[Show abstract][Hide abstract] ABSTRACT: We use a series of chemical transport model and chemistry climate model simulations to investigate the observed negative trends in MOPITT CO over several regions of the world, and to examine the consistency of time-dependent emission inventories with observations. We find that simulations driven by the MACCity inventory, used for the Chemistry Climate Modeling Initiative (CCMI), reproduce the negative trends in the CO column observed by MOPITT for 2000–2010 over the eastern United States and Europe. However, the simulations have positive trends over eastern China, in contrast to the negative trends observed by MOPITT. The model bias in CO, after applying MOPITT averaging kernels, contributes to the model-observation discrepancy in the trend over eastern China. The total ozone column plays a role in determining the simulated tropospheric CO trends. A large positive anomaly in the simulated total ozone column in 2010 leads to a negative anomaly in OH and hence a positive anomaly in CO, contributing to the positive trend in simulated CO.
Preview · Article · Feb 2016 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: Observations from long-term ozonesonde measurements show robust variations and trends in the evolution of ozone in the middle and upper troposphere over Réunion Island (21.1°S, 55.5°E) in June-August. Here we examine possible causes of the observed ozone variation at Réunion Island using hindcast simulations by the stratosphere-troposphere Global Modeling Initiative chemical transport model for 1992-2014, driven by assimilated Modern-Era Retrospective Analysis for Research and Applications meteorological fields. Réunion Island is at the edge of the subtropical jet, a region of strong stratospheric-tropospheric exchange. Our analysis implies that the large interannual variation (IAV) of upper tropospheric ozone over Réunion is driven by the large IAV of the stratospheric influence. The IAV of the large-scale, quasi-horizontal wind patterns also contributes to the IAV of ozone in the upper troposphere. Comparison to a simulation with constant emissions indicates that increasing emissions do not lead to the maximum trend in the middle and upper troposphere over Réunion during austral winter implied by the sonde data. The effects of increasing emission over southern Africa are limited to the lower troposphere near the surface in August-September.
No preview · Article · Dec 2015 · Journal of Geophysical Research Atmospheres
[Show abstract][Hide abstract] ABSTRACT: A low bias in carbon monoxide (CO) at high northern latitudes is a common feature of chemistry climate models (CCMs) that may indicate or contribute to a high bias in simulated OH and corresponding low bias in methane lifetime. We use simulations with CO tagged by source type to investigate the sensitivity of the CO bias to CO emissions, global mean OH, and the hemispheric asymmetry of OH. Our results show that reducing the hemispheric asymmetry of OH improves the agreement of simulated CO with observations. We use simulations with parameterized OH to quantify the impact of known model biases on simulated OH. Removing biases in ozone and water vapor as well as reducing Northern Hemisphere NOx does not remove the hemispheric asymmetry in OH, but brings the simulated methyl chloroform lifetime into agreement with observation-based estimates.
No preview · Article · Oct 2015 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: We present an analysis of joint balloon sonde profiles of water vapor and ozone made at Costa Rica from 2005–2011 using compositing techniques, tracer-tracer diagrams and back-trajectory methods. Our analysis reveals important seasonal differences in structure in the upper troposphere and lower stratosphere. Water vapor amounts in boreal winter at Costa Rica are much lower than expected from local ice saturation temperatures. The boreal summer data show both higher average water vapor amounts and a much higher level of variability than the winter data. To understand this seasonal contrast we consider three sources of tracer variability: wave-induced vertical motion across strong vertical gradients (‘wave variability’), differences in source air masses resulting from horizontal transport (‘source variability’), and changes induced along parcel paths due to physical processes (‘path variability’). The winter and summer seasons show different mixes of these three sources of variability with more air originating in the tropical western Pacific during winter.
