Chris A. McLinden

Stratospheric ozone in 3‐D models: A simple chemistry and the cross‐tropopause flux

Two simple and computationally efficient models for simulating stratospheric ozone in three-dimensional global transport models are presented. The first, linearized ozone (or Linoz), is a first-order Taylor expansion of stratospheric chemical rates in which the ozone tendency has been linearized about the local ozone mixing ratio, temperature, and the overhead column ozone density. The second, synthetic ozone (or Synoz), is a passive, ozone-like tracer released into the stratosphere at a rate equivalent to that of the cross-tropopause ozone flux which, based on measurements and tracer-tracer correlations, we have calculated to be 475±120 Tg/yr. Linoz and Synoz have been evaluated in the UC Irvine chemical transport model (CTM) with three different archived meteorological fields: the Goddard Institute for Space Studies (GISS) general circulation model (GCM) version II′, the GISS GCM version II, and merged forecast data from the European Centre forecast model (EC/Oslo). Linoz produced realistic annual, cross-tropopause fluxes of 421 Tg/yr for the GISS II′ winds and 458 Tg/yr for the EC/Oslo winds; the GISS II winds produced an unrealistic flux of 790 Tg/yr. Linoz and Synoz profiles in the vicinity of the tropopause using the GISS II′ and EC/Oslo winds were found to be in good agreement with observations. We conclude that either approach may be adequate for a CTM focusing on tropospheric chemistry but that Linoz can also be used for calculating ozone fields interactively with the stratospheric circulation in a GCM. A future version of Linoz will allow for evolving background concentrations of key source gases, such as CH4 and N2O, and thus be applicable for long-term climate simulations.

Sensitivity of ozone to bromine in the lower stratosphere

Measurements of BrO suggest that inorganic bromine (Br_y) at and above the tropopause is 4 to 8 ppt greater than assumed in models used in past ozone trend assessment studies. This additional bromine is likely carried to the stratosphere by short-lived biogenic compounds and their decomposition products, including tropospheric BrO. Including this additional bromine in an ozone trend simulation increases the computed ozone depletion over the past ∼25 years, leading to better agreement between measured and modeled ozone trends. This additional Br_y (assumed constant over time) causes more ozone depletion because associated BrO provides a reaction partner for ClO, which increases due to anthropogenic sources. Enhanced Br_y causes photochemical loss of ozone below ∼14 km to change from being controlled by HO_x catalytic cycles (primarily HO_2+O_3) to a situation where loss by the BrO+HO_2 cycle is also important.

Evidence for bromine monoxide in the free troposphere during the Arctic polar sunrise

During the Arctic polar springtime, dramatic ozone losses occur not only in the stratosphere but also in the underlying troposphere. These tropospheric ozone loss events have been observed over large areas, in the planetary boundary layer (PBL) throughout the Arctic. They are associated with enhanced concentrations of halogen species and are probably caused by catalytic reactions involving bromine monoxide (BrO) and perhaps also chlorine monoxide (ClO). The origin of the BrO, the principle species driving the ozone destruction, is thought to be the autocatalytic release of bromine from sea salt accumulated on the Arctic snow pack, followed by photolytic and heterogeneous reactions which produce and recycle the oxide. Satellite observations have shown the horizontal and temporal extent of large BrO enhancements in the Arctic troposphere, but the vertical distribution of the BrO has remained uncertain. Here we report BrO observations obtained from a high-altitude aircraft that suggest the presence of significant amounts of BrO not only in the PBL but also in the free troposphere above it. We believe that the BrO is transported from the PBL into the free troposphere through convection over large Arctic ice leads (openings in the pack ice). The convective transport also lifts ice crystals and water droplets well above the PBL, thus providing surfaces for heterogeneous reactions that can recycle BrO from less-reactive forms and thereby maintain its ability to affect the chemistry of the free troposphere.

The Relative Importance of Solar and Anthropogenic Forcing of Climate Change between the Maunder Minimum and the Present

Abstract The climate during the Maunder Minimum is compared with current conditions in GCM simulations that include a full stratosphere and parameterized ozone response to solar spectral irradiance variability and trace gas changes. The Goddard Institute for Space Studies (GISS) Global Climate/Middle Atmosphere Model (GCMAM) coupled to a q-flux/mixed-layer model is used for the simulations, which begin in 1500 and extend to the present. Experiments were made to investigate the effect of total versus spectrally varying solar irradiance changes; spectrally varying solar irradiance changes on the stratospheric ozone/climate response with both preindustrial and present trace gases; and the impact on climate and stratospheric ozone of the preindustrial trace gases and aerosols by themselves. The results showed that 1) the Maunder Minimum cooling relative to today was primarily associated with reduced anthropogenic radiative forcing, although the solar reduction added 40% to the overall cooling. There is no obv...

