Tropospheric Chemistry Overview

(or, 40 years in 20 minutes)

Jennifer A. Logan

Recent Results in Planetary Sciences, Atmospheric Chemistry, Climate and Energy Policy

A symposium in celebration of Michael McElroy's contributions

March 20, 2010.

A short history of tropospheric chemistry highlighting MBM’s contributions

1949: Migeotte - identifies CO in the atmosphere in solar absorption, ~100 ppb

1969: Weinstock – shows lifetime of CO is ~1 month, based on 14CO and its budget, and suggests removal by OH

1971: Levy - first model of tropospheric OH, ~3x106 molec/cm3 at noon; CO lifetime is ~2 months, and CH4 oxidation is a source of HCHO (15 reactions)

1971, 1972: McConnell, McElroy, and Wofsy - show that CH4 oxidation is a large source of CO (also suggest sources from terpenes) (1-d model, 32 rxns)• Also show that CH4 oxidation is a source of H2O in the stratosphere

1973: Chameides and Walker, Crutzen - first models of tropospheric ozone

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Science, Levy

Nature, McConnell et al.

CH4 data used to test model

Mid-late 1970’s – the stratosphere and Mars 1975: Two postdocs join MBM’s group

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1981: Logan, Prather, Wofsy and McElroy - global model of trop. chemistry, constrained by observations of O3, CO, CH4, HNO3, H2O (51 rxns)

• inventory for all sources of CO (P=2740 Tg)

• remote sources of NOx are ~10 Tg, based on almost no data

• used CH3CCl3 (MCF) and a box model to test OH

• used OH to infer global budget for CH4 (580 Tg) and other gases

• trop. ozone budget, P=3840 Tg, L=2820 Tg

All within ~20% of present day values

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CO observations

OH, HO2 vs. NO

NOx observations and model (lines)

Atmospheric Chemistry within a General Circulation Model

1978-1980: Mahlman & Moxim, Levy et al. - first tracer model of ozone using archived GCM fields, no chemistry

1981: McElroy and Wofsy propose to NASA to develop “a global 3-D model with realistic dynamics and chemistry”, using the GCM developed by Jim Hansen et al. at GISS, building on their tracer model(Section 4 of a 103 page proposal!)

Issues identified include• an accurate method for transport• tracer conservation• efficient techniques for chemical rates• use of activity data for anthropogenic emissions

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Strategy outlined in the 1981 proposal

Elucidate transport mechanisms, and include a strategy for model validation using observations

1. Use CFCs to test interhemispheric transport Simple chemistry Requires strict numerical accuracy

2. Use radioactive tracers to test downward transport from the stratosphere, including 7Be

3. Global model for OH – test with MCF Use observed distributions of CH4, CO, O3, NOx, and H2O Compute the time evolution of MCF

4. Global model for CO with sources prescribed5. Tropospheric ozone as an active tracer

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The holy grail – tropospheric ozone

“A reliable description of ozone presumes a model for NOx, for CO, for H2O, for OH and for heterogeneous chemistry, in addition to a satisfactory representation of the stratosphere and a valid description of troposphere-stratosphere exchange. It is unlikely that we can complete work on such a model within three years, but we expect to make substantial progress.”

MBM, 1981.

Wet and dry convection

Major steps in development of the chemical tracer model (CTM)

1986: Prather – 2nd order moments scheme for accurate non-diffusive 3-d advection

1987: Prather et al. – CFCs as Tracers of Air Motion• development of the CTM, many technical issues described• sub-grid diffusion needed for correct interhemispheric gradient

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Higher resolution window1985: Another new post-doc

1990: Spivakovsky, Wofsy and Prather – chemistry parameterization for computation of OH

1990: Spivakovsky et al. Tropospheric OH in a 3-d CTM – an assessment based on MCF• used observations of CO, CH4, O3, NOt, and column O3

• MCF lifetime with model OH is 5.5 y, MCF data implies 6.2 y

• OH fields provided to the communityUpdated and extended in Spivakovsky et al. (2000)

1990: Jacob and Prather – 222Rn as a test of convective transport in a GCM

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And finally, ozone, 12 years after the original proposal

1993: Jacob et al. – summertime ozone over the US• used 6 tracers, parameterized chemistry