[Show abstract][Hide abstract] ABSTRACT: The lifetime of nitrous oxide, the third-most-important human-emitted greenhouse gas, is based to date primarily on model studies or scaling to other gases. This work calculates a semiempirical lifetime based on Microwave Limb Sounder satellite measurements of stratospheric profiles of nitrous oxide, ozone, and temperature; laboratory cross-section data for ozone and molecular oxygen plus kinetics for O(1D); the observed solar spectrum; and a simple radiative transfer model. The result is 116 ± 9 years. The observed monthly-to-biennial variations in lifetime and tropical abundance are well matched by four independent chemistry-transport models driven by reanalysis meteorological fields for the period of observation (2005–2010), but all these models overestimate the lifetime due to lower abundances in the critical loss region near 32 km in the tropics. These models plus a chemistry-climate model agree on the nitrous oxide feedback factor on its own lifetime of 0.94 ± 0.01, giving N2O perturbations an effective residence time of 109 years. Combining this new empirical lifetime with model estimates of residence time and preindustrial lifetime (123 years) adjusts our best estimates of the human-natural balance of emissions today and improves the accuracy of projected nitrous oxide increases over this century.
Full-text · Article · May 2015 · Journal of Geophysical Research Atmospheres
[Show abstract][Hide abstract] ABSTRACT: Using a decade of Aura Microwave Limb Sounder observations, we show distinctly different N2O distributions in Southern Hemisphere winter that depend on the phase of the quasi-biennial oscillation (QBO). Composites of the nitrous oxide (N2O) anomalies calculated for westerly and easterly phases show that QBO-generated variability originating in the subtropical middle stratosphere fills the midlatitude surf zone by late winter. After the spring vortex breakup, the anomaly is transported to the Antarctic where it remains until the next vortex forms in fall. Trapped in the newly formed vortex, the anomaly descends in isolation through fall and winter, arriving in the Antarctic lower stratosphere in September—about 1 year after it formed. This transport pathway explains previously reported variability of N2O and inorganic chlorine (Cly) inside the Antarctic vortex and demonstrates that the middle stratosphere QBO affects ozone depletion by modulating Antarctic Cly.
No preview · Article · May 2015 · Geophysical Research Letters
[Show abstract][Hide abstract] ABSTRACT: Eight years of ozone measurements retrieved from the Ozone Monitoring Instrument (OMI) and the Microwave Limb Sounder, both on the EOS Aura satellite, have been assimilated into the Goddard Earth Observing System version 5 (GEOS-5) data assimilation system. This study evaluates this assimilated product, highlighting its potential for science. The impact of observations on the GEOS-5 system is explored by examining the spatial distribution of the observation-minus-forecast statistics. Independent data are used for product validation. The correlation of the lower-stratospheric (the tropopause to 50 hPa) ozone column with ozonesondes is 0.99 and the (high) bias is 0.5%, indicating the success of the assimilation in reproducing the ozone variability in that layer. The upper-tropospheric (500 hPa to the tropopause) assimilated ozone column is about 10% lower than the ozonesonde column but the correlation is still high (0.87). The assimilation is shown to realistically capture the sharp cross-tropopause gradient in ozone mixing ratio. Occurrence of transport-driven low ozone laminae in the assimilation system is similar to that obtained from the High Resolution Dynamics Limb Sounder (HIRDLS) above the 400 K potential temperature surface but the assimilation produces fewer laminae than seen by HIRDLS below that surface. Although the assimilation produces about 25% fewer occurrences per day during the three years of HIRDLS data, the interannual variability is captured correctly.This data-driven assimilated product is complementary to ozone fields generated from chemistry and transport models. Applications include study of the radiative forcing by ozone and tracer transport near the tropopause.
