Showing papers by "Paul J. Crutzen published in 1997"

Atmospheric aerosols: Biogeochemical sources and role in atmospheric chemistry

Abstract: Atmospheric aerosols play important roles in climate and atmospheric chemistry: They scatter sunlight, provide condensation nuclei for cloud droplets, and participate in heterogeneous chemical reactions. Two important aerosol species, sulfate and organic particles, have large natural biogenic sources that depend in a highly complex fashion on environmental and ecological parameters and therefore are prone to influence by global change. Reactions in and on sea-salt aerosol particles may have a strong influence on oxidation processes in the marine boundary layer through the production of halogen radicals, and reactions on mineral aerosols may significantly affect the cycles of nitrogen, sulfur, and atmospheric oxidants.

An inverse modeling approach to investigate the global atmospheric methane cycle

Abstract: Estimates of the global magnitude of atmospheric methane sources are currently mainly based on direct flux measurements in source regions. Their extrapolation to the entire globe often involves large uncertainties. In this paper, we present an inverse modeling approach which can be used to deduce information on methane sources and sinks from the temporal and spatial variations of atmospheric methane mixing ratios. Our approach is based on a three-dimensional atmospheric transport model which, combined with a tropospheric background chemistry module, is also employed to calculate the global distribution of OH radicals which provide the main sink for atmospheric methane. The global mean concentration of OH radicals is validated with methyl chloroform (CH3CCl3) observations. The inverse modeling method optimizes the agreement between model-calculated and observed methane mixing ratios by adjusting the magnitudes of the various methane sources and sinks. The adjustment is constrained by specified a priori estimates and uncertainties of the source and sink magnitudes. We also include data on the 13C/12C isotope ratio of atmospheric methane and its sources in the model. Focusing on the 1980s, two scenarios of global methane sources are constructed which reproduce the main features seen in the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) methane observations. Differences between these two scenarios may probably be attributed to underestimated a priori uncertainties of wetland emissions. Applying the inverse model, the average uncertainty of methane source magnitudes could be reduced by at least one third. We also examined the decrease in the atmospheric methane growth rate during the early 1990s but could not uniquely associate it with changes in particular sources.

Freezing of HNO3/H2SO4/H2O Solutions at Stratospheric Temperatures: Nucleation Statistics and Experiments

Abstract: Calorimetric freezing experiments with aqueous sulfuric and nitric acid solutions are presented and applied to the formation of polar stratospheric clouds (PSCs). We show that the nucleation of hydrates from these solutions is a stochastic process and that nucleation rates and their uncertainties can be determined using Poisson statistics. Under thermodynamic equilibrium conditions above the ice frost point, the homogeneous nucleation rates of stratospheric aerosols are exceedingly low, ruling out homogeneous freezing as a pathway for PSC formation. Several stratospherically important substrates were tested concerning their ability to induce heterogeneous nucleation. None of the experiments indicated a relevant enhancement of the freezing probability of liquid aerosols. Moreover, the experiments reveal that the freezing process of the solutions under stratospheric conditions is limited by the nucleation rates of the hydrates, rather than their crystal growth rates, thus ruling out the possibility of a gla...

Severe chemical ozone loss in the Arctic during the winter of 1995–96

Abstract: Severe stratospheric ozone depletion is the result of perturbations of chlorine chemistry owing to the presence of polar stratospheric clouds (PSCs) during periods of limited exchange of air between the polar vortex and midlatitudes and partial exposure of the vortex to sunlight1,2,3,4. These conditions are consistently encountered over Antarctica during the austral spring. In the Arctic, extensive PSC formation occurs only during the coldest winters, when temperatures fall as low as those regularly found in the Antarctic1,5,6. Moreover, ozone levels in late winter and early spring are significantly higher than in the corresponding austral season1,7,8, and usually strongly perturbed by atmospheric dynamics9,10,11,12. For these reasons, chemical ozone loss in the Arctic is difficult to quantify. Here we use the correlation between CH4 and O3 in the Arctic polar vortex to discriminate between changes in ozone concentration due to chemical and dynamical effects10. Our results indicate that 120–160 Dobson units (DU) of ozone were chemically destroyed between January and March 1996—a loss greater than observed in Antarctica in 1985, when the ‘ozone hole’ was first reported13,14. This loss outweighs the expected increase in total ozone over the same period through dynamical effects, leading to an observed6 net decrease of about 50 DU. This ozone loss arises through the simultaneous occurrence of extremely low Arctic stratospheric temperatures6,15 and large stratospheric chlorine loadings. Comparable depletion is likely to recur because stratospheric cooling16,17 and elevated chlorine concentrations5,18 are expected to persist for several decades.

