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In May 2003, both MODIS aerosol optical depth (AOD) and carbon monoxide (CO) measurements from MOPITT show significant trans-Pacific transport to North America. We apply the global chemical transport model, GEOS-Chem, to analyze the main features of the long-range transport events. Enhancements of MOPITT CO over the tropical Pacific are much broader than MODIS AOD enhancements. We find in model simulations that a major fraction of the CO enhancements in the subtropics in May is due to biomass burning in Southeast Asia in April. Biomass burning CO was recirculated into the subtropical high-pressure system and lingered for a much longer period than aerosols transported at higher latitudes. Simulated AOD enhancements are due to a combination of dust, sulfate, and organic and elemental carbons. Dust contribution dominates the AOD enhancements in early May. Model results indicate that dust transport takes place at higher altitude than the other aerosols. MODIS observations indicate a bias in model simulated pathway of dust transport in one out of the three cases analyzed. Sensitivities of dust transport pathways are analyzed in the model. The dipole structure of transport, consisting of the Aleutian Low to the north and the Pacific High to the south, over the Pacific is found to be a key factor. The placement of the dipole structure relative to model parameters such as up-stream wind field and source location may lead to the high sensitivity of simulated transport pathways.
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Uncertainties in aerosol radiative forcings, especially those associated with clouds, contribute to a large extent to uncertainties in the total anthropogenic forcing. The interaction of aerosols with clouds and radiation introduces feedbacks which can affect the rate of precipitation formation. In former assessments of aerosol radiative forcings, these effects have not been quantified. Also, with global aerosol-climate models simulating interactively aerosols and cloud microphysical properties, a quantification of the aerosol forcings in the traditional way is difficult to define properly. Here we argue that fast feedbacks should be included because they act quickly compared with the time scale of global warming. We show that for different forcing agents (aerosols and greenhouse gases) the radiative forcings as traditionally defined agree rather well with estimates from a method, here referred to as radiative flux perturbations (<i>RFP</i>), that takes these fast feedbacks and interactions into account. Based on our results, we recommend <i>RFP</i> as a valid option to compare different forcing agents, and to compare the effects of particular forcing agents in different models.
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Coal-fired power plants influence climate via both the emission of long-lived carbon dioxide (CO<sub>2</sub>) and short-lived ozone and aerosol precursors. Using a climate model, we perform the first study of the spatial and temporal pattern of radiative forcing specifically for coal plant emissions. Without substantial pollution controls, we find that near-term net global mean climate forcing is negative due to the well-known aerosol masking of the effects of CO<sub>2</sub>. Imposition of pollution controls on sulfur dioxide and nitrogen oxides leads to a rapid realization of the full positive forcing from CO<sub>2</sub>, however. Long-term global mean forcing from stable (constant) emissions is positive regardless of pollution controls. Emissions from coal-fired power plants until ~1970, including roughly 1/3 of total anthropogenic CO<sub>2</sub> emissions, likely contributed little net global mean climate forcing during that period though they may have induce weak Northern Hemisphere mid-latitude (NHml) cooling. After that time many areas imposed pollution controls or switched to low-sulfur coal. Hence forcing due to emissions from 1970 to 2000 and CO<sub>2</sub> emitted previously was strongly positive and contributed to rapid global and especially NHml warming. Most recently, new construction in China and India has increased rapidly with minimal application of pollution controls. Continuation of this trend would add negative near-term global mean climate forcing but severely degrade air quality. Conversely, following the Western and Japanese pattern of imposing air quality pollution controls at a later time could accelerate future warming rates, especially at NHmls. More broadly, our results indicate that due to spatial and temporal inhomogenaities in forcing, climate impacts of multi-pollutant emissions can vary strongly from region to region and can include substantial effects on maximum rate-of-change, neither of which are captured by commonly used global metrics. The method we introduce here to estimate regional temperature responses may provide additional insight.
