Showing papers by "Ulrich Platt published in 1999"

Short‐lived alkyl iodides and bromides at Mace Head, Ireland: Links to biogenic sources and halogen oxide production

Abstract: Automated in situ gas chromatograph/mass spectrometer (GC/MS) measurements of a range of predominantly biogenic alkyl halides in air, including CHBr3, CHBr2Cl, CH3Br, C2H5Br, CH3I, C2H5I, CH2ICl, CH2I2, and the hitherto unreported CH2IBr were made at Mace Head during a 3-week period in May 1997. C3H7I and CH3CHICH3 were monitored but not detected. Positive correlations were observed between the polyhalomethane pairs CHBr3/CHBr2Cl and CHBr3/CH2IBr and between the monohalomethane pair CH3I/C2H5I, which are interpreted in terms of common or linked marine sources. During periods when air masses were affected by emissions from local seaweed beds, the concentrations of CHBr3, CH2ICl, and CH2IBr not only showed remarkable correlation but also maximized at low water. These are the first field observations to provide evidence for a link between the tidal cycle, polyhalomethanes in air, and potential marine production. The calculated total flux of iodine atoms into the boundary layer at Mace Head from organic gaseous precursors was dominated by photolytic destruction of CH2I2. Photolysis of CH3I contributed less than 3%. The calculated peak flux of iodine atoms during the campaign coincided with the highest measured levels of iodine oxide radicals, as determined using Differential Optical Absorption Spectrometry (DOAS).

Iodine oxide in the marine boundary layer

Abstract: A striking example of the influence of halogen chemistry on tropospheric ozone levels is the episodic destruction of boundary-layer ozone during the Arctic sunrise by reactive halogen species1, 2. We detected iodine oxide in the boundary layer at Mace Head, Ireland (53°20′ N, 9°54′ W) during May 1997, which indicates that iodine chemistry is occurring in the troposphere.

DOAS Measurements of Tropospheric Bromine Oxide in Mid-Latitudes

Abstract: Episodes of elevated bromine oxide (BrO) concentration are known to occur at high latitudes in the Arctic boundary layer and to lead to catalytic destruction of ozone at those latitudes; these events have not been observed at lower latitudes. With the use of differential optical absorption spectroscopy (DOAS), locally high BrO concentrations were observed at mid-latitudes at the Dead Sea, Israel, during spring 1997. Mixing ratios peaked daily at around 80 parts per trillion around noon and were correlated with low boundary-layer ozone mixing ratios.

Chemistry of Halogen Oxides in the Troposphere: Comparison of Model Calculations with Recent Field Data

Abstract: Reactive halogen species (RHS = X, XO, HOX, OXO; X = Cl, Br, I) are known to have an important influence on the chemistry in the polar boundary layer (BL), where they are responsible for ozone depletion events in spring Recent field campaigns at Mace Head, Ireland, and the Dead Sea, Israel, identified for the first time iodine oxide (IO) at mixing ratios of up to 66 ppt and 90 ppt bromine oxide (BrO), respectively, by DOAS also at lower latitudes These results intensified the discussion about the role of the RHS in the mid-latitude BL Photochemical box model calculations show that the observed IO mixing ratios can destroy ~045 ppb ozone per hour This is comparable to the rates of the known O3-loss processes in the boundary layer The model studies also reveal that IO, at these levels, has a strong influence on the BL photochemistry, increasing the OH/HO2- and the NO2/NO - ratios In combination these changes lead to a reduction of the photochemical ozone formation, which - in addition - reduces ozone mixing ratios by up to 015 ppb/h The studies for the Dead Sea case give no information on the heterogeneous process responsible for the bromine release, but they show that a total of 2 – 4 ppb of total bromine have to be released to explain the observed complete depletion of 60 ppb ozone in 2 – 3 hours

Observation of Filterable Bromine Variabilities During Arctic Tropospheric Ozone Depletion Events in High (1hour) Time Resolution

Abstract: The halogen ions Br- and Cl- together with NO3 -, SO4 =, MSA- (methane sulfonate), Na+ and NH4 + were analysed by ion chromatography in extracts of more than 800 aerosol cellulose filter samples taken at Ny Alesund, Svalbard (79°N, 12°E) in spring 1996 (March 27 - May 16) within the European Union project ARCTOC (Arctic Tropospheric Ozone Chemistry). Anticorrelated variations between f-Br (filterable bromine, i.e. water soluble bromine species that can be collected by aerosol filters) and ozone within the arctic troposphere were evaluated at a resolution of 1 or 2 hours for periods with depleted ozone and 4 hours at normal ozone. A mean f-Br concentration of 11 ng m-3 (0.14 nmol m-3) was observed for the whole campaign, while maximum concentrations of 80 ng m-3 (1 nmol m-3) were detected during two total O3-depletion events (O3 drop to mixing ratios below the detection limit of < 2 ppb). Anticorrelation between f-Br and O3 was also seen during minor O3-depletion episodes (sudden drop in O3 by at least 10 ppb, but O3 still exceeding the detection limit) and even for ozone variations near its background level (40-50 ppb). A time lag of about 10 hours between the change of ozone and of f-Br concentrations could only be found during a total ozone depletion event, when f-Br reached its maximum values several hours after ozone was totally destroyed. Bromine oxide (BrO) concentrations, measured by DOAS (Differential Optical Absorption Spectroscopy), and f-Br showed a coincident variability during almost the entire campaign (except in the case of total O3-loss). Frequently enhanced anthropogenic nitrate and sulphate concentrations were observed during O3-depletion periods. At O3 concentrations < 10 ppb sulphate and nitrate exceed their typical mean level by 54% and 77%, respectively. This may indicate a possible connection between acidity and halogen release.