Temperature dependence of UV absorption cross sections and atmospheric implications of several alkyl iodides

Abstract: The ultraviolet absorption spectra of a number of alkyl iodides which have been found in the troposphere, CH3I, C2H5I, CH3CH2CH2I, CH3CHICH3, CH2I2, and CH2ClI, have been measured over the wavelength range 200–380 nm and at temperatures between 298 and 210 K. The absorption spectra of the monoiodides CH3I, C2H5I, CH3CH2CH2I, and CH3CHICH3 are nearly identical in shape and magnitude and consist of single broad bands centered near 260 nm. The addition of a chlorine atom in CH2ClI shifts its spectrum to longer wavelengths (σmax at 270 nm). The spectrum of CH2I2 is further red-shifted, reaching a maximum of 3.85×10−18 cm2 molecule−1 at 288 nm and exhibiting strong absorption in the solar actinic region, λ>290 nm. Atmospheric photolysis rate constants, J values, have been calculated assuming quantum efficiencies of unity for different solar zenith angles as a function of altitude using the newly measured cross sections. Surface photolysis rate constants, calculated from the absorption cross sections measured at 298 K, range from 3×10−6 s−1 for CH3I to 5×10−3 s−1 for CH2I2 at a solar zenith angle of 40°.

Tropospheric water‐vapour and ozone cross‐sections in a zonal plane over the central equatorial Pacific Ocean

Abstract: Tropospheric water-vapour and ozone measurements, using calibrated balloon-borne sensors, are reported from the Central Equatorial Pacific Experiment (CEPEX). the sensors were launched from the Research Vessel Vickers along 2°S latitude between 156°E (west of the international date line) and 155°W (east of the date line). These measurements are combined with those from water-vapour sondes launched over the western Pacific warm pool, during the Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). Taking the two experiments CEPEX and TOGA-COARE together, the sensors included frost-point hygrometers, Humicap-A Vaisala sondes, Humicap-H Vaisala sondes and electrochemical ozone-sondes. Taken together, the CEPEX and TOGA-COARE data provide over 150 vertical profiles of water vapour within the troposphere in varied conditions of convective activity ranging from disturbed to suppressed. the primary motivation behind the present analyses is to understand the role of tropical deep convection in the vertical distribution of water-vapour. With this in mind, the profiles have been analysed in relation to occasions of recent deep convection and occasions when convection was suppressed. We employ three different criteria to identify the profiles influenced by deep convection: brightness temperature in the infrared-window channel of the Japanese Geostationary Meteorological Satellite (GMS); ozone as a quasi-conservative tracer for deep convection; and using water vapour itself, that is the wettest versus the driest soundings. Irrespective of the criteria used, we report here that the atmosphere, while under the influence of active deep convection, was found to have relative humidities in excess of 75% over most of the troposphere between the surface and about 14 km. the sondes were launched routinely over a period of 45 days (between CEPEX and TOGA-COARE), without biasing the sample towards convectively disturbed conditions. A feature of the convectively disturbed profile is a distinct minimum in relative humidity at about 700 hPa, where it was as low as 65%. the low relative humidity was accompanied by relatively high ozone mixing ratios, which raises the possibility of long-range transport of dry sub-tropical air into the warm, convectively disturbed, regions of the equatorial Pacific Ocean. Inspection of the analysed fields, and the wind fields from the sondes, supports this assertion. It then follows that the omnipresent minimum of moist static energy and minimum relative humidity at 700 hPa in the inner tropics may be the result of long-range, inclined, transport of dry air from non-convective regions. This detection suggests a linkage between the large-scale circulation, deep convection and the thermodynamic structure within the equatorial troposphere. The results presented here demonstrate the applicability of ozone as a quasi-conservative tracer of transport in the context of deep convection. The ozone-based criterion is used to diagnose recent deep convection, independent of the GMS satellite observations, and allows one to follow the evolution of relative humidity and of water-vapour mixing ratio after the dissipation of the cloud anvil to optically thin conditions. We show that the troposphere dries to low humidity soon after anvil dissipation. This observation leads to the hypothesis that moistening of the atmosphere, away from the core of Cb convection, occurs by evaporation of precipitation falling out of the anvils. After anvil dissipation, the ensuing subsidence in clear air causes the relative humidity and the water mixing ratio to decrease.

HALOE observations of the vertical structure of chemical ozone depletion in the Arctic Vortex during winter and early spring 1996–1997

Abstract: We discuss observations by the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite in the lower stratosphere in the Arctic vortex during winter and spring 1996-1997. Using HF as a chemically conserved tracer, we identify chemical ozone depletion and chlorine activation, despite variations caused by dynamical processes. For the Arctic vortex region, significant chemical ozone loss (up to two thirds around 475 K potential temperature) due to extensive activation of the inorganic chlorine reservoir is deduced, as observed similarly for previous winters. Chemical reductions in column ozone of up to 70-80 Dobson units (DU) in the lower stratosphere are calculated. Both chlorine activation and ozone loss inside the vortex, however, are more variable than observed in previous years.

