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Ethanol is an important biogenic volatile organic compound, which is increasingly used as a fuel for motor vehicles; therefore, an improved understanding of its atmospheric cycle is important. In this paper we use three sets of observational data, measured emissions of ethanol from living plants, measured concentrations of ethanol in the atmosphere and measured hydroxyl concentrations in the atmosphere (by methyl chloroform titration), to make two independent estimates related to the rate of cycling of ethanol through the atmosphere. In the first estimate, simple calculations give the emission rate of ethanol from living plants as 26 (range, 10–38) Tg yr<sup>−1</sup>. This contributes significantly to the total global ethanol source of 42 (range, 25–56) Tg yr<sup>−1</sup>. In the second estimate, the total losses of ethanol from the global atmosphere are 70 (range, 50–90) Tg yr<sup>−1</sup>, with about three-quarters of the ethanol removed by reaction with hydroxyl radicals in the gaseous and aqueous phases of the atmosphere, and the remainder lost through wet and dry deposition to land. These values of both the source of ethanol from living plants and the removal of atmospheric ethanol via oxidation by hydroxyl radicals (derived entirely from observations) are significantly larger than those in recent literature. We suggest that a revision of the estimate of global ethanol emissions from plants to the atmosphere to a value comparable with this analysis is warranted.
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Submicron aerosol physical and chemical properties in remote marine air were measured from aircraft over the Southeast Pacific during VOCALS-REx in 2008 and the North Pacific during IMPEX in 2006, and aboard a ship in the Equatorial Pacific in 2009. A High Resolution – Particle Time of Flight Aerosol Mass Spectrometer (HR-ToF-AMS) measured non-refractory submicron aerosol composition during all campaigns. Sulfate (SO<sub>4</sub>) and organics (Org), during VOCALS and the cruise show lower absolute values than those reported for previous "clean air" studies. In the marine boundary layer, average concentrations for SO<sub>4</sub> were 0.52 μg m<sup>−3</sup> for the VOCALS region and 0.85 μg m<sup>−3</sup> for the equatorial region while average Org concentrations were 0.10 and 0.07 μg m<sup>−3</sup>, respectively. Campaign average Org/SO<sub>4</sub> ratios were 0.19 (VOCALS) and 0.08 (equatorial Pacific). Black carbon (BC) measurements from a single particle soot photometer (SP2) and carbon monoxide (CO) concentrations over the Southeast Pacific provided sensitive indicators of pollution. CO and BC were used to identify the least polluted air, which had average concentrations of SO<sub>4</sub> and Org of 0.14 and 0.01 μg m<sup>−3</sup>, respectively, with an average Org/SO<sub>4</sub> of 0.08. Data from IMPEX was constrained to similar clean air criterion, and resulted in an average Org/SO<sub>4</sub> ratio of 0.19. Under the cleanest MBL conditions during VOCALS, identified by CO below 61 ppbv, a robust linear relationship between Org and BC concentrations revealed that even at very low pollution levels, combustion sources dominated organic aerosol, suggesting little to no marine source of submicrometer Org to the atmosphere over the eastern South Pacific. This means marine organics cannot be identified by merely setting a standard for background conditions below which anthropogenic influence can be disregarded. Other methods must be used to exclude non-marine sources.
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The scattered sunlight measurements made by the Optical Spectrograph and InfraRed Imaging System (OSIRIS) on the Odin spacecraft are used to retrieve vertical profiles of stratospheric aerosol extinction at 750 nm. The recently released OSIRIS Version 5 data product contains the first publicly released stratospheric aerosol extinction retrievals, and these are now available for the entire Odin mission, which extends from the present day back to launch in 2001. A proof-of-concept study for the retrieval of stratospheric aerosol extinction from limb scatter measurements was previously published and the Version 5 data product retrievals are based on this work, but incorporate several important improvements to the algorithm. One of the primary changes is the use of a new retrieval vector that greatly improves the sensitivity to aerosol scattering by incorporating a forward modeled calculation of the radiance from a Rayleigh atmosphere. Additional improvements include a coupled retrieval of the effective albedo, a new method for normalization of the retrieval vector to improve signal-to-noise, and the use of an initial guess that is representative of very low background aerosol loading conditions, which allows for maximal retrieval range. Furthermore, the Version 5 data set is compared to Stratospheric Aerosol and Gas Experiment (SAGE) III 755 nm extinction profiles during the almost four years of mission overlap from 2002 to late 2005. The vertical structure in coincident profile measurements is well correlated and the statistics on a relatively large set of tight coincident measurements show agreement between the measurements from the two instruments to within approximately 10% throughout the 15 to 25 km altitude range, which covers the bulk of the stratospheric aerosol layer for the mid and high latitude cases studied here.