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Condensation trails (contrails) formed from water vapor emissions behind aircraft engines are the most uncertain components of the aviation impacts on climate change. To gain improved knowledge of contrail and contrail-induced cirrus cloud formation, understanding of contrail ice particle formation immediately after aircraft engines is needed. Despite many efforts spent in modeling the microphysics of ice crystal formation in jet regime (with a plume age <5 s), systematic understanding of parametric effects of variables affecting contrail ice particle formation is still limited. In this work, we apply a microphysical parcel modeling approach to study contrail ice particle formation in near-field aircraft plumes up to 1000 m downstream of an aircraft engine in the soot-rich regime (soot number emission index >1×10<sup>15</sup> (kg-fuel)<sup>−1</sup>) at cruise. The effects of dilution history, ion-mediated nucleation, ambient relative humidity, fuel sulfur contents, and initial soot emissions were investigated. Our simulation results suggest that ice particles are mainly formed by water condensation on emitted soot particles. The growth of ice coated soot particles is driven by water vapor emissions in the first 1000 m and by ambient relative humidity afterwards. The presence of chemi-ions does not significantly contribute to the formation of ice particles in the soot-rich regime, and the effect of fuel sulfur contents is small over the range typical of standard jet fuels. The initial properties of soot emissions play the most critical role, and our calculations suggest that higher number concentration and smaller size of contrail particle nuclei may be able to effectively suppress the formation of contrail ice particles. Further modeling and experimental studies are needed to verify if our findings can provide a possible approach for contrail mitigation.
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Primary Biological Aerosol Particles (PBAPs), including bacteria, spores and pollen, are essential for the spread of organisms and disease in the biosphere, and numerous studies have suggested that they may be important for atmospheric processes, including the formation of clouds and precipitation. The atmospheric abundance and size distribution of PBAPs, however, are largely unknown. At a semi-urban site in Mainz, Germany we used an Ultraviolet Aerodynamic Particle Sizer (UV-APS) to measure Fluorescent Biological Aerosol Particles (FBAPs), which provide an estimate of viable bioaerosol particles and can be regarded as an approximate lower limit for the actual abundance of PBAPs. Fluorescence of non-biological aerosol components are likely to influence the measurement results obtained for fine particles (<1 μm), but not for coarse particles (1–20 μm). <br><br> Averaged over the four-month measurement period (August–December 2006), the mean number concentration of coarse FBAPs was ~3×10<sup>−2</sup> cm<sup>−3</sup>, corresponding to ~4% of total coarse particle number. The mean mass concentration of FBAPs was ~1μg m<sup>−3</sup>, corresponding to ~20% of total coarse particle mass. The FBAP number size distributions exhibited alternating patterns with peaks at various diameters. A pronounced peak at ~3 μm was essentially always observed and can be described by the following campaign-average lognormal fit parameters: geometric mean diameter 3.2 μm, geometric standard deviation 1.3, number concentration 1.6×10<sup>−2</sup> cm<sup>−3</sup>. This peak is likely due to fungal spores or agglomerated bacteria, and it exhibited a pronounced diel cycle (24-h) with maximum intensity during early/mid-morning. FBAP peaks around ~1.5 μm, ~5 μm, and ~13 μm were also observed, but less pronounced and less frequent. These may be single bacterial cells, larger fungal spores, and pollen grains, respectively. <br><br> The observed number concentrations and characteristic sizes of FBAPs are consistent with microscopic, biological and chemical analyses of PBAPs in aerosol filter samples. To our knowledge, however, this is the first exploratory study reporting continuous online measurements of bioaerosol particles over several months and a range of characteristic size distribution patterns with a persistent bioaerosol peak at ~3 μm. The measurement results confirm that PBAPs account for a substantial proportion of coarse aerosol particle number and mass in continental boundary layer air. Moreover, they suggest that the number concentration of viable bioparticles is dominated by fungal spores or agglomerated bacteria with aerodynamic diameters around 3 μm rather than single bacterial cells with diameters around 1 μm.
