Atmospheric Chemistry in the Tropical Tropopause Layer

Atmospheric Chemistry in the Tropical Tropopause Layer Mark G. Lawrence Max Planck Institute for Chemistry Mainz, Germany SPARC/GEWEX/IGAC 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., forCH4) • 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 (Based on WMO, 2002) FBL-TT ~ 0.2-6% FBL-STT “potentially significant” TT, ~16-17 km  ~ months TTL  ~ 10 (5-20) d LS LS STT, ~11-13 km UT UT UT BL,  ~ 5 d

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)

CEPEXCentral Equatorial Pacific Experiment March, 1993

Observed and MATCH-MPIC vertical O3 profiles West Middle East Observations (balloon sondes) (Kley et al., Science, 1996; Lawrence et al., QJRMS, 1999) Modeled (MATCH-MPIC)

Effect of Convection on Modelled O3 With Convection No O3 Convection (Only convective Transport of ozone was turned off) Western CEPEX region Central CEPEX region Eastern CEPEX region (Lawrence et al., QJRMS, 1999)

Effect of Convection on Modelled O3 TTL – O3 reduced by convective transport WRF Simulation Results for TOGA COARE (Salzmann, 2005)

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

Ozone Sonde (SHADOZ) Observations black lines - average profiles for 1998–2004colored lines - illustrative reduced ozone events (Solomon et al., GRL, 2005)

Ozone Sonde (SHADOZ) Observations • Reduced ozone events (< 20 nmol/mol) most common over western Pacific • Very low ozone events (< 10 nmol/mol) nevertheless quite rare (Solomon et al., GRL, 2005)

Ozone Sonde (SHADOZ) Observations Ozone frequency distribution changing over time! Samoa, 200 hPa (Solomon et al., GRL, 2005)

Deep Convection – Outflow in the TTL 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 (Folkins and Martin, JAS, 2005)

Convective Transport Formulations Plume Ensemble Bulk Arakawa and Schubert, 1974; Lord et al., 1982; Hack et al., 1984; Grell, 1993 Yanai et al., 1973; Tiedtke, 1989; Grell, 1993; Pan and Wu, 1995; Zhang and McFarlane, 1995 (Lawrence and Rasch, JAS, 2005) Mark G. Lawrence, Max Planck Institute for Chemistry TTL Workshop, Victoria, Canada, 14 June 2006

Role of Convective Transport Formulations  = 1 d  = 2 d Zonal Mean, July 2001 (Lawrence and Rasch, JAS, 2005) Mark G. Lawrence, Max Planck Institute for Chemistry TTL Workshop, Victoria, Canada, 14 June 2006

Convective Mass Flux Characterizations -.1 -.05 0.0 .05 .1 Cloud mass flux (kg/m2/s) -.1 -.05 0.0 .05 .1 Cloud mass flux (kg/m2/s) -2 -1 0 1 2 3 Cloud mass flux (kg/m2/s) -2 -1 0 1 2 3 Cloud mass flux (kg/m2/s) Domain Means Cloudy-Area Means (Salzmann, 2005)

Influence of Precipitation Scavenging Ice Hx in M/atm: Hx= 0 Hx = 103 Hx = 104 Hx = 105 Hx = 106 (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 20 Sept., 1999 21 Sept., 1999 (Tuck et al., JGR, 2004; Ridley et al., Atmos. Env., 2004)

ACCENT TTL Chemistry Observations Wide variety of gases and aerosol components observed, large spatial variability (Tuck et al., JGR, 2004; Ridley et al., Atmos. Env., 2004)

ACCENT TTL Chemistry Observations • 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 (Tuck et al., JGR, 2004; Ridley et al., Atmos. Env., 2004)

ACCENT TTL Chemistry Observations • 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 (Tuck et al., JGR, 2004; Ridley et al., Atmos. Env., 2004)

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