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Halogen Occultation Experiment (HALOE) and balloon-borne in situ measurements of methane in stratosphere and their relation to the quasi-biennial oscillation (QBO)

Measurements of methane have been made from various observational platforms in the atmosphere. In this article, we have compared four high precision balloon-borne measurements from Hyderabad (17.5°N), India in the period of 1987 and 1998 with a part of HALOE/UARS global observations available since 1991. All the balloon measurements correspond to boreal spring (March and April) but belong to different years. A comparison shows fairly good agreement with each other. The gradient in CH4 profiles in the troposphere is controlled by the variation in vertical transport. The strongest effect of dynamical influence on methane vertical profiles is shown to be resulting from the dynamical quasi-biennial oscillation in the stratosphere, and that has been consistently derived from both the measurement techniques and chemistry-transport model simulations. It is observed that the QBO signal in CH4 anomaly exhibits interhemispheric asymmetry caused by the tropics to midlatitude circulation in the stratosphere. A mechanism is suggested to explain how and to what extent the methane vertical profiles over Hyderabad and higher latitudes could be modulated by the prevailing QBO winds in the tropics. We have also discussed how the same mechanism would affect ozone distribution in the stratosphere quite differently.
Laboratory study on heterogeneous decomposition of methyl chloroform on various standard aluminosilica clay minerals as a potential tropospheric sink

Methyl chloroform (1,1,1-trichloroethane, CH3CCl3) was found to decompose heterogeneously on seven types of standard clay minerals (23 materials) in dry air at 313 K in the laboratory. All reactions proceeded through the elimination of HCl; CH3CCl3 was converted quantitatively to CH2=CCl2. The activities of the clay minerals were compared via their pseudo-first-order reaction rate constants (k1). A positive correlation was observed between the k1 value and the specific surface area (S) of clay minerals, where the S value was determined by means of the general Brunauer-Emmett-Teller (BET) equation. The k1 value was anti-correlated with the value of n, which was a parameter of the general BET equation and related to the average pore size of the clay minerals, and correlated with the water content that can be removed easily from the clay minerals. The reaction required no special pretreatment of clay minerals, such as heating at high temperatures; hence, the reaction can be expected to occur in the environment. Photoillumination by wavelengths present in the troposphere did not accelerate the decomposition of CH3CCl3, but it induced heterogeneous photodecomposition of CH2=CCl2. The temperature dependence of k1, the adsorption equilibrium coefficient of CH3CCl3 and CH2=CCl2, and the surface reaction rate constant of CH3CCl3 were determined for an illite sample. The k1 value increased with increasing temperature. The amount of CH3CCl3 adsorbed on the illite during the reaction was proportional to the partial pressure of CH3CCl3. The reaction was sensitive to relative humidity and the k1 value decreased with increasing relative humidity. However, the reaction was found to proceed at a relative humidity of 22% at 313 K, although the k1 value was about one-twentieth of the value in non-humidified air. The conditions required for the reaction may be present in major desert regions of the world. A simple estimation indicates that the possible heterogeneous decomposition of CH3CCl3 on the ground surface in arid regions is worth taking into consideration when inferring the tropospheric lifetime of CH3CCl3 and global OH concentration from the global budget concentration of CH3CCl3.
Ultrathin Tropical Tropopause Clouds (UTTCs): II. Stabilization mechanisms