Stratospheric N2O-NOysystem: Testing uncertainties in a three-dimensional framework

Nitrous oxide (N2O) is an important greenhouse gas and the major source of stratospheric reactive nitrogen (NOy), an active participant in the stratospheric chemistry controlling ozone depletion. Tropospheric N2O abundances are increasing at nearly 0.3% yr−1 and this increase is expected to continue in the near future as are direct stratospheric NOy perturbations, for example, from aircraft. In order to test and gain confidence in three-dimensional (3-D) model simulations of the stratospheric N2O-NOy system, a simplified photochemistry for N2O and NOy is developed for use in chemistry transport models (CTMs). This chemical model allows for extensive CTM simulations focusing on uncertainties in chemistry and transport. We compare 3-D model simulations with measurements and evaluate the effect on N2O and NOy of potential errors in model transport, in column and local ozone, and in stratospheric temperatures. For example, with the three different 3-D wind fields used here, modeled N2O lifetimes vary from 173 to 115 years, and the unrealistically long lifetimes produce clear errors in equatorial N2O profiles. The impact of Antarctic denitrification and an in situ atmospheric N2O source are also evaluated. The modeled N2O and NOy distributions are obviously sensitive to model transport, particularly the strength of tropical upwelling in the stratosphere. Midlatitude, lower-stratospheric NO2/N2O correlations, including seasonal amplitudes, are well reproduced by the standard model when denitrification is included. These correlations are sensitive to changes in stratospheric chemistry but relatively insensitive to model transport. The lower stratospheric NOy/N2O correlation slope gives the correct net NOy production of about 0.5 Tg N yr−1 (i.e., the cross-tropopause flux as in the Plumb-Ko relation) only when N2O values from 250 to 310 ppb are used. As a consequence, the Synoz calibration of the flux of O3 from the stratosphere to the troposphere needs to be corrected to 550±140 Tg O3 yr−1.

Stratospheric ozone profiles retrieved from limb scattered sunlight radiance spectra measured by the OSIRIS instrument on the Odin satellite

[1]xa0Stratospheric ozone density profiles between 15 and 40 km altitude are derived from scattered sunlight limb radiance spectra measured with the Optical Spectrograph and InfraRed Imager System (OSIRIS) on the Odin satellite. The method is based on the analysis of limb radiance profiles in the centre and the wings of the Chappuis-Wulf absorption bands of ozone. It employs a non-linear Newtonian iteration version of Optimal Estimation (OE) coupled with the radiative transfer model LIMBTRAN. The derived zonally averaged ozone field for August 2001 is in excellent agreement with the main characteristics of the global morphology of stratospheric ozone, indicating that the limb scatter technique is capable of providing ozone profiles with high accuracy and high vertical resolution on a global scale and a daily basis.

Global modeling of the isotopic analogues of N2O: Stratospheric distributions, budgets, and the 17O–18O mass‐independent anomaly

14 N 14 N 16 O, 14 N 15 N 16 O, 15 N 14 N 16 O, 14 N 14 N 18 O, and 14 N 14 N 17 O. Two different chemistry models are used to derive photolysis cross sections for the analogues of N2O: (1) the zeropoint energy shift model, scaled by a factor of 2 to give better agreement with recent laboratory measurements and (2) the time-dependent Hermite propagator model. Overall, the CTM predicts stratospheric enrichments that are in good agreement with most measurements, with the latter model performing slightly better. Combining the CTMcalculated stratospheric losses for each N2O species with current estimates of tropospheric N2O sources defines a budget of flux-weighted enrichment factors for each. These N2O budgets are not in balance, and trends of � 0.04 to � 0.06 %/yr for the mean of 14 N 15 N 16 O and 15 N 14 N 16 O and � 0.01 to � 0.02 %/yr for 14 N 14 N 18 O are predicted, although each has large uncertainties associated with the sources. The CTM also predicts that 14 N 14 N 17 O and 14 N 14 N 18 O will be fractionated by photolysis in a manner that produces a nonzero massindependent anomaly. This effect can account for up to half of the observed anomaly in the stratosphere without invoking chemical sources. In addition, a simple one-dimensional model is used to investigate a number of chemical scenarios for the mass-independent composition of stratospheric N2O. INDEX TERMS: 0317 Atmospheric Composition and Structure: Chemical kinetic and photochemical properties; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0341 Atmospheric Composition and Structure: Middle atmosphere—constituent transport and chemistry (3334); 1040 Geochemistry: Isotopic composition/chemistry; 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation; KEYWORDS: global model, N2O, isotopomer, isotopologue, stratosphere, fractionation

