Peter S. Connell

The chemical and radiative effects of the Mount Pinatubo eruption

The eruption of Mount Pinatubo introduced large amounts of sulfur-containing particles into the stratosphere. Stratospheric ozone measured by ozonesondes and satellites is significantly lower following the June 1991 eruption and throughout 1992 and 1993. To clarify the mechanisms leading to effects on stratospheric ozone, time-dependent stratospheric aerosol and gas experiment II (SAGE II) and cryogenic limb array elaton spectrometer (CLAES) aerosol optical extinction data and SAGE II surface area density are used as parameters in a two-dimensional (2-D) zonally averaged chemical radiative transport model. The model was integrated with time from before the eruption through December 1993. The modeled impact on global ozone results from increased rates of heterogeneous reactions on sulfate aerosols and from the increased radiative heating and scattering caused by these aerosols. The models dynamical response to changes in forcing (from changes in radiatively active trace gas concentrations and from aerosol heating) is treated in one of three ways: (1) the stratospheric temperature is perturbed, with fixed seasonal circulation, (2) the circulation is perturbed, with fixed seasonal temperature, or (3) both circulation and temperature are unperturbed, when investigating only the impact of Mount Pinatubo increased aerosol surface area density (SAD) and aerosol scattering of actinic solar radiation, When the aerosol heating is allowed to modify the temperature distribution, the maximum change calculated in equatorial column ozone is −1.6%. The calculated equatorial temperature change and peak local ozone change in October 1991 are +6 K and −4%, respectively. When aerosol heating perturbs the circulation in the model, the maximum change in equatorial column ozone is −6%. Increased heterogeneous processing on sulfate aerosols is calculated to have changed equatorial column ozone in late 1991 by −1.5%. Global column ozone in the model in 1992 and 1993 changed by −2.8% and −2.4%, respectively. The relationship of ozone-controlling processes in the lower stratosphere is altered as well; HOx becomes the most important catalytic cycle, followed by ClOx and NOx. This is driven by significant changes in trace gas concentrations. In October 1991, lower stratospheric, equatorial NOx decreased by 40%, ClOx increased by 60%, and HOx increased by 25%. When the effect of heterogeneous chemical processing on sulfate aerosols is combined with aerosol heating, modifying either circulation or temperature, dramatically different ozone fingerprints with time and latitude are predicted. Model-derived changes in the equatorial region in column ozone best represented the observed data when perturbed circulation was combined with heterogeneous chemical effects. However, at high latitudes, the increased ozone production from the strengthening of the mean circulation tends to cancel the heterogeneous reduction of ozone. This is not in good agreement with observed data, especially in 1992 and 1993. When the circulation is held fixed and the temperature allowed to change, and heterogeneous chemical effects are included, the equatorial ozone decrease predicted was too small for 1991. However, the mid- to high-latitude decrease in 1992 and 1993 is in better agreement with observed data.

Observations of lower-stratospheric ClONO2, HNO3, and aerosol by the UARS CLAES experiment between January 1992 and April 1993

Abstract This paper discusses simultaneous measurements of stratospheric CIONO2, HNO3, temperature, and aerosol extinction coefficient by the Cryogenic Limb Array Etalon Spectrometer (CLAES) on the NASA Upper Atmosphere Research Satellite (UARS), obtained over the period 9 January 1992 through 23 April 1993. The discussion concentrates on the stratosphere region near 21 km of particular interest to heterogeneously driven ozone depletion. For periods between 12 June and 1 September 1992 at latitudes poleward of about 60°S, when temperatures were below type I polar stratospheric cloud (PSC) formation thresholds throughout the lower stratosphere, CLAES observed high levels of PSCs coincident with highly depleted fields of both HNO3 and CIONO2. By 17 September, the incidence of PSCs had greatly diminished in the lower stratosphere, but both CIONO2 and HNO3 remained highly depleted. These observations are consistent with the removal of gaseous HNO3 through the formation of nitric acid trihydrate (NAT) particle...

The Role of Atmospheric Chemistry in Climate Change

Surface emissions and concentrations of globally important trace gases are increasing. Climate models indicate significant temperature increases could occur in the next century due to increasing CO...