[Show abstract][Hide abstract] ABSTRACT: Aura OMI and MLS measurements are combined to produce daily maps of tropospheric ozone beginning October 2004. We show that El Ni no Southern Oscillation (ENSO) related inter-annual change in tropospheric ozone in the tropics is small compared to combined intra-seasonal/Madden–Julian Oscillation (MJO) and shorter timescale variability by a factor ~ 3–10 (largest in the Atlantic). Outgoing Longwave Radiation (OLR) indicates further that deep convection is the primary driver of the observed tropospheric ozone variability from ENSO down to weekly timescales. We compare tropospheric ozone and OLR satellite observations with two simulations: (1) the Goddard Earth Observing System (GEOS) chemistry-climate model (CCM) that uses observed sea surface temperatures and is otherwise free-running, and (2) the NASA Global Modeling Initiative (GMI) chemical transport model (CTM) that is driven by Modern-Era Retrospective Analysis for Research and Applications (MERRA) analyses. It is shown that the CTM-simulated ozone accurately matches measurements for timescales from ENSO to intra-seasonal/MJO and even 1–2 week periods; however (though not unexpected) the CCM simulation reproduces ENSO variability but not shorter timescales. These analyses suggest that using a model to delineate temporal/spatial properties of tropospheric ozone and convection in the tropics will require that the model reproduce the non-ENSO variability that dominates.
No preview · Article · Mar 2015 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: The atmospheric levels of human-produced chlorocarbons and bromocarbons are projected to make only small contributions to ozone depletion by 2100. Increases in carbon dioxide (CO2) and nitrous oxide (N2O) will become increasingly important in determining the future of the ozone layer. N2O increases lead to increased production of nitrogen oxides (NOx ), contributing to ozone depletion. CO2 increases cool the stratosphere and affect ozone levels in several ways. Cooling decreases the rate of many photochemical reactions, thus slowing ozone loss rates. Cooling also increases the chemical destruction of nitrogen oxides, thereby moderating the effect of increased N2O on ozone depletion. The stratospheric ozone level projected for the end of this century therefore depends on future emissions of both CO2 and N2O. We use a two-dimensional chemical transport model to explore a wide range of values for the boundary conditions for CO2 and N2O, and find that all of the current scenarios for growth of greenhouse gases project the global average ozone to be larger in 2100 than in 1960.
Preview · Article · Mar 2015 · Environmental Research Letters
[Show abstract][Hide abstract] ABSTRACT: We infer the interannual variability of inorganic chlorine in the Antarctic lower stratospheric vortex using nine years of Aura Microwave Limb Sounder (MLS) nitrous oxide (N2O) measurements and a previously measured compact correlation. Inorganic chlorine (Cly) is the sum of the destruction products of long-lived chlorine-containing source gases. Its correlation with N2O, derived from observations in the year 2000, is scaled to the years 2004-2012 to account for subsequent N2O growth and chlorofluorocarbon decline. The expected annual Cly change due to the Montreal Protocol is -20 ppt/yr, but the MLS-inferred Cly varies year-to-year from -200 to +150 ppt. Because of this large variability, attributing Antarctic ozone recovery to a statistically significant chlorine trend requires 10 years of chlorine decline. We examine the relationship between Equivalent Effective Stratospheric Chlorine (EESC) and ozone hole area. Temperature variations driven by dynamics are a primary contributor to area variability, but we find a clear linear relationship between EESC and area during years when Antarctic collar temperatures are 1σ or more below the mean. This relationship suggests that smaller ozone hole areas in recent cold years 2008 and 2011 are responding to decreased chlorine loading. Using ozone hole areas from 1979-2013, the projected EESC decline, and the inferred interannual Cly variability, we expect ozone hole areas greater than 20 million km2 will occur during very cold years until 2040. After that time, all ozone hole areas are likely to be below that size due to reduced EESC levels.
[Show abstract][Hide abstract] ABSTRACT: In the 30 years since the ozone hole was discovered, our understanding of the polar atmosphere has become much more complete. The worldwide response to the discovery was fast, but the recovery is slow.
