Correlation between cloud condensation nuclei concentration and aerosol optical thickness in remote and polluted regions

International audienceA large number of published and unpublished measurements of cloud condensation nuclei (CCN) concentrations and aerosol optical thickness (AOT) measurements have been analyzed. AOT measurements were obtained mostly from the AERONET network, and selected to be collocated as closely as possible to the CCN investigations. In remote marine regions, CCN0.4 (CCN at a supersaturation of 0.4%) are around 110 cm?3 and the mean AOT500 (AOT at 500 nm) is 0.057. Over remote continental areas, CCN are almost twice as abundant, while the mean AOT500 is ca. 0.075. (Sites dominated by desert dust plumes were excluded from this analysis.) Some, or maybe even most of this difference must be because even remote continental sites are in closer proximity to pollution sources than remote marine sites. This suggests that the difference between marine and continental levels must have been smaller before the advent of anthropogenic pollution. Over polluted marine and continental regions, the CCN concentrations are about one magnitude higher than over their remote counterparts, while AOT is about five times higher over polluted than over clean regions. The average CCN concentrations from all studies show a remarkable correlation to the corresponding AOT values, which can be expressed as a power law. This can be very useful for the parameterization of CCN concentrations in modeling studies, as it provides an easily measured proxy for this variable, which is difficult to measure directly. It also implies that, at least at large scales, the radiative and microphysical effects of aerosols on cloud physics are correlated and not free to vary independently. While this strong empirical correlation is remarkable, it must still be noted that that there is about a factor-of-four range of CCN concentrations at a given AOT, and that there remains considerable room for improvement in remote sensing techniques for measuring CCN abundance

Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols

Although the definition and measurement techniques for atmospheric 'black carbon' ('BC') or 'elemental carbon'' ('EC') have long been subjects of scientific controversy, the recent discovery of light-absorbing carbon that is not black ('brown carbon, C_brown') makes it imperative to reassess and redefine the components that make up light-absorbing carbonaceous matter (LAC) in the atmosphere. Evidence for the atmospheric presence of C_brown comes from (1) spectral aerosol light absorption measurements near specific combustion sources, (2) observations of spectral properties of water extracts of continental aerosol, (3) laboratory studies indicating the formation of light-absorbing organic matter in the atmosphere, and (4) indirectly from the chemical analogy of aerosol species to colored natural humic substances. We show that brown carbon may severely bias measurements of 'BC' and 'EC' over vast parts of the troposphere, especially those strongly polluted by biomass burning, where the mass concentration of C_brown is high relative to that of soot carbon. Chemical measurements to determine 'EC' are biased by the refractory nature of C_brown as well as by complex matrix interferences. Optical measurements of 'BC' suffer from a number of problems: (1) many of the presently used instruments introduce a substantial bias into the determination of aerosol light absorption, (2) there is no unique conversion factor between light absorption and 'EC' or 'BC' concentration in ambient aerosols, and (3) the difference in spectral properties between the different types of LAC, as well as the chemical complexity of C_brown, lead to several conceptual as well as practical complications. We also suggest that due to the sharply increasing absorption of C_brown towards the UV, single-wavelength light absorption measurements may not be adequate for the assessment of absorption of solar radiation in the troposphere. We discuss the possible consequences of these effects for our understanding of tropospheric processes, including their influence on UV-irradiance, atmospheric photochemistry and radiative transfer in clouds

The Aegean Sea as a source of atmospheric nitrous oxide and methane

During the EGAMES (Evasion of GAses from the MEditerranean Sea) expedition in July 1993 we determined the concentrations of nitrous oxide and methane in the atmosphere and in the surface waters of the Aegean Sea, the northwestern Levantine Basin, the eastern Ionian Sea and the Amvrakikos Bay. Both gases were found to be supersaturated in all sampled areas. Nitrous oxide was homogeneously distributed with a mean saturation of 105 ± 2%, showing no differences between shelf and open ocean areas, whereas methane saturation values ranged from about 1.2 times (northwestern Levantine Basin) to more than 5 times solubility equilibrium (Amvrakikos Bay estuary). Therefore the Aegean Sea and the adjacent areas were sources of atmospheric nitrous oxide and methane during the study period

Hydrogen Sulfide and Radon in and Over the Western North Atlantic Ocean

Atmospheric measurements of radon and hydrogen sulfide, and seawater measurements of total sulfide, free sulfide, and carbonyl sulfide, were made on a cruise in the western North Atlantic Ocean (October 24 to November 9, 1989). Measured values for 222Rn ranged from 3 to 70 pCi m−3, those for atmospheric hydrogen sulfide from 1 to 85 parts per trillion, and those for dissolved total and free sulfide in seawater from 33 to 930 pmol L−1 and 0 to 73 pmol L−1, respectively. A positive correlation between 222Rn and atmospheric H2S was observed. Both 222Rn and H2S were high in air masses traced back to North America. Measurements in seawater showed that uncomplexed sulfides were approximately 13% of total sulfide at 2 m depth. Atmospheric H2S and dissolved H2S in seawater were usually not far from saturation equilibrium. Our results indicate that the ocean acted at some times as a source of atmospheric H2S but more frequently as a sink. Hydrolysis of COS and atmospheric deposition of H2S both may contribute to the budget of dissolved sulfide in seawater of the western North Atlantic Ocean. On a global scale the ocean/atmosphere exchange of H2S appears to play a minor role in the atmospheric sulfur cycle