Chemical perturbation of the lowermost stratosphere through exchange with the troposphere

Abstract: In the troposphere, anthropogenic emissions of nitrogen oxides, hydrocarbons and carbon monoxide cause large-scale photochemical build up of ozone. In the stratosphere breakdown of anthropogenic halocarbons damages the ozone layer. In the extratropics a transition region between these air layers occurs, the lowermost stratosphere (below 12–14 km', in which about half the current subsonic air traffic takes place. Here, we report aircraft measurements of HNO3, O3 and CO over western Europe in July 1994 (5 flights of several hours during a 10-day period', at approximately 1–2 km above the tropopause. The HNO3 mixing ratios observed were highly variable (0.76–1.2 ppbv', while HNO3/O3 ratios seem relatively high (5.2–7.0 10−3'. Moreover, several times we observed very high levels of pollutant CO (up to ∼0.5 ppmv' that did not originate from aircraft exhausts. Instead, we pose that it had mixed-in from the troposphere. Cross-tropopause mixing also helps explaining the variable HNO3 and relatively high HNO3/O3 ratios. These measurements suggest that relatively short-lived surface emitted pollutants can reach the lowermost stratosphere. We expect that this contributes to O3 formation.

Re‐formation of chlorine reservoirs in southern hemisphere polar spring

Abstract: This paper focuses on the recovery of chlorine reservoir species in the lower stratosphere in late Antarctic spring. The investigations are based on measurements from the Halogen Occultation Experiment (HALOE) on board the Upper Atmosphere Research Satellite (UARS) and calculations by the Mainz photochemical box model and the NASA Langley Research Center trajectory model. During late Antarctic spring 1994, HALOE observed high HCl mixing ratios up to 2.7 ppbv at 20 km altitude in the ozone-depleted air inside the polar vortex. These values correspond approximately to the sum of all available inorganic chlorine species. In the preceding period of chlorine activation on polar stratospheric clouds (PSCs), the observed HCl mixing ratios in some cases were below 0.3 ppbv. This indicates a fast conversion of active chlorine species into the form of HCl after PSCs disappear with increasing stratospheric temperatures. Box model calculations are presented that assess the rate of HCl increase in late spring when heterogeneous chemistry on polar stratospheric clouds becomes insignificant. The calculations were performed along Lagrangian trajectories starting from HALOE measurements in September 1994. Sensitivity calculations are presented regarding uncertainties in input parameters of the calculations. In the vortex edge region, calculated HCl increase rates are significantly lower compared with HALOE observations. Introducing additional HCl-yielding branches of the reactions of ClO with OH and HO2 helps to reduce this discrepancy.

Observations of high concentrations of total reactive nitrogen (NOy) and nitric acid (HNO3) in the lower Arctic stratosphere during the Stratosphere-Troposphere Experiment by Aircraft Measurements (STREAM) II campaign in February 1995

Abstract: Simultaneous in situ measurements of NO y , HNO 3 , O 3 , N 2 O, and CO have been performed in the lower stratosphere during the Stratosphere-Troposphere Experiment by Aircraft Measurements (STREAM) II intensive winter campaign in February 1995 from Kiruna airport (northern Sweden) with a Cessna Citation II twinjet aircraft up to a maximum altitude of 12.8 km. The flights were coordinated with the Arctic Second European Stratospheric Arctic and Midlatitude Experiment (SESAME) winter campaign. Strongly elevated levels of total reactive nitrogen (NO Y ) and its most abundant contributing species, nitric acid (HNO 3 ), with mixing ratios up to 9 parts per billion by volume (ppbv), were observed during all flights at altitudes near 12 km. On average, the measured NO concentrations exceed the expected levels by a factor of 2-3. Normal background O Y has been calculated from observed N 2 O mixing ratios using the NO Y -N 2 O correlation reported for the undisturbed northern hemisphere. This indicates that subsidence of air in the vortex alone cannot explain these findings. We propose that the elevated NO Y concentrations were caused by nitrification of the lower stratosphere associated with sedimentation and evaporation of polar stratospheric cloud particles that carry down HNO 3 from higher altitudes, that is, from altitudes up to about 25 km.

