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Because of the climate and air quality effects of organic aerosol, it is important to quantify the influence of anthropogenic emissions on the aerosol burden, both globally and regionally, and both in terms of mass and number. Methods exist with which the fractions of organic aerosol resulting directly from anthropogenic and biogenic processes can be estimated. However, anthropogenic emissions can also lead to an enhancement in secondary organic aerosol formation from naturally emitted precursors. We term this enhanced biogenic secondary organic aerosol (eBSOA). Here, we review the mechanisms through which such an effect may occur in the atmosphere and describe a work flow via which it may be quantified, using existing measurement techniques. An examination of published data reveals support for the existence of the enhancement effect.
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Regional and global chemical transport models underpredict NO<sub>x</sub> (NO + NO<sub>2</sub>) in the upper troposphere where it is a precursor to the greenhouse gas ozone. The NO<sub>x</sub> bias has been shown in model evaluations using aircraft data (Singh et al., 2007) and total column NO<sub>2</sub> (molecules cm<sup>−2</sup>) from satellite observations (Napelenok et al., 2008). The causes of NO<sub>x</sub> underpredictions have yet to be fully understood due to the interconnected nature of simulated emission, transport, and chemistry processes. Recent observation-based studies, in the upper troposphere, identify chemical rate coefficients as a potential source of error (Olson et al., 2006; Ren et al., 2008). Since typical chemistry evaluation techniques are not available for upper tropospheric conditions, this study develops an evaluation platform from in situ observations, stochastic convection, and deterministic chemistry. We derive a stochastic convection model and optimize it using two simulated datasets of time since convection, one based on meteorology, and the other on chemistry. The chemistry surrogate for time since convection is calculated using seven different chemical mechanisms, all of which predict shorter time since convection than our meteorological analysis. We evaluate chemical simulations by inter-comparison and by pairing results with observations based on NO<sub>x</sub>:HNO<sub>3</sub>, a photochemical aging indicator. Inter-comparison reveals individual chemical mechanism biases and recommended updates. Evaluation against observations shows that all chemical mechanisms overpredict NO<sub>x</sub> removal relative to long-lived methanol and carbon monoxide. All chemical mechanisms underpredict observed NO<sub>x</sub> by at least 30%, and further evaluation is necessary to refine simulation sensitivities to initial conditions and chemical rate uncertainties.
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The Community Multiscale Air Quality (CMAQ) model version 4.6 has been revised with regard to the representation of chlorine (HCl, ClNO<sub>2</sub>) and sulfur (dimethylsulfide, or DMS, and H<sub>2</sub>S), and evaluated against observations and earlier published models. Chemistry parameterizations were based on published reaction kinetic data and a recently developed cloud chemistry model that includes heterogeneous reactions of organic sulfur compounds. Evaluation of the revised model was conducted using a recently enhanced data base of natural emissions that includes ocean and continental sources of DMS, H<sub>2</sub>S, chlorinated gases and lightning NO<sub>x</sub> for the continental United States and surrounding regions. Results using 2002 meteorology and emissions indicated that most simulated "natural" (plus background) chemical and aerosol species exhibit the expected seasonal variations at the surface. Ozone exhibits a winter and early spring maximum consistent with ozone data and an earlier published model. Ozone distributions reflect the influences of atmospheric dynamics and pollutant background levels imposed on the CMAQ simulation by boundary conditions derived from a global model. A series of model experiments reveals that the consideration of gas-phase organic sulfur chemistry leads to sulfate aerosol increases over most of the continental United States. Cloud chemistry parameterization changes result in widespread decreases in SO<sub>2</sub> across the modeling domain and both increases and decreases in sulfate. Most cloud-mediated sulfate increases occurred mainly over the Pacific Ocean (up to about 0.1 μg m<sup>−3</sup>) but also over and downwind from the Gulf of Mexico (including parts of the eastern US). Geographic variations in simulated SO<sub>2</sub> and sulfate are due to the link between DMS/H<sub>2</sub>S and their byproduct SO<sub>2</sub>, the heterogeneity of cloud cover and precipitation (precipitating clouds act as net sinks for SO<sub>2</sub> and sulfate), and the persistence of cloud cover (the largest relative sulfate increases occurred over the persistently cloudy Gulf of Mexico and western Atlantic Ocean). Overall, the addition of organic sulfur chemistry increased hourly surface sulfate levels by up to 1–2 μg m<sup>−3</sup> but reduced sulfate levels in the vicinity of high SO<sub>2</sub> emissions (e.g., wildfires). Simulated surface levels of DMS compare reasonably well with observations in the marine boundary layer where DMS oxidation product levels are lower than observed. This implies either a low bias in model oxidation rates of organic sulfur species or a low bias in the boundary conditions for DMS oxidation products. This revised version of CMAQ provides a tool for realistically simulating the influence of natural emissions on air quality.
