Atmospheric Chemistry in the Tropical Tropopause Layer

Atmospheric Chemistry in the Tropical Tropopause Layer

Mark G. Lawrence

Max Planck Institute for ChemistryMainz, Germany

SPARC/GEWEX/IGAC TTL WorkshopVictoria, Canada, 14 June 2006

Mark G. Lawrence, Max Planck Institute for Chemistry TTL Workshop, Victoria, Canada, 14 June 2006

Flashback to the “dark ages” (about 3-5 years ago)

Observations

• WMO (2002): “Currently very few chemical observations of species other than ozone are available in the TTL”

• Tuck et al. (2004): “There are no published horizontal observations of water, ozone, and tracers in the upper tropical troposphere [ TTL]… recent analysis of this region has worked in terms of single-valued ozone and water vertical profiles…”

Modelling

• WMO (2002): “These estimates…depend strongly on their respective model formulation…such modeling studies, however, are unfortunately rarely repeated once the first ones have been published.”


But we’ve not been completely in the dark…

Selected Topics For This Overview

• Low-Ozone Airmasses in the TTL

• Deep Convective Transport of Tracers

• Scavenging – Especially by Ice and Role for HNO3

• Extended Horizontal Observations of a Suite of Gases (ACCENT results)

Largely complementary to topics and more recent results

in other talks / posters (“excess” of MPI results for illustrations)


Why has it been so difficult?

• Tough region to observe in situ

– Only a few aircraft can reach well into the TTL Pickering talk: recent extensive efforts in TROCCINOX, SCOUT-O3, …

– O3-sonde situation improved by SHADOZ…but:• It’s only one gas (albeit important)

• It’s quite long-lived ( ~ 1 year in the TTL) probably tells more about transport than the real “chemical nature”

• Tough region to observe with remote sensing – Satellites: overlying stratospheric columns, steep gradients near the TT– Ground-based (FTIR, LIDAR): far away

• Tough region to model– Heavily dependent on:

• Deep convection and convective transport parameterizations• Representation of the Brewer-Dobson circulation (poor in tropospheric models with caps ~10 hPa)• Representation of complex NMHC chemistry (e.g., acetone as a HOx source) and scavenging, which

are typically poor in middle-atmosphere models


What TTL chemistry topics are we interested in for future observations and

model studies?• O3

– Chemistry “driver”– Radiative forcing

• HOx – Intriguing, though not expected to be critical for the global oxidizing

efficiency (e.g., for CH4)

• Halogenated VSLS – Stratospheric source– See WMO, 2002 (and upcoming assessment)

• Aerosols – Cirrus cloud effects – Influence on water transport into the stratosphere

• In situ emissions – Aircraft and Lightning


Timescales

UT

BL, ~ 5 d

UTUT

LSLS

TTL

TT, ~16-17 km

STT, ~11-13 km

~ 10 (5-20) d

~ months

“potentially significant”

FBL-TT ~ 0.2-6% FBL-STT

(Based on WMO, 2002)


Selected Topics for this Overview






CEPEXCentral Equatorial Pacific Experiment

March, 1993


Observed and MATCH-MPIC vertical O3 profiles

Observations (balloon sondes)Modeled (MATCH-MPIC)

(Kley et al., Science, 1996;Lawrence et al., QJRMS, 1999)

West Middle East


Western CEPEX regionCentral CEPEX regionEastern CEPEX region

With Convection No O3 Convection

Effect of Convection on Modelled O3

(Lawrence et al., QJRMS, 1999)

(Only convective Transport of ozone was turned off)


Effect of Convection on Modelled O3

(Salzmann, 2005)

WRF Simulation Results for TOGA COARE

TTL – O3 reduced by convective transport


Other factors possibly influencing the TTL O3 minima?

• Reactions on Ice (HO2, Cl/Br)?– Kley et al., Science, 1996; Lawrence, 1996

• Reactions in the MBL (Halogens)?– Lawrence et al., 1999

• Lightning NOx => Titration?– Wang and Prinn, 2000

Asman et al. (2003): Extreme O3 minima are rare in the MOZAIC data…But flights likely too low and not sampling Pacific enough


black lines - average profiles for 1998–2004 colored lines - illustrative reduced ozone events

Ozone Sonde (SHADOZ) Observations

(Solomon et al., GRL, 2005)



• Reduced ozone events (< 20 nmol/mol) most common over western Pacific

• Very low ozone events (< 10 nmol/mol) nevertheless quite rare





Ozone frequency distribution changing over time!