Mechanisms by which subvisible cirrus clouds (SVCs) might contribute to dehydration close to the tropical tropopause are not well understood. Recently Ultrathin Tropical Tropopause Clouds (UTTCs) with optical depths around 10-4 have been detected in the western Indian ocean. These clouds cover thousands of square kilometers as 200-300 m thick distinct and homogeneous layer just below the tropical tropopause. In their condensed phase UTTCs contain only 1-5% of the total water, and essentially no nitric acid. A new cloud stabilization mechanism is required to explain this small fraction of the condensed water content in the clouds and their small vertical thickness. This work suggests a mechanism, which forces the particles into a thin layer, based on upwelling of the air of some mm/s to balance the ice particles, supersaturation with respect to ice above and subsaturation below the UTTC. In situ measurements suggest that these requirements are fulfilled. The basic physical properties of this mechanism are explored by means of a single particle model. Comprehensive 1-D cloud simulations demonstrate this stabilization mechanism to be robust against rapid temperature fluctuations of +/- 0.5 K. However, rapid warming (Δ T > 2 K) leads to evaporation of the UTTC, while rapid cooling (Δ T < -2 K) leads to destabilization of the particles with the potential for significant dehydration below the cloud
The rate of water vapor evaporation from ice substrates in the presence of HCl and HBr: implications for the lifetime of atmospheric ice particles

Using a multidiagnostic approach the rate Rev [ molec cm-3 s-1] or flux Jev [ molec cm-2 s-1] of evaporation of H2O and its corresponding rate constant for condensation, kcond [s-1 ], on a 1 µm thick ice film have been studied in the temperature range 190 to 240 K as well as in the presence of small amounts of HCl and HBr that left the vapor pressure of H2O on ice unchanged. The resulting Arrhenius expressions for pure ice are Jev = 1.6 · 10 28 ± 1 · exp  (- 10.3 ± 1.2/ RT)  [ molec cm-2 s-1] , kcond = 1.7 · 10 - 2 ± 1 · exp  (+ 1.6 ± 1.5/ RT ) [s -1], in the presence of a HCl mole fraction in the range 3.2 · 10 - 5 - 6.4 · 10 - 3 : Jev = 6.4 · 10 26 ± 1 · exp  (- 9.7 ± 1.2/ RT)  [ molec cm-2 s-1] , kcond = 2.8 · 10 - 2 ± 1 · exp ( + 1.5 ± 1.6 /RT)  [s -1], and a HBr mole fraction smaller than 6.4 · 10 - 3 : Jev = 7.4 · 10 25 ± 1 · exp ( - 9.1 ± 1.2 /RT)  [ molec cm-2 s-1] , kcond = 7.1 · 10 - 5 ± 1 · exp (+ 2.6 ± 1.5/ RT) [s -1]. The small negative activation energy for H2O condensation on ice points to a precursor mechanism. The corresponding enthalpy of sublimation is DHsubl = Eev - Econd = 11.9 ± 2.7 kcal mol-1 , DHsubl = 11.2 ± 2.8 kcal mol-1, and DHsubl = 11.7 ± 2.8 kcal mol-1 whose values are identical within experimental uncertainty to the accepted literature value of 12.3 kcal mol-1 . Interferometric data at 633 nm and FTIR absorption spectra in transmission support the kinetic results. The data are consistent with a significant lifetime enhancement for HCl- and HBr-contaminated ice particles by a factor of 3–6 and 10–20, respectively, for submonolayer coverages of HX once the fraction of the ice not contaminated by HX has evaporated.
First observations of noctilucent clouds by lidar at Svalbard, 78°N

In summer 2001 a potassium lidar was installed near Longyearbyen (78° N) on the north polar island of Spitsbergen which is part of the archipelago Svalbard. At the same place a series of meteorological rockets ("falling spheres", FS) were launched which gave temperatures from the lower thermosphere to the stratosphere. The potassium lidar is capable of detecting noctilucent clouds (NLCs) and of measuring temperatures in the lower thermosphere, both under daylight conditions. In this paper we give an overview on the NLC measurements (the first at this latitude) and compare the results with temperatures from meteorological rockets which have been published recently (Lübken and Mülleman, 2003) NLCs were observed from 12 June (the first day of operation) until 12 August when a period of bad weather started. When the lidar was switched on again on 26 August, no NLC was observed. The mean occurrence frequency in the period 12 June -- 12 August ("lidar NLC period") is 77%. The mean of all individual NLC peak altitudes is 83.6 km (variability: 1.1 km). The mean peak NLC altitude does not show a significant variation with season. The average top and bottom altitude of the NLC layer is 85.1 and 82.5 km, respectively, with a variability of ~1.2 km. The mean of the maximum volume backscatter coefficient bmax at our wavelength of 770 nm is 3.9 x 10-10/m/sr with a large variability of ±3.8 x 10-10/m/sr. Comparison of NLC characteristics with measurements at ALOMAR (69° N) shows that the peak altitude and the maximum volume backscatter coefficient are similar at both locations but NLCs occur more frequently at higher latitudes. Simultaneous temperature and NLC measurements are available for 3 flights and show that the NLC layer occurs in the lower part of the height range with super-saturation. The NLC peak occurs over a large range of degree of saturation (S) whereas most models predict the peak at S = 1. This demonstrates that steady-state considerations may not be applicable when relating individual NLC properties to background conditions. On the other hand, the mean variation of the NLC appearance with height and season is in agreement with the climatological variation of super-saturation derived from the FS temperature measurements.
Occurrence of ozone anomalies over cloudy areas in TOMS version-7 level-2 data

