Water and Methane in the Upper Troposphere and
Stratosphere based on ACE-FTS Measurements
Ray Nassar, Chris Boone, Kaley A. Walker and Peter F. Bernath
Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
ray@acebox.uwaterloo.ca
The SCISAT-1 mission, also known as the Atmospheric Chemistry Experiment (ACE), is a Canadian satellite
mission to investigate chemical and dynamical processes that control the distribution of ozone in the
stratosphere and upper troposphere. The primary instrument on SCISAT-1 is the ACE-FTS, a high resolution
Fourier transform spectrometer operating in the 750-4500 cm-1 range. ACE-FTS solar occultation spectra
dating back to January 2004 have been processed to produce Version 1 temperature and pressure profiles from
.
the mid-troposphere to the mesopause and VMR profiles of H2O, N2O, CO, NO, HCl, HF, HNO3, CH4, O3,
NO2, HDO, SF6, COF2, N2O5, ClONO2, CCl3F, CCl2F2 and CHF2Cl. VMR profiles of many other molecules
will be included in our Version 2 data, which will soon be available. Here we present ACE-FTS H2O, HDO
and CH4 data and discuss their relationship to trends and measurements by other instruments as well as the
implications of these results for understanding processes relating to stratospheric water.
1. Background and introduction
The amount of water vapor in the stratosphere plays an important role in the climate and the chemistry of our atmosphere.
Elevated levels of water are known to enhance tropospheric warming and stratospheric cooling. Elevated levels of
stratospheric water accompanied by enhanced cooling, increase the formation of polar stratospheric clouds (PSCs) during
the polar winter enabling heterogeneous reactions that form species which can catalytically destroy stratospheric ozone
during the polar spring.
Water primarily enters the stratosphere through vertical transport across the tropical tropopause layer (TTL), then circulates
to midlatitudes and descends at the poles. The major photochemical source of water in the stratosphere is oxidation of CH4
by the reaction CH4 + 2O2 → 2H2O + CO2, in which one CH4 molecule creates two H2O molecules. A high level of
variability is often observed if one treats H2O and CH4 independently but [H2O]+2[CH4], often referred to as potential
water (PW) is conserved with CH4 oxidation.
Numerous studies indicate an increases in stratospheric water vapor occurring in the time frame of a few years up to the
past half century. This increase can partially be attributed to an increase in CH4 emissions but the remainder is attributed to
an increase in water entering the stratosphere across the TTL, known as [H2O]e. More recent evidence from instruments
such as HALOE indicate a halt or possible plateau in the increase in stratospheric [H2O] in the last few years.
2. H2O and CH4 measured by the ACE-FTS
We have determined [H2O], [CH4] and PW based on 83 extratropical
extravortex ACE-FTS occultations. Figure 1 shows the individual and
averaged profiles of the occultations used in the determination. Based on
these measurements, we obtain PW = 7.14±0.05 ppm and [H2O]e =
3.65±0.15 ppm. Due to high variability in measured values of water crossing
the TTL, [H2O]e is often calculated as:
[H2O]e = [H2O] -b ([CH4]e - [CH4]) or [H2O]e = PW - b[CH4]e
where b is the magnitude of the slope of the [H2O] vs. [CH4] correlation plot
(shown in Figure 2e) and [CH4]e is the methane entering the stratosphere
across the TTL which can be measured more accurately than [H2O]e.
3. Trends and comparison with other measurements
Figure 1: 83 extratropical extravortex
profiles of [CH4] and [H2O]. Individual
profiles are shown as thin colored lines,
averaged profiles are shown as black
dotted lines and potential water as PW =
[H2O]+2[CH4] is shown in black with 1s
error bars. Notice that [H2O] and [CH4]
variability from ~30-48 km is anticorrelated thus does not contribute to
uncertainty in the PW profile.
The ATMOS and ACE-FTS instruments, measurement techniques and
retrievals are similar enough to justify a direct comparison of results. The
occultations used above were selected to minimize seasonal affects during
comparison with ATMOS measurements recorded from 1985-1994. Figure 2
shows the [H2O] vs. [CH4] correlation plots and indicates that the increases in
PW and [H2O]e observed by ATMOS have clearly not continued to present.
We observe only a negligible change from 1994-2004, however we make no
prediction regarding how stratospheric water levels will unfold in the future.
f
e
g
Figure 2: [H2O] vs. [CH4] correlations from 4 ATMOS missions (1985-1994) and ACE (2004). (ATMOS plots were
reproduced from H.A. Michelsen et al. JGR 105, D18, 22713, 2000.) Tangent heights from 18-40 km (open circles) were
fit, with the slope giving –b and the intercept giving potential water as [H2O] + b[CH4]. Our results indicate that increases
in potential water and water entering the stratosphere [H2O]e have not continued.
B
4. HDO and H2O near the tropical tropopause layer
The relative importance of processes that control the quantity of water entering
the stratosphere are still not well understood. In gradual dehydration, assumed to
be the major process, condensation occurs gradually and the humidity of the
stratosphere is controlled by the coldest temperature (or lowest saturation water
VMR) that the air has experienced. In convective dehydration, air entering the
TTL by deep convection is already dehydrated below stratospheric abundances.
Since HDO has a lower vapor pressure than H2O, the isotopic composition of
water vapor in the TTL region can act as a useful tracer for determining the type
of dehydration processes involved. When vapor is in equilibrium with the
condensed phases, HDO is preferentially removed relative to H2O, a process
called fractionation. The convention is to express the result relative to VSMOW
(Vienna Standard Mean Ocean Water) using the following relationship:
 [ HDO]/[ H 2O]sample

d D  1000  
 1
 [ HDO]/[ H 2O]VSMOW 
Kuang et al. determined a near-vertical dD profile with dD ~ -670‰ based on 11
tropical ATMOS occultations, cited as evidence for convective dehydration. Our
preliminary work with ACE-FTS data gives dD ~ -673‰ from 45 winter/spring
occultations and dD ~ -648‰ based on 25 summer occultations above the TTL
(14-18 km). Furthermore, we observe an increase in [HDO]/[H2O] ratios with
altitude in the stratosphere and some seasonal variability above the TTL. We plan
to do a more in depth analysis of dD profiles based on [HDO]/[H2O] in the future.
Figure 3: (A) ATMOS ATLAS-3
[HDO]/[H2O] dD profiles taken from Z.
Kuang et al. GRL, 30, 7, 1372, 2003.
(B) ACE-FTS [HDO]/[H2O] dD
profiles from 25°N-25°S, which show a
similar dD value at the TTL and exhibit
evidence of seasonal variation.
Acknowledgements: The Canadian Space Agency (CSA) is the primary source of funding for this work. Other sources of funding include the Meteorological Service of Canada (MSC), Natural
Sciences and Engineering Research Council of Canada (NSERC), Canadian Foundation for Innovation (CFI) and the University of Waterloo.