[Show abstract][Hide abstract] ABSTRACT: We examine the seasonal behavior of ozone by using measurements from various instruments including ozonesondes, Aura MLS, and SAGE II. We find that the magnitude of the annual variation in ozone, as a percentage of the mean ozone, exhibits a maximum at or slightly above the tropical tropopause. The maximum is larger in the northern tropics than in the southern tropics, and the annual maximum of ozone in the southern tropics occurs two months later than that in the northern tropics, in contrast to usual assumption that the tropics can be treated as a horizontally homogeneous region. The seasonal cycles of ozone and other species in this part of the lower stratosphere result from a combination of the seasonal variation of the Brewer-Dobson Circulation and the seasonal variation of tropical and mid-latitude mixing. In the northern hemisphere, the impacts of upwelling and mixing between the tropics and midlatitudes on ozone are in phase and additive. In the southern hemisphere, they are not in phase. We apply a tropical leaky pipe model independently to each hemisphere to examine the relative roles of upwelling and mixing in the northern and southern tropical regions. Reasonable assumptions of the seasonal variation of upwelling and mixing yield a good description of the seasonal magnitude and phase in both the southern and northern tropics. The differences in the tracers and transport between the northern and southern tropical stratosphere suggest that the paradigm of well-mixed tropics needs to be revised to consider latitudinal variations within the tropics.
[Show abstract][Hide abstract] ABSTRACT: Measurements from the Ozone Monitoring Instrument (OMI) and Microwave Limb Sounder (MLS), both onboard the Aura spacecraft, have been used to produce daily global maps of column and profile ozone since August 2004. Here we compare and evaluate three strategies to obtain daily maps of tropospheric and stratospheric ozone from OMI and MLS measurements: trajectory mapping, direct profile retrieval, and data assimilation. Evaluation is based on an assessment that includes validation using ozonesondes and comparisons with the Global Modeling Initiative (GMI) chemical transport model (CTM). We investigate applications of the three ozone data products from near-decadal and inter-annual timescales to day-to-day case studies. Inter-annual changes in zonal mean tropospheric ozone from all of the products in any latitude range are of the order 1-2 Dobson Units while changes (increases) over the 8-year Aura record investigated vary by 2-4 Dobson Units. It is demonstrated that all of the ozone products can measure and monitor exceptional tropospheric ozone events including major forest fire and pollution transport events. Stratospheric ozone during the Aura record has several anomalous inter-annual events including split stratospheric warmings in the Northern Hemisphere extra-tropics that are well captured using the data assimilation ozone profile product. Data assimilation with continuous daily global coverage and vertical ozone profile information is the best of the three strategies at generating a global tropospheric and stratospheric ozone product for science applications.
[Show abstract][Hide abstract] ABSTRACT: The evolution of ozone is examined in the latest version of the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM) using old and new ozone depleting substances (ODS) scenarios. This version of GEOSCCM includes a representation of the Quasi-Biennial Oscillation, a more realistic implementation of ozone chemistry at high solar zenith angles, an improved air/sea roughness parameterization, and an extra 5 ppt of CH3Br to account for brominated very short-lived substances. Together these additions improve the representation of ozone compared to observations. This improved version of GEOSCCM was used to simulate the ozone evolution for the A1 2010 and the new SPARC 2013 ODS scenario derived using the SPARC Lifetimes Report 2013. This new ODS scenario results in a maximum Cltot increase of 65 pptv, decreasing slightly to 60 pptv by 2100. Approximately 72% of the increase is due to the longer lifetime of CFC-11. The quasi-global (60°S-60°N) total column ozone difference is relatively small and less than 1 DU on average and consistent with the 3-4% larger 2050-2080 average Cly in the new SPARC 2013 scenario. Over high latitudes, this small change in Cly compared to the relatively large natural variability makes it not possible to discern a significant impact on ozone in the 2nd half of the 21st century in a single set of simulations.
[Show abstract][Hide abstract] ABSTRACT: Chemistry climate models (CCMs) are used to project future evolution of stratospheric ozone as concentrations of ozone-depleting substances (ODSs) decrease and GHGs increase, cooling the stratosphere. CCM projections exhibit many common features, but also a broad range of values for quantities such as year of ozone-return-to-1980 and global ozone level at the end of the 21st century. Multiple linear regression is applied to each of fourteen CCMs to separate ozone response to ODS concentration change from that due to climate change. We show that the sensitivity of lower stratospheric ozone to chlorine change ΔO3/ΔCly is a near linear function of partitioning of total inorganic chlorine (Cly) into its reservoirs; both Cly and its partitioning are largely controlled by lower stratospheric transport. CCMs with best performance on transport diagnostics agree with observations for chlorine reservoirs and produce similar ozone responses to chlorine change. After 2035 differences in ΔO3/ΔCly contribute little to the spread in CCM projections as the anthropogenic contribution to Cly becomes unimportant. Differences among upper stratospheric ozone increases due to temperature decreases are explained by differences in ozone sensitivity to temperature change ΔO3/ΔT due to different contributions from various ozone loss processes, each with its own temperature dependence. Ozone decrease in the tropical lower stratosphere caused by a projected speed-up in the Brewer-Dobson circulation may or may not be balanced by ozone increases in the middle and high latitude lower stratosphere and upper troposphere. This balance, or lack thereof, contributes most to the spread in late 21st century projections.