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Water vapor in the tropical tropopause layer (TTL) has a local radiative cooling effect. As a source for ice in cirrus clouds, however, it can also indirectly produce infrared heating. Using NASA A-Train satellite measurements of CALIPSO and Aura/MLS we calculated the correlation of water vapor, ice water content and temperature in the TTL. We find that temperature strongly controls water vapor (correlation <i>r</i>=0.94) and cirrus clouds at 100 hPa (<i>r</i>=−0.9). Moreover we observe that the cirrus seasonal cycle is highly (<i>r</i>=−0.9) anticorrelated with the water vapor variation in the TTL, showing higher cloud occurrence during December-January-February. We further investigate the anticorrelation on a regional scale and find that the strong anticorrelation occurs generally in the ITCZ (Intertropical Convergence Zone). The seasonal cycle of the cirrus ice water content is also highly anticorrelated to water vapor (<i>r</i>=−0.91) and our results support the hypothesis that the total water at 100 hPa is roughly constant. Temperature acts as a main regulator for balancing the partition between water vapor and cirrus clouds. Thus, to a large extent, the depleting water vapor in the TTL during DJF is a manifestation of cirrus formation.
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Possible aerosol-cloud-precipitation effects over Germany are investigated using the COSMO model in a convection-permitting configuration close to the operational COSMO-DE. Aerosol effects on clouds and precipitation are modeled by using an advanced two-moment microphysical parameterization taking into account aerosol assumptions for cloud condensation nuclei (CCN) as well as ice nuclei (IN). Simulations of three summer seasons have been performed with various aerosol assumptions, and are analysed regarding surface precipitation, cloud properties, and the indirect aerosol effect on near-surface temperature. We find that the CCN and IN assumptions have a strong effect on cloud properties, like condensate amounts of cloud water, snow and rain as well as on the glaciation of the clouds, but the effects on surface precipitation are – when averaged over space and time – small. This robustness can only be understood by the combined action of microphysical and dynamical processes. On one hand, this shows that clouds can be interpreted as a buffered system where significant changes to environmental parameters, like aerosols, have little effect on the resulting surface precipitation. On the other hand, this buffering is not active for the radiative effects of clouds, and the changes in cloud properties due to aerosol perturbations may have a significant effect on radiation and near-surface temperature.
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We discuss the use of a two-dimensional volatility-oxidation space (2-D-VBS) to describe organic-aerosol chemical evolution. The space is built around two coordinates, volatility and the degree of oxidation, both of which can be constrained observationally or specified for known molecules. Earlier work presented the thermodynamics of organics forming the foundation of this 2-D-VBS, allowing us to define the <i>average</i> composition (C, H, and O) of organics, including organic aerosol (OA) based on volatility and oxidation state. Here we discuss how we can analyze experimental data, using the 2-D-VBS to gain fundamental insight into organic-aerosol chemistry. We first present a well-understood "traditional" secondary organic aerosol (SOA) system – SOA from <i>α</i>-pinene + ozone, and then turn to two examples of "non-traditional" SOA formation – SOA from wood smoke and dilute diesel-engine emissions. Finally, we discuss the broader implications of this analysis.
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During the CIRCLE-2 experiment carried out over Western Europe in May 2007, combined in situ and remote sensing observations allowed to describe microphysical and optical properties near-top of an overshooting convective cloud (11 080 m/−58 °C). The airborne measurements were performed with the DLR Falcon aircraft specially equipped with a unique set of instruments for the extensive in situ cloud measurements of microphysical and optical properties (Polar Nephelometer, FSSP-300, Cloud Particle Imager and PMS 2-D-C) and nadir looking remote sensing observations (DLR WALES Lidar). Quasi-simultaneous space observations from MSG/SEVIRI, CALIPSO/CALIOP-WFC-IIR and CloudSat/CPR combined with airborne RASTA radar reflectivity from the French Falcon aircraft flying above the DLR Falcon depict very well convective cells which overshoot by up to 600 m the tropopause level. Unusual high values of the concentration of small ice particles, extinction, ice water content (up to 70 cm<sup>−3</sup>, 30 km<sup>−1</sup> and 0.5 g m<sup>−3</sup>, respectively) are experienced. The mean effective diameter and the maximum particle size are 43 μm and about 300 μm, respectively. This very dense cloud causes a strong attenuation of the WALES and CALIOP lidar returns. The SEVIRI retrieved parameters confirm the occurrence of small ice crystals at the top of the convective cell. Smooth and featureless phase functions with asymmetry factors of 0.776 indicate fairly uniform optical properties. Due to small ice crystals the power-law relationship between ice water content (IWC) and radar reflectivity appears to be very different from those usually found in cirrus and anvil clouds. For a given equivalent reflectivity factor, IWCs are significantly larger for the overshooting cell than for the cirrus. Assuming the same prevalent microphysical properties over the depth of the overshooting cell, RASTA reflectivity profiles scaled into ice water content show that retrieved IWC up to 1 g m<sup>−3</sup> may be observed near the cloud top. Extrapolating the relationship for stronger convective clouds with similar ice particles, IWC up to 5 g m<sup>−3</sup> could be experienced with reflectivity factors no larger than about 20 dBZ. This means that for similar situations, indication of rather weak radar echo does not necessarily warn the occurrence of high ice water content carried by small ice crystals. All along the cloud penetration the shape of the ice crystals is dominated by chain-like aggregates of frozen droplets. Our results confirm previous observations that the chains of ice crystals are found in a continental deep convective systems which are known generally to generate intense electric fields causing efficient ice particle aggregation processes. Vigorous updrafts could lift supercooled droplets which are frozen extremely rapidly by homogeneous nucleation near the −37 °C level, producing therefore high concentrations of very small ice particles at upper altitudes. They are sufficient to deplete the water vapour and suppress further nucleation as confirmed by humidity measurements. These observations address scientific issues related to the microphysical properties and structure of deep convective clouds and confirm that particles smaller than 50 μm may control the radiative properties in convective-related clouds. These unusual observations may also provide some possible insights regarding engineering issues related to the failure of jet engines commonly used on commercial aircraft during flights through areas of high ice water content. However, large uncertainties of the measured and derived parameters limit our observations.
