THE ATMOSPHERE: OXIDIZING MEDIUM

IN GLOBAL BIOGEOCHEMICAL CYCLES

EARTH

SURFACE

Emission

Reduced gas Oxidized gas/

aerosol

Oxidation

Uptake

Reduction

Atmospheric oxidation is critical for removal of many pollutants, e.g.

• methane (major greenhouse gas)

• CO (toxic pollutant)

• HCFCs (Clx sources in stratosphere)

Oxidation is the loss of electrons or an increase in oxidation state

by a molecule, atom, or ion.

RADICAL REACTION CHAINS IN THE ATMOSPHERE

non-radical radical + radical Initiation: photolysis

thermolysis

oxidation by O(1D)

radical + non-radical non-radical + radical Propagation: bimolecular

reactions

non-radical + non-radical Termination: radical

reaction radical + radical

non-radical + M radical + radical + M 3-body recombination

Recycling: non-radical radical + radical photolysis

thermolysis

oxidation by O(1D)

Preparation: non-radical available for photolysis

Main oxidant: OH

• Known since 1950s to be produced in the stratosphere

• O3 + hν -> O2 + O(1D) R1 λ<320 nm

• O(1D) + H2O -> OH + OH R3

• Known since 1970s to be produced also in the troposphere

• POH = 2 k3 [O(1D)] [H2O]

Main oxidant: OH

• Main loss reactions

• CO + OH -> CO2 + H R4

• CH4 + OH -> CH3 + H2O R5

• Life time typically 1 second, highly variable in space and time

• No production during night (e.g. polar night), and zero

concentrations

We need to know concentrations and budgets of CO and CH4

CO: 50 – 150 ppbv in remote areas

CH4: increased from 800 to 1800 ppbv

THE TROPOSPHERE WAS VIEWED AS

CHEMICALLY INERT UNTIL 1970 • “The chemistry of the troposphere is mainly that of of a large number of

atmospheric constituents and of their reactions with molecular

oxygen…Methane and CO are chemically quite inert in the troposphere”

[Cadle and Allen, Atmospheric Photochemistry, Science, 1970]

• Lifetime of CO estimated at 2.7 years (removal by soil) leads to concern

about global CO pollution from increasing car emissions [Robbins and

Robbins, Sources, Abundance, and Fate of Gaseous Atmospheric

Pollutants, SRI report, 1967]

FIRST BREAKTHROUGH:

• Measurements of cosmogenic 14CO place a constraint of ~ 0.1 yr on the

tropospheric lifetime of CO [Weinstock, Science, 1969]

SECOND BREAKTHROUGH:

• Tropospheric OH ~1x106 cm-3 predicted from O(1D)+H2O, results in

tropospheric lifetimes of ~0.1 yr for CO and ~2 yr for CH4 [Levy, Science,

1971, J. Geophys. Res. 1973]

THIRD BREAKTHROUGH:

• Methylchloroform observations provide indirect evidence for OH at levels

of 2-5x105 cm-3 [Singh, Geophys. Res. Lett. 1977]

…but direct measurements of tropospheric OH had to wait until the 1990s

WHY WAS TROPOSPHERIC OH SO DIFFICULT TO FIGURE OUT?

Production of O(1D) in troposphere takes place in narrow band [290-320 nm]

solar flux I

ozone absorption

cross-section s

O(1D)

quantum

yield f

fsI

Isaksen, I.S.A. and P.J.

Crutzen, 1977: Uncertainties

in calculated hydroxyl radical

densities in the troposphere

and stratosphere.

Geophysica Norvegica, 31, 4,

1-10.

Målinger av OH

Schlosser et al.,

ACPD, 2009

OH-trender basert på CH3CCl3. Montzka et al. Science, 2011

CO oxidation mechanism (low NOx)

• Reaction chain

• CO + OH (+O2) -> CO2 + HO2 R4+R6

• HO2 + O3 -> OH + 2O2 R13

• Net: CO + O3 -> CO2 + O2

• OH&HO2 catalysts in loss of O3 in the troposphere in clean environments (low NOx)

• Reaction chain

• CO + OH (+O2) -> CO2 + HO2 R4+R6

• HO2 + NO -> OH + NO2 R10

• NO2 + hν (+O2) -> NO + O3 R11

• Net: CO + 2 O2 + hν -> CO2 + O3

CO oxidation mechanism (high NOx)

• OH&HO2, NO&NO2 catalysts in production of O3 in the troposphere

• Termination

• HO2 + HO2 -> H2O2 (soluble) + O2 R7

Hvilke

komponenter

er

katalysatorer

her?

