Europe are exported and the rest is removed by deposition. The anthropogenic aerosols are removed mostly by rain (95%) and only 5% is removed by dry deposition. For the larger natural aerosols, especially sea salt, a larger fraction is removed by dry processes (sea salt: 70%, mineral dust: 35%). We model transport of aerosols in the jet stream in the higher atmosphere and an import of Sahara dust from the south at high altitudes. Comparison with optical measurements shows that the model reproduces the Ångström parameter very well, which indicates a correct simulation of the aerosol size distribution. However, we underestimate the aerosol optical depth. Because the surface concentrations are close to the observations, the shortage of aerosol in the model is probably at higher altitudes. We show that the discrepancies are mainly caused by an overestimation of wet-removal rates. To match the observations, the wet-removal rates have to be scaled down by a factor of about 5. In that case the modelled ground-level concentrations of sulphate and sea salt increase by 50% (which deteriorates the match), while other components stay roughly the same. Finally, it is shown that in particular events, improved fire emission estimates may significantly improve the ability of the model to simulate the aerosol optical depth. We stress that discrepancies in aerosol models can be adequately analysed if all models would provide (regional) aerosol budgets, as presented in the current study.