Technical Note: Characterization of a static thermal-gradient CCN counter

International audienceThe static (parallel-plate thermal-gradient) diffusion chamber (SDC) was one of the first instruments designed to measure cloud condensation nuclei (CCN) concentrations as a function of supersaturation. It has probably also been the most widely used type of CCN counter. This paper describes the detailed experimental characterization of a SDC CCN counter, including calibration with respect to supersaturation and particle number concentration. In addition, we investigated the proposed effect of lowered supersaturation because of water vapor depletion with increasing particle concentration. The results obtained give a better understanding why and in which way it is necessary to calibrate the SDC CCN counter. The calibration method is described in detail and can, in parts, be used for calibrations also for other types of CCN counters. We conclude the following: 1) it is important to experimentally calibrate SDC CCN counters with respect to supersaturation, and not only base the supersaturation on the theoretical description of the instrument; 2) the number concentration calibration needs to be performed as a function of supersaturation, also for SDC CCN counter using the photographic technique; and 3) we observed no evidence that water vapor depletion lowered the supersaturation

Water vapor release from biofuel combustion

International audienceWe report on the emission of water vapor from biofuel combustion. Concurrent measurements of carbon monoxide and carbon dioxide are used to scale the concentrations of water vapor found, and are compared to carbon in the biofuel. Fuel types included hardwood (oak and African musasa), softwood (pine and spruce, partly with green needles), and African savanna grass. The session-averaged ratio of H2O to the sum of CO and CO2 in the emissions from 16 combustion experiments ranged from 1.2 to 3.7 on average, indicating the presence of water that is not chemically bound. This biofuel moisture content ranged from 33% in the dry African hardwood, musasa, to 220% in fresh pine branches with needles. The moisture content from fresh biofuel contributes distinctly to the water vapor in biomass burning emissions, and its influence on meteorology needs to be evaluated

Additional global climate cooling by clouds due to ice crystal complexity

Ice crystal submicron structures have a large impact on the optical properties of cirrus clouds and consequently on their radiative effect. Although there is growing evidence that atmospheric ice crystals are rarely pristine, direct in situ observations of the degree of ice crystal complexity are largely missing. Here we show a comprehensive in situ data set of ice crystal complexity coupled with measurements of the cloud angular scattering functions collected during a number of observational airborne campaigns at diverse geographical locations. Our results demonstrate that an overwhelming fraction (between 61 % and 81 %) of atmospheric ice crystals sampled in the different regions contain mesoscopic deformations and, as a consequence, a similar flat and featureless angular scattering function is observed. A comparison between the measurements and a database of optical particle properties showed that severely roughened hexagonal aggregates optimally represent the measurements in the observed angular range. Based on this optical model, a new parameterization of the cloud bulk asymmetry factor was introduced and its effects were tested in a global climate model. The modelling results suggest that, due to ice crystal complexity, ice-containing clouds can induce an additional short-wave cooling effect of −1.12 W m2 on the top-of-the-atmosphere radiative budget that has not yet been considered

Dark Production: A Significant Source of Oceanic COS

Carbonyl sulfide (COS) in air and dissolved in seawater was determined during a cruise in August 1999 in the Sargasso Sea in the northwest Atlantic Ocean. Dissolved concentrations at the sea surface displayed only a weak diel cycle with a mean of 8.6 ± 2.8 pmol dm−3 owing to low abundance of photochemical precursors and high temperatures causing rapid hydrolysis. Depth profiles measured over the oceanic mixed layer revealed significant vertical gradients of COS concentration with higher values at the surface, suggesting that the rate of photochemical production at the surface exceeds the rate of vertical mixing. The mean atmospheric mixing ratio was 486 ± 40 ppt, and calculated sea-air fluxes ranged from 0.03 to 0.8 g COS km−2 d−1. COS dark production, estimated from the predawn COS concentration at the surface and the hydrolysis constant, contributed significantly to the total amount of COS produced. A strong temperature dependence of the COS dark production rate q was found by comparing previously published values. The data further indicate an approximately first-order relationship between q and chromophoric dissolved organic matter (CDOM) absorbance at 350 nm, a350, which is used as a proxy for the CDOM content of the water but is likely to covary with other parameters, such as biological activity, that could also affect COS dark production. Together with known functions for COS hydrolysis and solubility, the parameterization of dark production as a function of temperature and a350 allows for the prediction of COS concentrations and saturation ratios as a function of physical and optical seawater properties in the absence of photoproduction. This is used to estimate a lower limit of 0.056 Tg COS yr−1 to the annual COS flux from the ocean to the atmosphere