This study investigates anomalous ozone distributions over cloudy areas in Nimbus-7 (N7) and Earth-Probe (EP) TOMS version-7 data and analyzes the causes for ozone anomaly formation. A 5°-longitude by 5°-latitude region is defined to contain a Positive Ozone Anomaly (POA) or Negative Ozone Anomaly (NOA) if the correlation coefficient between total ozone and reflectivity is > 0.5 or < -0.5. The average fractions of ozone anomalies among all cloud fields are 31.8 ± 7.7% and 35.8 ± 7.7% in the N7 and EP TOMS data, respectively. Some ozone anomalies are caused by ozone retrieval errors, and others are caused by actual geophysical phenomena. Large cloud-height errors are found in the TOMS version-7 algorithm in comparison to the Temperature Humidity Infrared Radiometer (THIR) cloud data. On average, cloud-top pressures are overestimated by ~200 hPa (THIR cloud-top pressure < 200 hPa) for high-altitude clouds and underestimated by ~150 hPa for low-altitude clouds (THIR cloud-top pressure > 750 hPa). Most tropical NOAs result from negative errors induced by large cloud-height errors, and most tropical POAs are caused by positive errors due to intra-cloud ozone absorption enhancement. However, positive and negative errors offset each other, reducing the ozone anomaly occurrence in TOMS data. Large ozone/reflectivity slopes for mid-latitude POAs show seasonal variation consistent with total ozone fluctuation, indicating that they result mainly from synoptic and planetary wave disturbances. POAs with an occurrence fraction of 30--60% occur in regions of marine stratocumulus off the west coast of South Africa and off the west coast of South America. Both fractions and ozone/reflectivity slopes of these POAs show seasonal variations consistent with that in the tropospheric ozone. About half the ozone/reflectivity slope can be explained by ozone retrieval errors over clear and cloudy areas. The remaining slope may result from there being more ozone production because of rich ozone precursors and higher photolysis rates over high-frequency, low-altitude clouds than in clear areas. Ozone anomalies due to ozone retrieval errors have important implications in TOMS applications such as tropospheric ozone derivation and analysis of ozone seasonal variation.
A quantitative test of infrared optical constants for supercooled sulphuric and nitric acid droplet aerosols

In situ Fourier transform infrared (FTIR) extinction spectra of supercooled H2SO4/H2O and HNO3/H2O solution droplets were recorded in the large coolable aerosol chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe for a range of aerosol compositions and at temperatures extending down to 192 K. The measured spectra were quantitatively analysed in terms of aerosol composition and mass concentration by using Mie theory in combination with published refractive index data as input parameters. Simultaneously, total sulphuric acid and nitric acid mass concentrations from filter analysis and total water concentrations measured with the Lyman-a hygrometer of Forschungszentrum Jülich were used to calculate the aerosol composition at thermodynamic equilibrium inside the aerosol chamber. By comparing these measured aerosol parameters with those retrieved from the analysis of the FTIR spectra, the accuracy of the literature data sets of refractive indices could be assessed. In summary, four data sets were tested in the H2SO4/H2O system as well as two data sets in the HNO3/H2O system, partly revealing significant discrepancies in the retrieved aerosol properties. Potential explanations for these differences are discussed in this paper.
The role of volatile organic compounds in the polluted urban atmosphere of Bristol, England