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Polar ecosystems are considered to be the last pristine environments of the earth relatively uninfluenced by human activities. Antarctica in particular, compared to the Arctic is considered to be even less affected by any kind of anthropogenic influences. Once contaminants reach the Polar Regions, their lifetime in the troposphere depends on local removal processes. Atmospheric mercury, in particular, has unique characteristics that include long-range transport to Polar Regions and the transformation to more toxic and water-soluble compounds that may potentially become bioavailable. These chemical-physical properties have placed mercury on the priority list of an increasing number of International, European and National conventions, and agreements, aimed at the protection of the ecosystems including human health (i.e. GEO, UNEP, AMAP, UN-ECE, HELCOM, OSPAR). This interest, in turn, stimulates a significant amount of research including measurements of gaseous elemental mercury reaction rate constant with atmospheric oxidants, experimental and modelling studies in order to understand the cycling of mercury in Polar Regions, and its impact to these ecosystems. Special attention in terms of contamination of Polar Regions is paid to the consequences of the springtime phenomena, referred to as "Atmospheric Mercury Depletion Events" (AMDEs), during which elemental mercury through a series of photochemically-initiated reactions involving halogens, may be converted to a reactive form that may accumulate in polar coastal, or sea ice, ecosystems. The discovery of the AMDEs, first noted in the Arctic, has also been observed at both poles and was initially considered to result in an important net input of atmospheric mercury into the polar surfaces. However, recent studies point out that complex processes take place after deposition that may result in less significant net-inputs from the atmosphere since a fraction, sometimes significant, of deposited mercury may be recycled. Therefore, the contribution of this unique reactivity occurring in polar atmospheres to the global budget of atmospheric mercury, and the role played by snow and ice surfaces of these regions, are important issues. This paper presents a review of atmospheric mercury studies conducted in the Antarctic troposphere, both at coastal locations and on the Antarctic Plateau since 1985. Our current understanding of atmospheric reactivity in this region is also presented.
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The effect of hygroscopic seeding on warm rain clouds was examined using a hybrid cloud microphysical model combining a Lagrangian Cloud Condensation Nuclei (CCN) activation model, a semi-Lagrangian droplet growth model, and an Eulerian spatial model for advection and sedimentation of droplets. This hybrid cloud microphysical model accurately estimated the effects of CCN on cloud microstructure and suggested the following conclusions for a moderate continental air mass (an air mass with a large number of background CCN). (1) Seeding can hasten the onset of surface rainfall and increase the accumulated amount of surface rainfall if the amount and radius of seeding particles are appropriate. (2) The optimal radius of monodisperse particles to increase rainfall becomes larger with the increase in the total mass of seeding particles. (3) Seeding with salt micro-powder can hasten the onset of surface rainfall and increase the accumulated amount of surface rainfall if the amount of seeding particles is sufficient. (4) Seeding by a hygroscopic flare decreases rainfall in the case of large updraft velocity (shallow convective cloud) and increases rainfall slightly in the case of small updraft velocity (stratiform cloud). (5) Seeding with hygroscopic flares including ultra-giant particles (<i>r</i>>5 μm) hastens the onset of surface rainfall but may not significantly increase the accumulated surface rainfall amount. (6) Hygroscopic seeding increases surface rainfall by two kinds of effects: the "competition effect" by which large soluble particles prevent the activation of smaller particles and the "raindrop embryo effect" in which giant soluble particles can immediately become raindrop embryos. In some cases, one of the effects works, and in other cases, both effects work, depending on the updraft velocity and the amount and size of seeding particles.