• sub-grid power-plant and urban plumes (Sillman et al., 1990)

• observations for boundary conditions

• gridded emissions from EPA, isoprene emissions

• dry deposition, wet deposition (Balkanski et al. 1993)

• evaluated with observations

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And 17 years after MBM’s proposal – global ozone

1998: Wang, Jacob, and Logan – 3 papers on global model of O3-NOx-hydrocarbons• 15 chemical tracers, new parameterizations• global emission inventories from fossil fuel/industry• biomass burning inventory, biogenic emissions• lightning NOx• stratospheric ozone flux• extensive evaluation with observations

All of the above work used the GISS GCM fields

2001: Bey et al. – Global tropospheric chemistry with assimilated meteorology – GEOS-Chem model• GEOS met. fields from NASA• Gear solver for chemistry (in window model earlier)• Adopted many features from GCM based CTM

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Déjà vu: from a “window” in 1987 to a nested grid formulation

2004: Yuxuan Wang, McElroy et al. – nested grid model for Asia.

1° x 1° resolution

CO datamodel,1° x 1° Model, 4° x 5°

Aircraft observations downwind of Asia in 2001

Applications to CO, NOx, ozone – tomorrow, China Project talks.

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Methane hindcast with a CTM, 1987-1998

2004: J. Wang, Logan, McElroy et al. – causes of the slowdown and variability in the CH4 growth rate

Data

Slowdown in growth rate:• slower growth in sources - the economic downturn in the former

Eastern bloc• increases in OH - column ozone decr. (solar cycle + trends)

Variability • wetland emissions + OH (especially post-Pinatubo)

Results also showed model OH is too high (Wang et al., 2008)

CH4 growth rate, ppb/yr

Tropospheric chemistry in the 21st century

The NASA “A-Train”

Surface sites

Models

Satellites

Aircraft, ships,sondes, lidars

Satellites provide a global continuous mapping of atmospheric composition, augmenting the otherwise sparse observing system

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Terra – CO data since 2000; Aura – CO, O3, NO2, HCHO since 2004

D.J. Jacob

NITROGEN DIOXIDE POLLUTION MEASURED FROM SPACE BY OMINITROGEN DIOXIDE POLLUTION MEASURED FROM SPACE BY OMI13 x 24 km pixels13 x 24 km pixels

USED to CONSTRAIN SOURCESUSED to CONSTRAIN SOURCES

March 2006

MAPPING OF REACTIVE HYDROCARBON EMISSIONS FROM SPACEMAPPING OF REACTIVE HYDROCARBON EMISSIONS FROM SPACEusing measurements of formaldehyde columnsusing measurements of formaldehyde columns

Millet et al. [2008]

Biogenic isoprene is the main reactive hydrocarbon precursor of ozone…and a major source of organic particles

hydro-carbons

340 nm

formaldehyde

Jacob slide

Model inversion of CO sources using data from three satellite instruments – Kopacz et al., 2010

AIRS

TES

SCIAMACHY

MOPITT

Annual emissions

Correction factors from inversion

Errors of up to a factor of 2!

CO data for the upper and lower troposphere: a test of model transport

MLS data GEOS-4 modelCO (ppb) at 200 hPa

Oct

Nov

Fires in Aug/Sept. are a large source of CO• satellite CO data show timing is correct in the lower troposphere

Convection moves south in October, lifting CO to the UT• model peak is 1-3 months too late (GEOS-5 is worse than GEOS-4)

Detailed analysis shows:• convection over South America detrains at too low an altitude• too strong export of CO to the eastern Pacific in Aug./Sept• isoprene is too large a source of CO

These transport problems will impact inversion studies, which cannot account for systematic errors in transport

J. Liu, draft paper

Concluding remarks

CTM studies often identify significant problems with model transport – but they don’t necessarily lead to improvements in parameterizations inherent in global GCMs – an issue for 20+ years – 2 communities

Need for a holistic approach with satellite data – a tendency for one species per paper, and global data for CO, O3, NO2, HCHO, aerosols now available (also CH4)

CTMs now used for policy – e.g., long-range transport from Asia to the US, the US to Europe

Coupled chemistry-climate models used for projections

The stakes high – we would like to get things right!

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