Intercomparison of measured and modelled BrO slant column amounts for the Arctic winter and spring 1994/95

Abstract: Diurnal variations of the slant column amount of BrO (BrO-SCD) measured at Kiruna/Esrange (67.9°N, 21.1°E) in winter/spring 1994/95 are compared with photochemical calculations using the SLIMCAT model. SLIMCAT assumes a total stratospheric bromine of 20 ppt, JPL-97 kinetics but no tropospheric inorganic bromine. Observed local noon BrO-SCDs are underestimated by the model by 20% to 40%. Sensitivity test of existing uncertainties in the stratospheric chemistry of bromine can not resolve the discrepancy. A tropospheric contribution to the total atmospheric BrO, corresponding to a vertical BrO column of 1 to 2×1013/cm², (or mixing ratios of 0.75 ppt to 2 ppt assuming an uniform tropospheric height profile) is suggested.

Heterogeneous and homogeneous chemistry of reactive halogen compounds in the lower troposphere

Abstract: This issue of the Journal of Atmospheric Chemistry contains a series of contributions describing recent developments in our understanding of the heterogeneous and homogeneous chemistry of reactive halogen compounds in the lower Troposphere. Twelve papers describe recent results in this very active area of research, were reported at the Annual Assembly of the European Geophysical Society 1998 in Nice, France, 20-24 April 1998. Considerable progress is documented on the elucidation of the various processes involved in the sources, transformation pathways and chemical consequences of the Reactive Halogen Species (RHS) in the troposphere.

Short‐term variations in the 13C/12C ratio of CO as a measure of Cl activation during tropospheric ozone depletion events in the Arctic

Abstract: Analyses of the CO stable isotopic composition in surface air samples collected at high northern latitudes during Arctic spring reveal significant depletions in the 13C content of atmospheric CO, which coincide with episodes of tropospheric O3 loss. This isotope signal can be explained by production of small amounts of highly 13C-depleted CO, formed in Cl-initiated oxidation of CH4 due to the presence of enhanced Cl levels during O3 depletion events. Using the recently measured fractionation factor for the CH4 + Cl reaction, the observed 13C depletion is employed to estimate the time-integrated amount of Cl radicals encountered by the O3 depleted air mass. The results support findings obtained from measurements of hydrocarbon mixing ratios during O3 depletion events. The 13C content of atmospheric CO can thus be used as a sensitive indicator for enhanced Cl radical levels in the atmosphere.

Fate of long‐lived trace species near the Northern Hemispheric tropopause: Carbon dioxide, methane, ozone, and sulfur hexafluoride

Abstract: The mixing ratios of CO2, CH4, O-3, and SF6 were measured in 118 whole air samples collected onboard a Transall C-160 aircraft around the NH extratropical tropopause in the winters 1993-1994 and 1994-1995. The samples originate mainly from the upper troposphere (similar to 7 km) and partly from the lowermost stratosphere up to altitudes of 12 km. With help of 7-day back trajectories the potential temperature theta of an air mass in the free troposphere was established to be a reliable measure of the average latitude the air mass spent the last week before sampling, The potential temperature are thus interpreted in terms of a "representative latitude" allowing -1- for the creation of representative meridional trace gas profiles in the troposphere that show a lower scatter than if the data were plotted versus the sampling latitude itself and -2- for the distinction of different types of air masses, For example, by categorizing the sampled air masses by their potential temperatures and by their trace gas composition, the 300 K theta-surface was identified to separate cold polar air from warmer subtropical air. The 300 K theta - surface is thus found to mark the polar front in the wintertime upper troposphere and can be viewed as the lowest isentrope that allows quasi-isentropic cross-tropopause transport. The midlatitude upper troposphere was frequently affected by surface air resulting in a positive correlation of CH4, SF6, and O-3. In contrast, in the arctic upper troposphere and in the lowermost stratosphere, both CH4 and SF6 were negatively correlated with O-3. Surprisingly, variable CO2 levels (spanning similar to 14 ppm) found just above the ozonopause point to intensive mixing of tropospheric air into the lowermost stratosphere. From the high SF6 growth rate of nearly 7% per year a clear aging of the observed air masses since their entry into the stratosphere could be inferred. This SF6 age was rather high, for example around 2 years at an altitude of 11.5 km and thus just 3.5 km above the dynamical tropopause.

