Cloud Base Signature in Transmission Spectra of Exoplanet Atmospheres

We present an analytical model for the transmission spectrum of a transiting exoplanet, showing that a cloud base can produce an observable inflection point in the spectrum. The wavelength and magnitude of the inflection can be used to break the degeneracy between the atmospheric pressure and the abundance of the main cloud material, however, the abundance still depends on cloud particle size. An observed inflection also provides a specific point on the atmospheric P-T profile, giving us a "thermometer" to directly validate or rule out postulated cloud species. We apply the model to the transit spectrum of HD 189733b

Characterization of Secondary Organic Aerosol (SOA) formed by the Reaction of β-caryophyllene, Soot and Ozone: Climate Impact

Diesel soot (black carbon, BC) is an important light absorbing aerosol component in atmosphere that can cause tropospheric heating. Laboratory studies have found it to be unreactive to ozone at ambient temperature. The low uptake coefficient i.e., γ 300 K = 2× 10-7 , of the soot-O3 reaction indicates a low probability of irreversible O3 loss from gas phase to surface-adsorbed product (Particle phase). This shows clearly that at low temperature soot is not reactive with atmospheric oxidants. In contrast, sesquiterpenes (SQT) such as β-caryophyllene (C15H24), which are produced primarily by plants, are extremely reactive with ozone. For example, the residence time of β- caryophyllene in the atmosphere is only 2 min in the presence of 60 ppb ozone. Thus, ozonolysis reaction of β-caryophyllene is expected to be a significant source of biogenic secondary organic aerosols. These oxidized products may condense onto soot particles, and a question arises as to how they will partition between the soot surface, vapor phase, and aqueous aerosol phases. Liquid chromatography- mass spectrometry (LC/MS), Fourier transform infrared (FTIR) and UVVis spectroscopies are being used to study the β- caryophyllene-dark ozonolysis reaction at low ozone levels (40-60 ppb). Products identified include low molecular weight highly volatile and water soluble products such as formaldehyde, acetaldehyde, acetone, and acetic acid. Also identified are high molecular weight components (~350 Dalton) with lower water solubility and vapor pressures. The SOA coatings of these SQTs on soot are being evaluated to determine their hygroscopicity. As these compounds absorb in the IR and UV-Vis they can add to radiative forcing by submicron aerosols and need to be better understood for climate modeling

An assessment of the tea bag index method as a proxy for organic matter decomposition in intertidal environments

This work was supported by the Natural Environment Research Council (grant NE/R010846/1) Carbon in Storage in Intertidal Environments (C-SIDE) project.Intertidal wetlands capture and store carbon (C) for long periods of time, helping to reduce the concentration of CO2 in the atmosphere. Yet the processes which govern the decomposition and subsequent long‐term storage of organic matter (OM) and C in these habitats remains poorly understood. The Tea Bag Index (TBI) uses a standardized OM (green and Rooibos tea) and has the potential to shed light on OM decomposition across habitats, including saltmarshes. Here, we apply the TBI method at two saltmarshes within the same estuary with the aim of (i) reducing the influence of climatic variables and (ii) determining the role of the environment, including the soil characteristics, in the decomposition of OM. We extended the standard (3 months) incubation period over a full year in order to investigate the longer‐term decomposition processes at each site. The initial results partially support previous studies that the early stages of decomposition (leaching of the water‐soluble fraction) is governed by climatic conditions, but may be further enhanced by tidal flushing in saltmarshes. By extending the incubation period, we observed the initiation of mid‐stage OM decomposition (Cellulose degradation) upon which the soil characteristics appear to be the dominant control. These results highlight the importance of long‐term TBI incubations to understand early‐stage OM decomposition. The relationship between tea mass (OM) loss and C loss in these intertidal environments is not straightforward and we would caution the use of the TBI as a direct universal proxy for soil C degradation in such intertidal wetlands.Publisher PDFPeer reviewe