[Show abstract][Hide abstract] ABSTRACT:  The 2011 Arctic stratospheric final warming was characterized by a large-scale frozen-in anticyclone (FrIAC) that rapidly displaced the winter polar vortex, establishing unusually strong polar easterlies. A comprehensive overview of the 2011 FrIAC is provided using meteorological analyses, Microwave Limb Sounder (MLS) N2O observations, and N2O simulations from the Global Modeling Initiative (GMI) 3-D chemistry and transport model and the Van Leer Icosahedral Triangular Advection (VITA) 2-D (latitude × longitude) isentropic transport model. A vortex edge diagnostic is used to determine the FrIAC boundary, allowing quantification of several FrIAC properties. The 2011 FrIAC originated over North Africa in late March and traveled eastward and poleward over 2 weeks, forming a strong anticyclone that extended from ~580–2100 K potential temperature (~25–50 km). Low potential vorticity (PV) was transported to the pole with the FrIAC in early April; during May, most of the PV signature decayed due to diabatic processes. A small remnant negative PV anomaly persisted near the pole until mid-June. Tracer equivalent latitude was low initially and remained low throughout the summer. GMI, VITA, and MLS showed elevated N2O in the FrIAC, although the peak value was smaller in GMI due to a subtropical low bias. The high-resolution (~20 km) VITA filamentary structure quantitatively matched most of the features observed by MLS when smoothed to match the MLS resolution. The high-N2O anomaly persisted in the middle stratosphere over 4 months until late August, when it was destroyed by horizontal and vertical shearing, combined with photochemical processes.
[Show abstract][Hide abstract] ABSTRACT: Observations have shown that the mass of nitrogen dioxide decreased at both southern and northern midlatitudes in the year following the eruption of Mt. Pinatubo, indicating that the volcanic aerosol had enhanced nitrogen dioxide depletion via heterogeneous chemistry. In contrast, the observed ozone response showed a northern midlatitude decrease and a small southern midlatitude increase. Previous simulations that included an enhancement of heterogeneous chemistry by the volcanic aerosol but no other effect of this aerosol produce ozone decreases in both hemispheres, contrary to observations. The authors' simulations show that the heating due to the volcanic aerosol enhanced both the tropical upwelling and Southern Hemisphere extratropical downwelling. This enhanced extratropical downwelling, combined with the time of the eruption relative to the phase of the Brewer–Dobson circulation, increased Southern Hemisphere ozone via advection, counteracting the ozone depletion due to heterogeneous chemistry on the Pinatubo aerosol.