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The pattern of evaporative sources and the direction of the large-scale circulation over the Indian Ocean during the boreal summer raises the question of whether atmospheric conditions in Australia could influence conditions over the Indian subcontinent, despite the long passage of air over the Indian Ocean. The authors propose that such an influence is sometimes possible when there is unusually low temperature over inland Australia during the austral winter, through the mechanism where such a temperature extreme enhances evaporation rate over the eastern tropical Indian Ocean and hence enhances rainfall over two regions in western India after 13–19 days. Results from trajectory calculations indicate that such an influence is mechanistically feasible, with air of Australian origin contributing 0.5–1.5% of the climatological net precipitation for monsoon seasonal rainfall over western India. Statistics performed on reanalysis, satellite and in situ data are consistent with such a mechanism. Since extreme winter temperature in Australia is often associated with cold-air outbreaks, the described mechanism may be an example of how southern hemispheric mid-latitude weather can influence northern hemispheric monsoon rainfall. Further study is recommended through modelling and comparison with various known causes of atmospheric variability to confirm the existence of such a mechanism and determine the extent of its influence during specific low temperature episodes.
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Soluble iron in fine atmospheric particles has been identified as a public health concern by participating in reactions that generate reactive oxygen species (ROS). The mineralogy and oxidation state (speciation) of iron have been shown to influence fractional iron solubility (soluble iron/total iron). In this study, iron speciation was determined in single particles at urban and rural sites in Georgia USA using synchrotron-based techniques, such as X-ray Absorption Near-Edge Structure (XANES) spectroscopy and microscopic X-ray fluorescence measurements. Soluble and total iron content (soluble + insoluble iron) of these samples was measured using spectrophotometry and synchrotron-based techniques, respectively. These bulk measurements were combined with synchrotron-based measurements to investigate the relationship between iron speciation and fractional iron solubility in ambient aerosols. XANES measurements indicate that iron in the single particles was present as a mixture of Fe(II) and Fe(III), with Fe(II) content generally between 5 and 35% (mean: ~25%). XANES and elemental analyses (e.g. elemental molar ratios of single particles based on microscopic X-ray fluorescence measurements) indicate that a majority (74%) of iron-containing particles are best characterized as Al-substituted Fe-oxides, with a Fe/Al molar ratio of 4.9. The next most abundant group of particles (12%) was Fe-aluminosilicates, with Si/Al molar ratio of 1.4. No correlation was found between fractional iron solubility (soluble iron/total iron) and the abundance of Al-substituted Fe-oxides and Fe-aluminosilicates present in single particles at any of the sites during different seasons, suggesting solubility largely depended on factors other than differences in major iron phases.
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Tropospheric ozone is the third strongest greenhouse gas, and has the highest uncertainty in radiative forcing of the top five greenhouse gases. Throughout the troposphere, ozone is produced by radical oxidation of nitrogen oxides (NO<sub>x</sub> = NO + NO<sub>2</sub>). In the upper troposphere (8–10 km), current chemical transport models under-estimate nitrogen dioxide (NO<sub>2</sub>) observations. Improvements to simulated NO<sub>x</sub> production from lightning have increased NO<sub>2</sub> predictions, but the predictions in the upper troposphere remain biased low. The upper troposphere has low temperatures (<i>T</i> < 250 K) that increase the uncertainty of many important chemical reaction rates. This study constrains uncertain reaction rates by combining model predictions with measurements from the Intercontinental Chemical Transport Experiment-North America observational campaign. The results show that the nitric acid formation rate, which is the dominant sink of NO<sub>2</sub> and radicals, is currently over-estimated by 22% in the upper troposphere. The results from this study suggest that the temperature sensitivity of nitric acid formation is lower than currently recommended. Since the formation of nitric acid removes nitrogen dioxide and radicals that drive the production of ozone, the revised reaction rate will affect ozone concentrations in upper troposphere impacting climate and air quality in the lower troposphere.