Samoa, 200 hPa








Deep Convection – Outflow in the TTL

(Folkins and Martin, JAS, 2005)

Airmasses from convection detraining above the transition from radiative cooling to radiative warming (~15 km) have a much greater chance of being transported into the stratosphere


Yanai et al., 1973; Tiedtke, 1989; Grell, 1993; Pan and Wu, 1995; Zhang and McFarlane, 1995

BulkPlume Ensemble

Arakawa and Schubert, 1974; Lord et al., 1982; Hack et al., 1984; Grell, 1993


Convective Transport Formulations

(Lawrence and Rasch, JAS, 2005)

Mark G. Lawrence, Max Planck Institute for Chemistry TTL Workshop, Victoria, Canada, 14 June 2006Mark G. Lawrence, Max Planck Institute for Chemistry TTL Workshop, Victoria, Canada, 14 June 2006

Role of Convective Transport Formulations

= 2 d

= 1 d

Zonal Mean, July 2001 (Lawrence and Rasch, JAS, 2005)


-2 -1 0 1 2 3 Cloud mass flux (kg/m2/s)

-2 -1 0 1 2 3 Cloud mass flux (kg/m2/s)

-.1 -.05 0.0 .05 .1 Cloud mass flux

(kg/m2/s)

-.1 -.05 0.0 .05 .1 Cloud mass flux

(kg/m2/s)

Domain Means Cloudy-Area Means

Convective Mass Flux Characterizations

(Salzmann, 2005)








Influence of Precipitation Scavenging

Hx in M/atm:

Hx = 0Hx = 103

Hx = 104

Hx = 105

Hx = 106

Ice

(Crutzen and Lawrence, J. Atmos. Chem., 2000)


Influence of Precipitation Scavenging

Hx in M/atm:

Hx = 0Hx = 103

Hx = 104

Hx = 105

Hx = 106

Ice

(Crutzen and Lawrence, J. Atmos. Chem., 2000)


Influence of Precipitation Scavenging: Role of Uptake of HNO3 on Ice

• Large sensitivity to uptake formulation• Based on comparison to observations, some degree of

uptake and scavenging seems very likely, quantification still difficult

• Effects on ozone minor

Focus shifted towards HNO3 itself, especially role for cloud microphys., e.g., -ice (Gao et al., Science, 2004)

• Further laboratory and field work needed!

(von Kuhlmann and Lawrence, ACP, 2006)








ACCENT TTL Chemistry Observations

(Tuck et al., JGR, 2004; Ridley et al., Atmos. Env., 2004)

20 Sept., 1999 21 Sept., 1999




Wide variety of gases and aerosol components observed, large spatial variability




• Samples collected between 10 and 19 km altitude (both UT/LS and TTL)

• Chloroform: Continental tracer

• Methyl Nitrate: Marine tracer

• Both continental and marine origins observed at “essentially all latitudes” covered by the flights




• Particularly informative: correlations with O3

• Correlations found with tracers with wide range of lifetimes (in this figure: 10-2, 10-1, 10, 102 and 103 years)

• Major identifiable influences:– Marine convection– Continental convection– Stratospheric descent (10% admixture)– Biomass burning– In situ chemistry

• “No large-scale division of these signatures into separate airmasses”

• Key shortcoming: degree of influences only partially quantifiable from the limited data


Summary/Outlook

• We do know some about TTL chemistry, though much of this is focused on ozone, especially the reduced-ozone observations (CEPEX and SHADOZ sondes)

• Due to the long lifetimes of most key gases in the TTL (e.g., O3 ~ 1 year, NOx ~ 1 week, etc.), transport processes are critical, especially

– Deep convection– Uptake into condensate, especially ice (including subvisible cirrus?), and further lofting

or sedimentation/precipitation – Slow upwelling, especially in the upper TTL– Exchange with the stratosphere (both from above and horizontally)

• More observations needed: major very recent advances through SCOUT-O3, TROCCINOX and others overview in Ken Pickering’s talk

• Modeling studies still tend to be very individual case studies (see several other talks/posters) hope given in Mary Barth’s talk on what we can learn from intercomparisons?


Outlook: New ECHAM5/MESSy O3 Simulations

• Consistent simulation from surface to 0.01 hPa• No upper boundary near the TTL,

full tropospheric (NMHC) chemistry• Submitted manuscript:

Jöckel et al.,The atmospheric chemistry general circulation model ECHAM5/MESSy1: Consistent simulation of ozone from the surface to the mesosphere, submitted to ACPD. 2006.


Final Food (and Wine) for Thought

• Adrian Tuck (ACCENT Observations): “As far as transport is concerned, the small-scale variation is not noise, it is music”

• Frank Zappa: “The difference between music and noise is that music is organized – even if it seems like noise to some people”

Atmospheric Chemistry in the Tropical Tropopause Layer

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