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Atmospheric aerosol particles serving as Cloud Condensation Nuclei (CCN) are key elements of the hydrological cycle and climate. We measured and characterized CCN in polluted air and biomass burning smoke during the PRIDE-PRD2006 campaign from 1–30 July 2006 at a rural site ~60 km northwest of the mega-city Guangzhou in southeastern China. <br><br> CCN efficiency spectra (activated fraction vs. dry particle diameter; 20–290 nm) were recorded at water vapor supersaturations (<i>S</i>) in the range of 0.068% to 1.27%. The corresponding effective hygroscopicity parameters describing the influence of particle composition on CCN activity were in the range of κ≈0.1–0.5. The campaign average value of κ=0.3 equals the average value of κ for other continental locations. During a strong local biomass burning event, the average value of κ dropped to 0.2, which can be considered as characteristic for freshly emitted smoke from the burning of agricultural waste. At low <i>S</i> (≤0.27%), the maximum activated fraction remained generally well below one, indicating substantial portions of externally mixed CCN-inactive particles with much lower hygroscopicity – most likely soot particles (up to ~60% at ~250 nm). <br><br> The mean CCN number concentrations (<i>N</i><sub>CCN,S</sub>) ranged from 1000 cm<sup>−3</sup> at <i>S</i>=0.068% to 16 000 cm<sup>−3</sup> at <i>S</i>=1.27%, which is about two orders of magnitude higher than in pristine air. Nevertheless, the ratios between CCN concentration and total aerosol particle concentration (integral CCN efficiencies) were similar to the ratios observed in pristine continental air (~6% to ~85% at <i>S</i>=0.068% to 1.27%). Based on the measurement data, we have tested different model approaches for the approximation/prediction of <i>N</i><sub>CCN,S</sub>. Depending on <i>S</i> and on the model approach, the relative deviations between observed and predicted <i>N</i><sub>CCN,S</sub> ranged from a few percent to several hundred percent. The largest deviations occurred at low <i>S</i> with a simple power law. With a Köhler model using variable κ values obtained from individual CCN efficiency spectra, the relative deviations were on average less than ~10% and hardly exceeded 20%, confirming the applicability of the κ-Köhler model approach for efficient description of the CCN activity of atmospheric aerosols. Note, however, that different types of κ-parameters must be distinguished for external mixtures of CCN-active and -inactive aerosol particles (κ<sub>a</sub>, κ<sub>t</sub>, κ<sub>cut</sub>). Using a constant average hygroscopicity parameter (κ=0.3) and variable size distributions as measured, the deviations between observed and predicted CCN concentrations were on average less than 20%. In contrast, model calculations using variable hygroscopicity parameters as measured and constant size distributions led to much higher deviations: ~70% for the campaign average size distribution, ~80% for a generic rural size distribution, and ~140% for a generic urban size distribution. These findings confirm earlier studies suggesting that aerosol particle number and size are the major predictors for the variability of the CCN concentration in continental boundary layer air, followed by particle composition and hygroscopicity as relatively minor modulators. Depending on the required and applicable level of detail, the information and parameterizations presented in this study should enable efficient description of the CCN activity of atmospheric aerosols in detailed process models as well as in large-scale atmospheric and climate models.
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China disposes of bulk Municipal Solid Waste (MSW) by landfilling, resulting in a large quantity of mercury that enters landfills through waste. A detailed study on atmospheric mercury emissions from MSW landfills in China is necessary to understand mercury behavior from this source. Between 2003 and 2006, mercury airborne emissions through different pathways, as well as mercury speciation in Landfill Gas (LFG) were measured at 5 MSW landfills in Guiyang and Wuhan, China. The results showed that mercury content in the substrate increased the magnitude of mercury emissions, with the highest emission rate measured at the working face and in uncovered waste areas, and the lowest measured near soil covers and vegetated areas. Meteorological parameters, especially solar radiation, influenced the diurnal pattern of mercury surface-air emissions. Total Gaseous Mercury (TGM) in LFG varied from 2.0 to 1406.0 ng m<sup>−3</sup>, Monomethyl Mercury (MMHg) and Dimethyl Mercury (DMHg) in LFG averaged at 1.93 and 9.21 ng m<sup>−3</sup>, and accounted for 0.51% and 1.79% of the TGM in the LFG, respectively. Total mercury emitted from the five landfills ranged from 17 to 3300 g yr<sup>−1</sup>, with the highest from the working face, then soil covering, and finally the vent pipes.