Hvilke

komponenter

er

katalysatorer

her?

CH4 oxidation mechanism

• Reaction chain starting with

• CH4 + OH -> CH3 + H2O R5

• The chain proceeds through several hydrocarbons, in different pathways, to

produce O3 and HOx

• Maximum yield (high NOx)

• CH4 + 10 O2 -> CO2 + H2O + 5 O3 + 2 OH

• Minimum yield (low NOx)

• CH4 + 3 OH + 2 O2 -> CO2 + 3 H2O + HO2

RADICAL CYCLE CONTROLLING TROPOSPHERIC OH

AND OZONE CONCENTRATIONS

O3

O2 hn

O3

OH HO2 hn, H2O

Deposition

NO

H2O2

CO, CH4

NO2 hn

STRATOSPHERE

TROPOSPHERE

8-18 km

SURFACE

IPCC [2013]

IPCC [2013]

Tropospheric ozone

Is the third most

important

anthropogenic

greenhouse gas

Modellert total masse av ozon i troposfæren

IPCC [2013]

IPCC [2013]

Utslippsscenarier RCP, brukt av

IPCC, 2013

CARBON MONOXIDE IN ATMOSPHERE

Source: incomplete combustion

Sink: oxidation by OH (lifetime of 2 months)

Estimert utvikling av utslipp av CO og VOC 1980-2010

Monks et al., ACP, 2015

Betydningen av teknologistandarder for biler

Her hovedsakelig diesel

Monks et al., ACP, 2015

Utslipp av biogene VOC

Monks et al., ACP, 2015

Monks et al., ACP, 2015

SATELLITE OBSERVATION OF CARBON MONOXIDE

MOPITT

CO columns

(Mar-Apr 01)

150-250 ppb

50-70 ppb

CO i Arktis, Observert og modellert

(Shindell et al., ACP, 2008)

GLOBAL METHANE SOURCES, Tg y-1 [IPCC, 2007]

ANIMALS

80-90

LANDFILLS

40-70

GAS

50-70

COAL

30-50 RICE

30-110

TERMITES

20-30

WETLANDS

100-230

BIOMASS

BURNING

10-90

GLOBAL DISTRIBUTION OF METHANE NOAA/CMDL surface air measurements

Sink: oxidation by OH (lifetime of 10 years)

HISTORICAL TRENDS IN METHANE

The last 1000 years

The last 30 years

IPCC [2007]

Recent changes in CO2,N2O, CFCs and

CH4

http://www.esrl.noaa.gov/gmd/aggi/

Metan fra smeltende Permafrost i

Arktis – Er det en stor trussel?

Økte utslipp fra skifergass?

Metanutslipp

fra skifergass

Monks et al., ACP, 2015

600

800

700

Scenarios

A1B

A1T

A1F1

A2

B1

B2

IS92a

900

Year

IPCC [2001] Projections of Future

CH4 Emissions (Tg CH4) to 2050

2000 2020 2040

NOx EMISSIONS (Tg N yr-1) TO TROPOSPHERE

FOSSIL FUEL

23.1

AIRCRAFT

0.5

BIOFUEL

2.2

BIOMASS

BURNING

5.2

SOILS

5.1

LIGHTNING

5.8

STRATOSPHERE

0.2

Global budget of NOx

• Emitted mainly as NO

• Fast chain (null cycle)

• NO + O3 -> NO2 + O2

• NO2 + hv (+O2) -> NO + O3

• Main loss day:

• NO2 + OH + M -> HNO3 + M

• Main loss night:

• NO2 + O3 -> NO3 + O2

• NO3 + NO2 + M -> N2O5 + M

• N2O5 + H2O (on aerosols, if available) -> 2 HNO3

Fuktige aerosoler v/RH<100%

USING SATELLITE OBSERVATIONS OF NO2 TO MONITOR NOx EMISSIONS

SCIAMACHY data. May-

Oct 2004

(R.V. Martin, Dalhousie U.)