A polar stratospheric cloud parameterization for the global modeling initiative three‐dimensional model and its response to stratospheric aircraft

We describe a new parameterization of polar stratospheric clouds (PSCs) which was written for and incorporated into the three-dimensional (3-D) chemistry and transport model (CTM) developed for NASAs Atmospheric Effects of Aviation Project (AEAP) by the Global Modeling Initiative (GMI). The parameterization was designed to respond to changes in NOy and H2O produced by high-speed civilian transport (HSCT) emissions. The parameterization predicts surface area densities (SADs) of both Type 1 and Type 2 PSCs for use in heterogeneous chemistry calculations. Type 1 PSCs are assumed to have a supercooled ternary sulfate (STS) composition, and Type 2 PSCs are treated as water ice with a coexisting nitric acid trihydrate (NAT) phase. Sedimentation is treated by assuming that the PSC particles obey lognormal size distributions, resulting in a realistic mass flux of condensed phase H2O and HNO3. We examine a simulation of the Southern Hemisphere high-latitude lower stratosphere winter and spring seasons driven by temperature and wind fields from a modified version of the National Center for Atmospheric Research (NCAR) Middle Atmosphere Community Climate Model Version 2 (MACCM2). Predicted PSC SADs and median radii for both Type 1 and Type 2 PSCs are consistent with observations. Gas phase HNO3 and H2O concentrations in the high-latitude lower stratosphere qualitatively agree with Cryogenic Limb Array Etalon Spectrometer (CLAES) HNO3 and Microwave Limb Sounder (MLS) H2O observations. The residual denitrification and dehydration of the model polar vortex after polar winter compares well with atmospheric trace molecule spectroscopy (ATMOS) observations taken during November 1994. When the NOx and H2O emissions of a standard 500-aircraft HSCT fleet with a NOx emission index of 5 are added, NOx and H2O concentrations in the Southern Hemisphere polar vortex before winter increase by up to 3%. This results in earlier onset of PSC formation, denitrification, and dehydration. Active Cly increases and produces small (∼1%) decreases in lower stratospheric vortex O3 concentrations during the spring relative to the HSCT-free run.

Aviation Fuel Tracer Simulation: Model Intercomparison and Implications

An upper limit for aircraft-produced perturbations to aerosols and gaseous exhaust products in the upper troposphere and lower stratosphere (UT/LS) is derived using the 1992 aviation fuel tracer simulation performed by eleven global atmospheric models. Key findings are that subsonic aircraft emissions: 1) have not be responsible for the observed water vapor trends at 40°N; 2) could be a significant source of soot mass near 12 km, but not at 20 km, 3) might cause a noticeable increase in the background sulfate aerosol surface area and number densities (but not mass density) near the northern mid-latitude tropopause, and 4) could provide a global, annual mean top of the atmosphere radiative forcing up to +0.006 W/m² and −0.013 W/m² due to emitted soot and sulfur, respectively.

Validation of CLAES ClONO2 measurements

The cryogenic limb array etalon spectrometer (CLAES) aboard the Upper Atmosphere Research Satellite has made extensive measurements of thermal infrared radiation from the Earths limb from which vertical concentration profiles of several stratospheric gases and multiwavelength aerosol absorption coefficients have been retrieved for the period from January 9, 1992, to May 5, 1993. This work examines stratospheric ClONO2 concentrations from the current calibration and retrieval software which are designated version 7 data. These data provide the first near-global view of this stratospheric species. This work evaluates data quality through (1) an analysis of estimated uncertainties and biases in the remote sensing process, (2) comparison with calculations using a two-dimensional chemical model, (3) comparison with correlative data, and (4) an examination of various known limitations. The precision of CLAES ClONO2 volume mixing ratio retrievals are within 15% in the range (10 30 mbar). A similar effect associated with thick polar stratospheric clouds is identified. Overall, this validation study indicates that the majority of these data are of good quality and should be very useful in quantitative and qualitative chemical studies of the stratosphere.

Effect of vibrationally excited oxygen on ozone production in the stratosphere

Photolysis of vibrationally excited oxygen produced by ultraviolet photolysis of ozone in the upper stratosphere is incorporated into the Lawrence Livermore National Laboratory two-dimensional zonally averaged chemical-radiative-transport model of the troposphere and stratosphere. The importance of this potential contributor of odd oxygen to the concentration of ozone is evaluated based on recent information on vibrational distributions of excited oxygen and on preliminary studies of energy transfer from the excited oxygen. When energy transfer rate constants similar to those of Toumi et al. (1991) are assumed, increases in model ozone concentrations of up to 4.0% in the upper stratosphere are found, and the model ozone concentrations are found to agree slightly better with measurements, including recent data from the Upper Atmosphere Research Satellite. However, the ozone increase is only 0.3% when the larger energy transfer rate constants indicated by recent experimental work are applied to the model. An ozone increase of 1% at 50 km requires energy transfer rate constants one-twentieth those of the preliminary observations. As a result, vibrationally excited oxygen processes probably do not contribute enough ozone to be significant in models of the upper stratosphere. 41 refs., 10 figs., 3 tabs.

