SESSION OVERVIEW

Topic 3: Which chemical / physical processes are important in governing UTLS composition? Photochemistry, humidity and microphysics

Session convenors: Ross Salawitch & Bernd Kärcher
(rjs@caesar.jpl.nasa.gov & Bernd.Kaercher@dlr.de)

Science issues

upper tropospheric NOx and HOx
very short-lived organic halogen decomposition
heterogeneous chemistry (CIOx, NOx)
transport and distribution of humidity
relative humidity, vertical air motion variability
aerosol chemical composition
ice nucleation in cirrus clouds
vertical transport of chemically active gases and ice-forming aerosols

What was known 5 years ago?

a) Many measurements up to ~12 km altitude and a hand full of measurements through the tropopause demonstrate that peroxides and / or non methane hydrocarbons constitute a significant source of HOx. The long-lived NOy reservoir PAN likely plays an important role in the transport of NOy species above ~12 km altitude, but the role of PAN and lightning on NOx, for altitudes between ~12 km and the tropopause, is essentially untested by observations. Simulations of future evolution of tropospheric ozone are quite dependent on the strength of the lightning NOx production used in models.

b) There has long been speculation regarding the role of decomposition products of VSL chlorine and bromine species in the UTLS. Observations from GOME show much higher values of column BrO that found in standard models (eg. discrepancies as high as a factor of 3 or 4!) There has been much discussion suggesting this excess bromine resides in the troposphere, due to a ubiquitous, background level of 1 to 2 ppt of BrO. Recently, it has been suggested that both the stratosphere and troposphere might contribute to the GOME signal. These excess levels of bromine have important implications for photochemistry of tropospheric and / or stratospheric O3, depending on where the excess bromine resides. The role of VSL chlorine containing compounds has received less attention than bromine counterparts.

c) There is still uncertainty regarding the role of heterogeneous chemistry in the UTLS. Cirrus particles are known to exist in the lowermost stratosphere. It is unclear, however, if CLy levels are high enough for cirrus activation of chlorine to play an appreciable role in regulating abundances of ClO. For the UT, it is unclear whether production of NO2 or HONO from organic aerosols (in the presence of HNO3) plays a significant role in affecting UT photochemistry on a global scale.

d) The transport pathways, sinks, sources, and abundance of water vapor in the extra tropical UTLS have been studied with the help of satellite (HALOE, SAGE, MLS) and in-situ data (mainly ER-2). Isentropic transport from the subtropics and tropical TTL, large-scale diabatic descent of stratospheric air, small-scale vertical turbulent mixing through the tropopause, and sedimenting cirrus particles as a microphysical sink all contribute to the concentration and pronounced seasonal variation of water in the lowermost stratosphere. Recently new satellite sensors and improved in-situ measurements became available that - along with observations of exchange and mixing near the tropopause - could lead to a more robust characterization of the water budget.

e) Early measurements of H2O and RH suggested that ice supersaturation occurs at times in the UTLS, with low but non-negligible frequency also above the local midlatitude tropopause.  The body of observational evidence (including data from aircraft and satellites) for this occurrence has grown in the last years, but a global climatology is not available.  Mountain-induced mesoscale gravity waves and apparently ubiquituous small-scale variability in vertical winds and temperatures was known to affect lower stratospheric aerosol composition and to strongly contribute to Arctic PSC formation.  Only recently it has been recognized that mesoscale temperature fluctuations also play a crucial role in controlling the total ice crystal number density and other cirrus properties.

f) Until recently, it was commonly accepted that SO4 (in the form of sulfuric acid or ammonium sulfate) is the predominant aerosol species in the UTLS.  However, in-situ analyses indicate that crustal material, strongly absorbing (soot) material, and organics are further chemical constituents of high altitude aerosol particles.  A few global general circulation or chemistry transport models exist which are capable of predicting the lifecycle of the tropospheric aerosol, but the main focus of such studies is usually the lower troposphere, where most of the aerosol optical depth resides and warm cloud modification occurs.  The lack of detailed observations so far has precluded a critical evaluation of results of such global aerosol models regarding UTLS aerosols.

g) Homogeneous freezing of ice in liquid supercooled aerosol droplets is thought to be the predominant formation path of cirrus clouds below the water stability temperature (~235 K).  A few studies suggest significant potential impact of heterogeneous ice nuclei from natural (dust) or anthropogenic (aircraft soot) sources on cirrus formation and development.  Because of the difficulty to quantify the heterogeneous ice nucleation activity of aerosol particles in cirrus conditions and the paucity of in-situ observations, this issue has mostly been addressed with model studies that are largely untested by observations.

i) Deep convective clouds can cause significant transport of gases and particles from the boundary layer to the UTLS region within very short timescales, depending on their solubilities and hygroscopic properties.  Quantification of this effect is not yet possible in part because of the great sensitivity of cloud development on initial profiles (e.g., stability, humidity) and location (maritime, continental) and the highly complex suite of microphysical interactions between gases, aerosol particles, and liquid and solid cloud elements.  The lack of adequate parameterizations of gas-aerosol-cloud interactions does not allow the scale gap between cloud and global models to be closed.

Relevant sources of information include WMO 2002 and 2003, EU/THESEO 2000, IPCC 2001 (Climate Change), and IPCC 1999 (Aviation and the Global Atmosphere) assessment reports.

What is better understood now than 5 years ago?

Where are the remaining uncertainties?

When addressing the above two questions at the workshop, speakers are encouraged to provide an overview, rather than focus solely on their own contributions.  They should ideally cover instrument development, observations, and models.  Time allocated for invited contributions is kept at an acceptable minimum to ensure sufficient flexibility for the open discussion.

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