No preview · Article · Mar 2013 · Journal of the Atmospheric Sciences
[Show abstract][Hide abstract] ABSTRACT: Stratospheric and total columns of Arctic O-3 (63-90 degrees N) in late March 2011 averaged 320 and 349 DU, respectively, 50-100 DU lower than any of the previous 6 years. We use Aura Microwave Limb Sounder (MLS) O-3 observations to quantify the roles of chemistry and transport and find there are two major reasons for low O-3 in March 2011: heterogeneous chemical loss and a late final warming that delayed the resupply of O-3 until April. Daily vortex-averaged partial columns in the lowermost stratosphere (p > 133 hPa) and middle stratosphere (p < 29 hPa) are largely unaffected by local heterogeneous chemistry, according to model calculations. Very weak transport into the vortex between late January and late March contributes to the observed low ozone. The lower stratospheric (LS) column (133-29 hPa, similar to 370-550 K) is affected by both heterogeneous chemistry and transport. Because MLS N2O data show strong isolation of the vortex, we estimate the contribution of vertical transport to LS O-3 using the descent of vortex N2O profiles. Simulations with the Global Modeling Initiative (GMI) chemistry and transport model (CTM) with and without heterogeneous chemical reactions show 73 DU vortex averaged O-3 loss; the loss derived from MLS O-3 is 84 +/- 12 DU. The GMI simulation reproduces the observed O-3 and N2O with little error and demonstrates credible transport and chemistry. Without heterogeneous chemical loss, March 2011 vortex O-3 would have been at least 40 DU lower than climatology due to the late final warming that did not resupply O-3 until mid-April. Citation: Strahan, S. E., A. R. Douglass, and P. A. Newman (2013), The contributions of chemistry and transport to low arctic ozone in March 2011 derived from Aura MLS observations, J. Geophys. Res. Atmos., 118, 1563-1576, doi:10.1002/jgrd.50181.
[Show abstract][Hide abstract] ABSTRACT: The extratropical stratosphere-troposphere exchange (STE) of ozone from
2005 to 2010 is estimated by combining Microwave Limb Sounder ozone
observations and MERRA reanalysis meteorological fields in an
established direct diagnostic framework. The multiyear mean ozone STE is
275 Tg yr-1 and 214 Tg yr-1 in the Northern and
Southern Hemispheres, respectively. The year-to-year variability is
greater in the Northern Hemisphere, where the difference between the
highest and the lowest annual flux is 15% of the multiyear mean compared
with 6% in the Southern Hemisphere. Variability of lower stratospheric
ozone and variability of the net mass flux both contribute to
interannual variability in the Northern Hemisphere ozone flux. The flux
across the extratropical 380 K surface determines the amount of flux
across the extratropical tropopause, and the greatest seasonal
variability of the 380 K ozone flux occurs in the late winter/early
spring, around the time of greatest flux. Both the mass flux and the
ozone mixing ratios on the 380 K surface show recurring spatial
patterns, but interannual variability of these quantities and their
alignment contribute to the ozone flux variability. The spatial and
temporal variability are not well represented when zonal and/or monthly
mean fields are used to calculate the ozone STE, although this results
in a small high bias of the seasonal amplitude and annual magnitude. If
the climatological variability over these 6 years is representative, the
estimated number of years required to detect a 2 - 3%
decade-1 trend in ozone STE using this diagnostic is 35 - 39
No preview · Article · Jan 2013 · Journal of Geophysical Research Atmospheres
[Show abstract][Hide abstract] ABSTRACT: The El Niño-Southern Oscillation (ENSO) is the dominant mode of
inter-annual variability in the tropical ocean and troposphere. Its
impact on tropospheric circulation causes significant changes to the
distribution of ozone. Here we derive the lower tropospheric to lower
stratospheric ozone response to ENSO from observations by the
Tropospheric Emission Spectrometer (TES) and the Microwave Limb Sounder
(MLS) instruments, both on the Aura satellite, and compare to the
simulated response from the Goddard Earth Observing System
Chemistry-Climate Model (GEOSCCM). Measurement ozone sensitivity is
derived using multiple linear regression to include variations from ENSO
as well as from the first two empirical orthogonal functions of the
quasi-biennial oscillation. Both measurements and simulation show
features such as the negative ozone sensitivity to ENSO over the
tropospheric tropical Pacific and positive ozone sensitivity over
Indonesia and the Indian Ocean region. Ozone sensitivity to ENSO is
generally positive over the midlatitude lower stratosphere, with greater
sensitivity in the Northern Hemisphere. GEOSCCM reproduces both the
overall pattern and magnitude of the ozone response to ENSO obtained
from observations. We demonstrate the combined use of ozone measurements
from MLS and TES to quantify the lower atmospheric ozone response to
ENSO and suggest its possible usefulness in evaluating chemistry-climate
No preview · Article · Jan 2013 · Journal of Geophysical Research Atmospheres