The Global Modeling Initiative assessment model: Application to high-speed civil transport perturbation

(3-D) chemical transport model (CTM) was applied to assess the impact of a fleet of high-speed civil transports (HSCTs) on abundances of stratospheric ozone, total inorganic nitrogen (NOv), and H20. This model is specifically designed to incorporate a diversity of approaches to chemical and physical processes related to the stratosphere in a single computing framework, facilitating the analysis of model component differences, modeling intercomparison and comparison with data. A proposed HSCT fleet scenario was adopted, in which the aircraft cruise in the lower stratosphere, emitting nitrogen oxides (NOx) and water (H20). The model calculated an HSCT-induced change in Northern and Southern Hemisphere total column ozone of +0.2% and +0.05%, respectively. This change is the result of a balance between an increase in local ozone below approximately 25 km and a decrease above this altitude. When compared to available NOy observations, we find that the model consistently underestimates lower stratospheric NO v. This discrepancy is consistent with the model bias toward less negative ozone impact, when cohapared to results from other models. Additional analysis also indicates that for an HSCT assessment it is equally important for a model to accurately represent the lower stratospheric concentrations of ozone and H20. The GMI model yields good agreement in comparisons to ozone data for present-day conditions, while H20 is constrained by climatology as much as possible; thus no further biases would be expected from these comparisons. Uncertainties due to discrepancies in the calculated age of air compared to that derived from measurements, and of the impact of emissions on heterogeneous and polar chemistry, are difficult to evaluate at this point.

Variations in the free chlorine content of the stratosphere (1991-1997) : Anthropogenic, volcanic, and methane influences

Remote sensing of chlorine monoxide (ClO) by the Microwave Limb Sounder experiment aboard the Upper Atmosphere Research Satellite (UARS) has provided global measurements of variations in stratospheric free chlorine for 1991-1997. Linear trends were obtained from a multiple regression analysis of this data set at low latitudes and midlatitudes. ClO increases in the upper stratosphere (2 hPa) are significantly larger than expected from trends in chlorine source gases alone. Much of the upper stratospheric CIO variability can be explained by changes in CH 4 , as measured by the UARS Halogen Occultation Experiment. Decreasing ClO in the lower stratosphere is consistent with a relaxation from a chemically perturbed state attributed to the 1991 Mt. Pinatubo eruption.

Global CF2Cl2 measurements by UARS cryogenic limb array etalon spectrometer: Validation by correlative data and a model

The cryogenic limb array etalon spectrometer (CLAES) onboard the Upper Atmosphere Research Satellite (UARS) has obtained the first global measurements of CF 2 Cl 2 over six seasons, for which 388 days have been processed in data version 7 for the period from January 9, 1992, to May 5, 1993. The CLAES measurements provide a near-global view of this stratospheric species, greatly extending the altitude, latitude, and seasonal coverage of previous measurements. This work evaluates CLAES version 7 data set quality. To arrive at estimates of experimental error, we compared the CLAES CF 2 Cl 2 profiles with all of the available correlative data from balloon and space-borne sensors, and we looked at the repeatability of multiple profiles in the same location. In addition, we carried out empirical estimates of experimental error based on knowledge of instrument characteristics, and we performed consistency checks using the Lawrence Livermore National Laboratory two-dimensional (LLNL-2D) model and the CLAES N 2 O data set. Both the range of mean differences between CLAES and the available correlative data and the empirical estimates of the instrument systematic error indicate a CF 2 Cl 2 profile systematic error of 14% over the range of 18.7 through 32 km and 22% for 33-35 km and 16-18 km. These systematic errors are applicable in the spring through fall seasons for the midlatitude region and all seasons in the tropical region. The CF 2 Cl 2 estimated random errors, which are close to the observed data repeatability, are 32 to 11 pptv between 17 and 32 km and translate to an average 9% in the low to midstratosphere. The altitude range of best confidence for the CF 2 Cl 2 mixing ratio profiles is 18.7 to 32 km (∼68 to 10 mbar), to which we assign an accuracy (root-sum-square of systematic and random errors) of ∼17%. In the tropics the profiles from 16 to about 26 km may sometimes be biased low by up to 14% due to a data processing algorithm constraint. The CLAES CF 2 Cl 2 global fields show generally good spatial correlation and exhibit the major morphological and seasonal features seen in other global tracer field data. Overall, the results of this validation exercise indicate that the CLAES version 7 CF 2 Cl 2 data set, within the limitations discussed in the paper, can be used for quantitative and qualitative studies of stratospheric structure and dynamics.