Stratospheric profiles of nitrogen dioxide observed by Optical Spectrograph and Infrared Imager System on the Odin satellite

[1] Vertical profiles of nitrogen dioxide in the 19–40 km altitude range are successfully retrieved over the globe from Optical Spectrograph and Infrared Imager System (OSIRIS) limb scatter observations in late 2001 and early 2002. The inclusion of multiple scattering in the radiative transfer model used in the inversion algorithm allows for the retrieval of NO2 down to 19 km. The slant column densities, which represent the observations in the inversion, are obtained by fitting the fine structure in normalized radiance spectra over the 435–449 nm range, where NO2 electronic absorption is readily observable because of long light paths through stratospheric layers rich in this constituent. Details of the spectral fitting and inversion algorithm are discussed, including the discovery of a pseudo-absorber associated with pixelated detectors and a new method to verify altitude registration. Comparisons are made with spatially and temporally coincident profile measurements of this photochemically active trace gas. Better than 20% agreement is obtained with all correlative measurements over the common retrieval altitude range, confirming the validity of OSIRIS NO2 profiles. Systematic biases in the number densities are not observed at any altitude. A ‘‘snapshot’’ meridional cross section between 40� N and 70� S is shown from observations during a fraction of an orbit. INDEX TERMS: 0340 Atmospheric Composition and Structure: Middle atmosphere—composition and chemistry; 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0394 Atmospheric Composition and Structure: Instruments and techniques; 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); KEYWORDS: optical, Sun-synchronous, polar-orbiting, Fraunhofer, Ring effect, iterative onion peel

OBSERVATIONS OF STRATOSPHERIC AEROSOL USING CPFM POLARIZED LIMB RADIANCES

Abstract The authors have used CPFM (composition and photodissociative flux measurement) polarized limb radiance measurements combined with a vector radiative transfer model to estimate stratospheric aerosol number density, extinction coefficient profiles, and size distribution. The CPFM spectroradiometer is flown on board the NASA ER-2 high-altitude research aircraft. The vertical and horizontal polarization components of limb radiance, nadir radiance, and horizontal flux are measured in the wavelength range 300–770 nm from approximately 5°–10° above to 5°–10° below the local horizon. Results from two flights during April and May 1997 as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer campaign are presented. Aerosol characteristics are determined by forcing the model radiances and polarization to match the measurements. Results indicate number densities at 20 km are roughly 5–6 cm−3 with an effective radius of 0.17–0.20 μm. Number, surface area, and volume densities compare favora...

Understanding trends in stratospheric NOy and NO2

Nitrous oxide (N2O), an important greenhouse gas, has been increasing since 1980 at a rate of about +3% per decade. Recently, a notably greater rate of increase of about +5% per decade since 1980 was reported for measurements of stratospheric nitrogen dioxide (NO2) over Lauder, New Zealand. Since N2O is the dominant source of odd-nitrogen compounds in the stratosphere, including NO2, this presents an obvious conundrum. Analysis here shows that these apparently conflicting trends are generally consistent when viewed in a global-change framework, specifically, when concurrent trends in stratospheric ozone and halogens are included. Using a combination of photochemical and three-dimensional chemistry-transport models, we predict a 1980–2000 trend in the NO2, as measured over Lauder, New Zealand, of +4.3%/decade when these concurrent trends are considered. Of this, only +2.4%/decade is attributed directly to the increase in N2O; the remainder includes +2.5%/decade due to the ozone change and −0.6%/decade to the increased halogens impact on odd-nitrogen partitioning. The slant column densities of NO2, as measured from the zenith scattered sunlight during twilight, are found to (1) overestimate the trend by +0.4%/decade as compared to the true vertical column densities and (2) display a diurnally varying trend with a maximum during the night and large gradients through sunrise and sunset in good agreement with measurement. Nonetheless, measurements such as these are essential for identifying global change and provide a lesson in understanding it: careful simulation of the time, location, and geometry of measurements must be combined with concurrent trends in related chemical species and climate parameters.