These tropospheric NO2 columns are derived from satellite

observations based on slant column NO2 retrievals with the DOAS

technique, and the KNMI combined modelling/retrieval/assimilation

approach

DOAS: Differential optical absorption spectroscopy

NO2 columns can be retrieved from SCIAMACHY spectra with high accuracy in the 425-450 nm region

using the DOAS method. The satellite measurements contain both tropospheric and stratospheric

contributions, and a separation algorithm has to be used if tropospheric columns are the quantity of

interest. In the case of NO2, the simplest method is to use the Pacific sector as a clean background

value and to assume that stratospheric NO2 is zonally homogeneous. The difference between the

actual measurement and the value determined in the reference sector on the same day at the same

latitude is interpreted as tropospheric excess column

http://www.temis.nl/airpollution/no2col/no2regioomimonth_v2.php?Region=1&Year=2014&Month=04

NITROGEN DIOXIDE FROM THE OMI SATELLITE (MARCH 2006)

March 2006

LIGHTNING FLASHES SEEN FROM SPACE (2000)

DJF

JJA

Monks et al., ACP, 2015

Nesten lik årstidsvariasjon, men motsatt halvkule. Hvorfor?

Cape Grim 41°S

Arkona 54°N

Lite NOx i sør Katalytisk ozontap med HOx

OH+O3HO2+O2

HO2+O3OH+2O2

Observerte ozonkonsentrasjoner ved bakken

Variasjon i bidraget til ozon fra forurensingsutslipp for

Mace Head 53°N (Irland)

PEROXYACETYLNITRATE (PAN) AS RESERVOIR

FOR LONG-RANGE TRANSPORT OF NOx

NOAA/ITCT-2K2 AIRCRAFT CAMPAIGN IN APRIL-MAY 2002

Monterey, CA

Asian pollution plumes transported to California

CO

O3

PAN

HNO3

May 5 plume at 6 km:

High CO and PAN,

no O3 enhancement

May 17 subsiding

plume at 2.5 km:

High CO and O3,

PAN gNOxgHNO3

Hudman et al. [2004]

NOx

NOx

HNO3

PAN

O3

CO

SATELLITE OBSERVATIONS OF TROPOSPHERIC OZONE AND CO

TES satellite instrument measurements at 5 km altitude, July 2006

GLOBAL BUDGET OF TROPOSPHERIC OZONE (MODEL)

O3

O2 hn

O3

OH HO2 hn, H2O

Deposition

NO

H2O2

CO, VOC

NO2 hn

STRATOSPHERE

TROPOSPHERE

8-18 km

Chem prod in

troposphere,

Tg y-1

4300

1600

Chem loss in

troposphere,

Tg y-1

4000

1600

Transport from

stratosphere,

Tg y-1

400

400

Deposition,

Tg y-1 700

400

Burden, Tg 360

230

Lifetime, days 28

42

Present-day

Preindustrial

Beregnet romlig fordeling ozon produksjon og ozontap

(kun kjemi, ikke avsetning) Fra Monks et al., ACP, 2015

Monks et al., ACP, 2015

Monks et al., ACP, 2015

Monks et al., ACP, 2015

Estimert Utvikling i utslipp fra Asia

Monks et al., ACP, 2015

IPCC [2013]

Monks et al., ACP, 2015

IPCC RADIATIVE FORCING ESTIMATE FOR TROPOSPHERIC

OZONE (0.35 W m-2) RELIES ON GLOBAL MODELS

Preindustrial

ozone models

}

Observations at mountain

sites in Europe

[Marenco et al., 1994]

…but these underestimate the observed rise in ozone over the 20th century

Fitting to observations would imply a radiative forcing of 0.8 W m-2

Modelled TRENDS IN TROPOSPHERIC OH

Voulgarakis et al., ACP, 2013.

Monks et al., ACP, 2015

QUESTIONS

1. How does the thinning of the stratospheric ozone layer affect the

source of OH in the troposphere?

2. If CO emission to the atmosphere were to double, would you expect CO

concentrations to (a) double, (b) less than double, (c) more than double?