The Near-Infrared and Optical Spectra of Methane Dwarfs and Brown Dwarfs

We identify the pressure--broadened red wings of the saturated potassium resonance lines at 7700 \AA as the source of anomalous absorption seen in the near-infrared spectra of Gliese 229B and, by extension, of methane dwarfs in general. This conclusion is supported by the recent work of Tsuji {\it et al.} 1999, though unlike them we find that dust need not be invoked to explain the spectra of methane dwarfs shortward of 1 micron. We find that a combination of enhanced alkali abundances due to rainout and a more realistic non-Lorentzian theory of resonant line shapes may be all that is needed to properly account for these spectra from 0.5 to 1.0 microns. The WFPC2 $I$ measurement of Gliese 229B is also consistent with this theory. Furthermore, a combination of the blue wings of this K I resonance doublet, the red wings of the Na D lines at 5890 \AA, and, perhaps, the Li I line at 6708 \AA can explain in a natural way the observed WFPC2 $R$ band flux of Gliese 229B. Hence, we conclude that the neutral alkali metals play a central role in the near-infrared and optical spectra of methane dwarfs and that their lines have the potential to provide crucial diagnostics of brown dwarfs. We speculate on the systematics of the near-infrared and optical spectra of methane dwarfs, for a given mass and composition, that stems from the progressive burial with decreasing \teff of the alkali metal atoms to larger pressures and depths.Comment: Revised and accepted to Ap.J. volume 531, March 1, 2000, also available at http://jupiter.as.arizona.edu/~burrows/papers/BMS.p

Modeling the choices of individual decision-makers by combining efficient choice experiment designs with extra preference information

We show how to combine statistically efficient ways to design discrete choice experiments based on random utility theory with new ways of collecting additional information that can be used to expand the amount of available choice information for modeling the choices of individual decision makers. Here we limit ourselves to problems involving generic choice options and linear and additive indirect utility functions, but the approach potentially can be extended to include choice problems with non-additive utility functions and non-generic/labeled options/attributes. The paper provides several simulated examples, a small empirical example to demonstrate proof of concept, and a larger empirical example based on many experimental conditions and large samples that demonstrates that the individual models capture virtually all the variance in aggregate first choices traditionally modeled in discrete choice experiments

Comparison of measurements of peroxyacyl nitrates and primary carbonaceous aerosol concentrations in Mexico City determined in 1997 and 2003

International audiencePeroxyacetyl nitrate (PAN) concentrations in ambient air can be a good indicator of air quality and the effectiveness of control strategies for reducing ozone levels in urban areas. As PAN is formed by the oxidation of reactive hydrocarbons in the presence of nitrogen dioxide (NO2), it is a direct measure of the peroxyacyl radical levels produced from reactive organic emissions in the urban air shed. Carbon soot, known as black carbon or elemental carbon, is a primary atmospheric aerosol species and is a good indicator of the levels of combustion emissions, particularly from diesel engines, in major cities. Mexico City is the second largest megacity in the world and has long suffered from poor air quality. Reported here are atmospheric measurements of PAN and black carbon obtained in Mexico City during the Mexico Megacity 2003 field study. These results are compared with measurements obtained earlier during the Investigación sobre Materia Particulada y Deterioro Atmosférico ? Aerosol and Visibility Research (IMADA-AVER) campaign in 1997 to obtain an estimate of the changes in emissions in Mexico City and the effectiveness of control strategies adopted during that time. Concentrations of PAN in 1997 reached a maximum of 34 ppb with an average daily maximum of 15 ppb. The PAN levels recorded in 2003 were quite different, with an average daily maximum of 3 ppb. This dramatic reduction in PAN levels observed in 2003 indicate that reactive hydrocarbon emissions have been reduced in the city due to controls on olefins in liquefied petroleum gas (LPG) and also due to the significant number of newer vehicles with catalytic converters that have replaced older higher emission vehicles. In contrast, black/elemental carbon levels were similar in 1997 and 2003 indicating little improvement likely due to the lack of controls on diesel vehicles in the city. Thus, while air quality and ozone production has improved, Mexico City and other megacities continue to be a major source of black carbon aerosols, which can be an important species in determining regional radiative balance and climate