The results of a field campaign carried out from early spring through to the late summer of 2000, in Bristol, England, are presented. Continuous measurements of over 40 hydrocarbons have been made at an urban background site, located at Bristol University, for approximately nine months using a Gas Chromatography - Flame Ionisation Detection (GC-FID) system and for a selection of halocarbons for approximately one month using a Gas Chromatography - Electron Capture Detection (GC-ECD) system. In this paper we present the time-series of the nine halocarbons and selected hydrocarbons. Daytime and night-time hydroxyl radical concentrations have been estimated based on the diurnal variations of a selection of the measured hydrocarbons. The average summer daytime concentration of OH was found to be 2.5x106 molecules cm-3 and the night-time concentration to be in the range 104 to 105 molecules cm-3. In addition, the role played by certain VOCs in the formation of ozone is assessed using the POCP (Photochemical Ozone Creation Potential) concept.
Out of Africa: High aerosol concentrations in the upper troposphere over Africa

In the year 2000, six flights (three southbound and three northbound) of the CARIBIC project were conducted between Germany and two destinations in the southern hemisphere (Windhoek, Namibia and Cape Town, South Africa). In the present report, results on particle number concentrations are discussed in three size ranges (>4 nm, >12 nm, and >18 nm particle diameter) during the unique transequatorial Africa flights. The flights covered a total of about 80 h in May, July, and December. Thus, no claim can be made for long-term representativeness of the aerosol data. Nevertheless, they are the first upper systematic tropospheric transequatorial aerosol profiles over Africa. The average aerosol results show a broad maximum, roughly symmetrical to the equator, which compares well in latitudinal extent to a maximum of CO concentrations measured on the same flights. This export of continental surface aerosol to the upper troposphere will be dispersed on a global scale both with the easterly flow near the equator and with the westerlies in the adjacent subtropical regions. There was strong evidence of recent new particle formation before aerosol arrival at flight level, in particular during the time periods between 9:00 and 13:00 local time over Africa. Direct and indirect climate effects of the respective particulate matter remain to be investigated by future flights with the ongoing extension of the CARIBIC payload towards size-resolved measurements above 100 nm particle diameter. At the same time global chemical transport models and aerosol dynamics models need to be extended to be able to reproduce the CARIBIC findings over Africa.
Atmospheric impact of the 1783–1784 Laki Eruption: Part II Climatic effect of sulphate aerosol

The long 1783-1784 eruption of Laki in southern Iceland, was one of the first eruptions to have been linked to an observed climate anomaly, having been held responsible for cold temperatures over much of the Northern Hemisphere in the period 1783-1785. Results from the first climate model simulation of the impact of a similar eruption to that of 1783-1784 are presented. Using sulphate aerosol fields produced in a companion chemical transport model simulation by Stevenson et al. (2003), the radiative forcing and climate response due to the aerosol are calculated here using the Reading Intermediate General Circulation Model (IGCM). The peak Northern Hemisphere mean direct radiative forcing is -5.5 Wm-2 in August 1783. The radiative forcing dies away quickly as the emissions from the volcano decrease; however, a small forcing remains over the Mediterranean until March 1784. There is little forcing in the Southern Hemisphere. There is shown to be an uncertainty of at least 50% in the direct radiative forcing due to assumptions concerning relative humidity and the sophistication of the radiative transfer code used. The indirect effects of the Laki aerosol are potentially large but essentially unquantifiable at the present time. In the IGCM at least, the aerosol from the eruption produces a climate response that is spatially very variable. The Northern Hemisphere mean temperature anomaly averaged over the whole of the calendar year containing most of the eruption is -0.21 K, statistically significant at the 95% level and in reasonable agreement with the available observations of the temperature during 1783.