Photochemical calculations along air mass trajectories during ASHOE/MAESA

Abstract: The practicality of conducting photochemical calculations along trajectories of air masses is investigated. An isentropic trajectory package is used in conjunction with a detailed photochemical model to compare predictions of the mean chemical content of air masses initialized with the Halogen Occultation Experiment (HALOE) data with coincident in situ observations from instruments onboard the ER-2 aircraft. Comparisons are made for 10 ER-2 flights originating from Christchurch, New Zealand, during the May to June and October 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) deployments. Between 54 and 84 coincidences are found, depending on the species measured. Correlations between the ER-2 and HALOE air mass/box model calculations are high (0.56–0.90) for most species considered except for H2O (0.14) and HCl (0.24). Statistically significant low biases in the prediction of HCl, H2O, and OH are found. Kolmogorov-Smirnov (KS) significance tests are used to quantify the agreement between the distribution of species observed by the ER-2 and predicted by the HALOE trajectory/ photochemical model. The model predictions agree with the observed variance within the distributions at significance levels greater than 0.80 (greater than 80% confidence that the predicted and observed variance are identical) for H2O, ClO, O3, and NOy. The impact of computational errors in the trajectory calculations and measurement uncertainty in the computed confidence levels are investigated using Monte Carlo techniques. Computational trajectory errors are found to play a small role in reducing confidence levels. The error analysis shows that the HALOE trajectory/photochemical model calculations reproduce the large-scale variability found in the in situ ER-2 constituent measurements to within the expected uncertainties in the HALOE observations for all species considered. It is concluded that the combined trajectory/photochemical model is an effective tool for interpreting in situ aircraft observations within the global perspective provided by remote satellite measurements.

Ozone clouds over the Atlantic

Abstract: Ozone in the atmosphere has many useful functions. Most of it is in the stratosphere (above 10 to 20 km) and only ten per cent is in the troposphere (the region down to ground level), where it helps to remove undesirable emissions. The two regions are usually separated, dynamically, by the tropopause, but new measurements taken on airliners show that pockets of extremely high ozone concentration are somehow getting from the stratosphere into the troposphere over the tropical Atlantic.

Converting potential temperature to altitude in the stratosphere

Abstract: In many atmospheric studies it has become useful and customary to use the quasiconservative property potential temperature (Θ) as the vertical coordinate, often without giving a corresponding, approximate altitude.

First results on tropospheric observations by the Global Ozone Monitoring Experiment, GOME, on ERS 2

Abstract: ERS2 based GOME provides UV-VIS-spectra of nadir scattered sunlight which carries the absorption signals of atmospheric compounds. So far ozone, nitrogen dioxide, chlorine dioxide, and bromine monoxide have been identified. As tropospheric chemistry is of high interest for the science community some effort was made to identify formaldehyde in the spectra taken by GOME. Formaldehyde as an important intermediate in the oxidation cycle of hydrocarbons is expected to occur in regions of strong and extended photochemical activity which are mostly found in the lower layers of the troposphere. In this contribution first observations of formaldehyde are described in plumes of burning savannas in Africa and in a smog episode which covered the upper Rhein area around Frankturt/Ludwigshaten in Germany. So far the weak formaldehyde absorptions could only be identified against a spectrum from the same or from close orbits containing background levels of formaldehyde absorptions. The plume above Africa and above the Atlantic Ocean contains an additional vertical column of 1.6 x 1016 molec/cm2 which corresponds to 2.5 or 1.4 ppb in a layer of 3 or 6 Km from ground, respectively.

Individual Reports from GLOMAC Contributors

Abstract: The Stockholm contribution to GLOMAC has been focused on the development of models to simulate the distribution of sulfur compounds (DMS, SO2 and aerosol sulfate) and nitrogen compounds (NO, HNO3, NO 3 − and PAN) in the global troposphere and on estimating the environmental impact of these distributions. The models include natural and anthropogenic sources, chemical transformations in the atmosphere, and wet and dry removal processes. During the first years, the main tool has been the transport model MOGUNTIA developed at the Max-Planck-Institute for Chemistry in Mainz. This model uses a coarse grid and is based on average meteorological conditions. During the past two years, the chemical schemes have been implemented in a meteorologically more advanced model (ECHAM) which uses meteorological parameters with higher spacial and temporal resolution.

Global ozone monitoring experiment, GOME : Global distribution of BrO

Abstract: ERS2 based GOME provides UV-VIS-spectra of nadir scattered sunlight. The spectral observations yielded column data of BrO besides those of ozone, NO2 and OClO. Stratospheric BrO at solar zenith angles of 80°and higher in polar regions are unambiguous because the long optical paths give rise to strong absorptions. The vertical column for BrO in the equatorial region is found to be (4.0 ± 1.5) x 1013 molec/cm 2 . GOME data are compared to a number of groundbased differential optical spectroscopy observations of BrO by zenith scattered sunlight at Sondre Stromfjord, Greenland.