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Inter-hemispheric coupling between the polar summer mesosphere and planetary-wave activity in the extra-tropical winter stratosphere has recently been inferred using Polar Mesospheric Cloud (PMC) properties as a proxy for mesospheric temperature (Karlsson et al., 2007). Here we confirm these results using a ten-year time series of July mesospheric temperatures near 60° N derived from the hydroxyl (OH) nightglow. In addition, we show that the time-lagged correlation between these summer mesospheric temperatures and the ECMWF winter stratospheric temperatures displays a strong Quasi-Biennial Oscillation (QBO). The sign and phase of the correlation is consistent with the QBO modulation of the extra-tropical stratospheric dynamics in the Southern Hemisphere via the Holton-Tan mechanism (Holton and Tan, 1980). This lends strength to the identification of synoptic and planetary waves as the driver of the inter-hemispheric coupling, and results in a strong QBO modulation of the polar summer mesospheric temperatures.
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Aerosol properties were measured during an airborne campaign experiment that took place in June 2006 in West Africa within the framework of the African Monsoon Multidisciplinary Analyses (AMMA). The goal of the present study was to investigate a dynamical mechanism able to facilitate the sedimentation of dust particles from the Saharan Air Layer (SAL) into the boundary layer. A significant change in the dust particle concentration measured along the meridian between Niamey (Niger) and Cotonou (Benin) was found in the boundary layer (~700 m), where the dust particle concentration increased in a zone where local emission is not possible. Moreover, the boundary layer top observed with the dropsondes launched with the F-F20 shows a strong relationship with the surface cover anomalies, with higher Boundary Layer (BL) tops over the warmer surfaces, such as croplands, as opposed to adjacent forest. A mesoscale atmospheric model with a new on-line dust parameterization, resulting from the Alfaro and Gomes (2001) parametrisation and AMMA observations, was used to interpret the impact of vegetation anomalies on dust particle sedimentation. The results of the simulation are consistent with the observations, with higher dust concentration over the warm surface cover anomalies.
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Satellite observations show that ice cloud effective radius (<i>r</i><sub>e</sub>) increases with ice water content (IWC) but decreases with aerosol optical thickness (AOT). Using least-squares fitting to the observed data, we obtain an analytical formula to describe the variations of <i>r</i><sub>e</sub> with IWC and AOT for several regions with distinct characteristics of <i>r</i><sub>e</sub>-IWC-AOT relationships. As IWC directly relates to convective strength and AOT represents aerosol loading, our empirical formula provides a means to quantify the relative roles of dynamics and aerosols in controlling <i>r</i><sub>e</sub> in different geographical regions, and to establish a framework for parameterization of aerosol effects on <i>r</i><sub>e</sub> in climate models.