Radiative smoothing in zenith‐scattered skylight transmitted through optically thick clouds to the ground

Abstract: A ground-based photometer was used to monitor the temporal variation of the zenith-scattered skylight radiance (at A = 752 nm) with a temporal res- olution of 2 Hz. In detail, eight data sets for different optically thick (re _ 1/(1-g) with re being the cloud's optical thickness and g the asymmetry factor for Mie scattering) cloud covers were analyzed with standard spectral methods. A general feature of all cloudy sky radiance power spectra is the presence of a remarkable scale break occurring at spatial scales that approxi- mately correspond to the vertical cloud extension. This finding is consistent with theoretical estimates by Mar- shaket al. (1995) and Davis et al. (1997b) based on radiative smoothing by photon diffusion in vertically extended, optically thick and homogeneous clouds.

Occurrence of polar stratospheric clouds at Kiruna

Abstract: Polar stratospheric clouds (PSCs) are often observed in the Kiruna region in northern Sweden, east of the Scandinavian mountain range, during wintertime. PSC occurrence can be detected by ground-based optical instruments. Most of these require clear tropospheric weather. By applying the zenith-sky colour index technique, which works under most weather conditions, the data availability can be extended. The observations suggest that PSC events, especially of type II (water PSCs) may indeed more common than predicted by synoptic models, which is expected because of the frequent presence of mountain-induced leewaves. However, it will be of importance to increase the density of independent observations.

Intercomparison of the influence of tropospheric clouds on UV-visible absorptions detected during the NDSC intercomparison campaign at OHP in June 1996

Abstract: The influence of tropospheric clouds on zenith sky light (or brief ZSL-DOAS) measurements of stratospheric gases is investigated. From a large set of intercomparison studies including six simultaneously operated UV/visible spectrometers, the zenith sky absorptions of O3, O4, NO2, and H2O are found to increase considerably under the investigated Cumulonimbus (Cb) cloud. The accuracy of the inferred visible O3 absorptions, however, are affected by interfering H2O absorptions. The increased cloudy sky absorptions are attributed to increased pathlengths due to multiple Mie scattering and hence interstitial gaseous absorptions inside the cloud. The absorptions detected for chemically inert gases like O4 (and H2O) are found to be inconsistent with those detected for NO2 and O3. This finding indicates that O3 and NO2 are modified by cloud related transport or chemical processes.

Observation of the Stratospheric NO2 Latitudinal Distribution in the Northern Winter Hemisphere

Abstract: During a series of flights in the winters 1991/92 to 1994/95 total stratospheric NO2 was measured by means of the DOAS (Differential Optical Absorption Spectroscopy) technique on board a C160 (Transall) aircraft. In an area covering 60°W to 60°E, and 16°N to 86°N, the total stratospheric NO2 was observed to vary markedly with latitude and season (winter and spring). In the mid-winter Arctic vortex extremely low total stratospheric NO2 (< 3.1014/cm2) was always found, generally larger amounts of NO2 occurred outside the vortex in winter and towards the spring both inside and outside the vortex. This behaviour of stratospheric NO2 can be explained by the denoxification of the wintertime polar stratosphere. Ambient to the vortex in mid-winter however, ‘sudden’ increases of total stratospheric NO2 by about a factor of 3 were observed. These sudden increases in stratospheric NO2 coincide with a change in the wavenumber 2 of the geopotential height at 60°N, which indicates that most likely the events are caused by planetary waves efficiently transporting air masses rich in NOx from lower to higher latitudes. The monitoring of stratospheric NO2, during latitudinal traverses ranging from the Arctic (80°N) to the Subtropics (18°N) in spring also unexpectedly showed a large variability in total stratospheric NO2 at mid-latitudes. Since photochemistry almost certainly can be excluded, it is proposed that the observed variability may be due to the planetary wave activity of the stratospheric surf zone, known to dynamically connect the tropical and the polar stratosphere.

Optical studies of polar stratospheric clouds

Abstract: Polar Stratospheric Clouds (PSC) appear in the polar zones of the Earth in the winter. These clouds are known to cause enhanced chemical ozone destruction. Methods for optical remote-sensing of PSC in use or under development at the Swedish Institute of Space Physics are discussed with respect to their advantages and limitations. Especially multistatic imaging may become a valuable additional tool for PSC studies.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Freie Radikale — die treibende Kraft der Stoffumsetzungen in der Atmosphäre

Abstract: Bei der Selbstreinigung der Atmosphare von Fremdstoffen, die durch menschliche, aber auch naturliche Aktivitaten in die Atmosphare gelangen, spielen sogenannte freie Radikale eine zentrale Rolle. Bereits in geringsten Spuren (ein Radikal auf 1013 Luftmolekule) entfalten sie ihre Wirkung. Diese extreme Seltenheit der Teilchen erklart auch, das Radikale — obwohl seit Jahrzehnten postuliert — erst in jungster Zeit durch neue physikalische Mesverfahren eindeutig in der Umgebungsluft nachgewiesen werden konnten.