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Between April and December 2008 the CARIBIC commercial aircraft conducted monthly measurement flights between Frankfurt, Germany and Chennai, India. These flights covered the period of the Asian summer monsoon (June–September), during which enhancements in a number of atmospheric species were observed in the upper troposphere over southwestern Asia. In addition to in situ measurements of trace gases and aerosols, whole air samples were collected during the flights, and these were subsequently analyzed for a suite of trace gases that included a number of C<sub>2</sub>–C<sub>8</sub> non-methane hydrocarbons. Non-methane hydrocarbons are relatively short-lived compounds and the large enhancements in their mixing ratios in the upper troposphere over southwestern Asia during the monsoon, sometimes more than double their spring and fall means, provides qualitative evidence for the influence of convectively uplifted boundary layer air. The particularly large enhancements of the combustion tracers benzene and ethyne, along with the similarity of their ratios with carbon monoxide and emission ratios from the burning of household biofuels, indicate a strong influence of biofuel burning to NMHC emissions in this region. Conversely, the ratios of ethane and propane to carbon monoxide, along with the ratio between i-butane and n-butane, indicate a significant source of these compounds from the use of fossil fuels, and comparison to previous campaigns suggests that this source could be increasing. Photochemical aging patterns of NMHCs showed that the CARIBIC samples were collected in two distinctly different regions of the monsoon circulation: a southern region where air masses had been recently influenced by low level contact and a northern region, where air parcels had spent substantial time in transit in the upper troposphere before being probed. Estimates of age using ratios of individual NMHCs have ranges of 3–6 days in the south and 9–12 days in the north.
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Emissions of CO<sub>2</sub> from fossil fuel combustion are a critical quantity that must be accurately given in established flux inversion frameworks. Work with emerging satellite-based inversions requires spatiotemporally-detailed inventories that permit analysis of regional natural sources and sinks. Conventional approaches for disaggregating national emissions beyond the country and city levels based on population distribution have certain difficulties in their application. We developed a global 1 km×1 km annual fossil fuel CO<sub>2</sub> emission inventory for the years 1980–2007 by combining a worldwide point source database and satellite observations of the global nightlight distribution. In addition to estimating the national emissions using global energy consumption statistics, emissions from point sources were estimated separately and were spatially allocated to exact locations indicated by the point source database. Emissions from other sources were distributed using a special nightlight dataset that had fewer saturated pixels compared with regular nightlight datasets. The resulting spatial distributions differed in several ways from those derived using conventional population-based approaches. Because of the inherent characteristics of the nightlight distribution, source regions corresponding to human settlements and land transportation were well articulated. Our distributions showed good agreement with a high-resolution inventory across the US at spatial resolutions that were adequate for regional flux inversions. The inventory can be extended to the future using updated data, and is expected to be incorporated into models for operational flux inversions that use observational data from the Japanese Greenhouse Gases Observing SATellite (GOSAT).
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Aerosol particles in the atmosphere are important participants in the formation of cloud droplets and have significant impact on cloud albedo and global climate. According to the Köhler theory which describes the nucleation and the equilibrium growth of cloud droplets, the surface tension of an aerosol droplet is one of the most important factors that determine the critical supersaturation of droplet activation. In this paper, with specific interest to remote marine aerosol, we predict the surface tension of aerosol droplets by performing molecular dynamics simulations on two model systems, the pure water droplets and glycine in water droplets. The curvature dependence of the surface tension is interpolated by a quadratic polynomial over the nano-sized droplets and the limiting case of a planar interface, so that the so-called Aitken mode particles which are critical for droplet formation could be covered and the Köhler equation could be improved by incorporating surface tension corrections.
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A layered structure of aerosol particles from surface to 1080 m was observed in Beijing during daytime on 8 December 2007. Under about 700 m, particles were well mixed vertically. From 700 to 1000 m was an elevated aerosol layer (EAL), in which aerosol concentration was remarkably larger than those in the lower and upper layers. Electron microscopic analysis of particles in the size range of 0.2~1.3 μm at different altitudes revealed that soot particles were predominant in all layers. There were fresh, young and aged soot particles in the lower layer. In contrast, soot particles in the EAL were all well aged, showing the structure of shrunk soot inclusions coated with weak absorbing species (the so-called core-shell structure). The geometric mean equivalent diameter of soot particles in the EAL was approximately 0.4~0.6 μm while that of their soot inclusions was about 0.1 μm. The EAL coincided with the remaining nocturnal layer aloft, which was the residual left by the daytime upward convective mixing in the boundary layer. The lapse rate in the lower part of the EAL had an obvious decrease in the morning, indicating the absorption effect of soot particles there. These results suggest that the fate of soot particles was closely dependent on the evolution of the boundary layer. While particles emitted from surface were efficiently mixed upward in daytime, residual nocturnal layer as a cap lid produced an EAL abundant in well-aged soot particles, in which the aerosols, as a feedback, enhanced the stability of the layer by absorbing solar radiation.
