Quantifying Photolysis Rates in the Troposphere and

Stratosphere(An Overview)

William H. SwartzDepartment of Chemistry and Biochemistry

Friday, November 1, 2002

Important Chemical Processes in the Troposphere and

Stratosphere

Tropospheric Ozone:

j-values are critical

P : jNO2 (polluted)

L : jO3 (remote)

Important Chemical Processes in the Stratosphere (continued)

Stratospheric Ozone:

HCl + ClONO2 HNO3(s) + Cl2(g)

Cl2 2Cl

PSC

h

j-values are critical

P : jO2 (tropics)

L : jClOOCl (polar vortex)

“j-Values”: Definition

NO2 + h NO + O ( < 424 nm)

][][

2NO2 NO

NO2

jdt

d

“actinic” flux (photons cm-2 s-1 nm-1)

absorption cross section (cm2)

photolysis quantum yield (photons-1)

dTTFj ),(),()( ONONONONO 222

Components of the Radiation Field

Actinic Flux F =

A (direct attenuated flux) +

B (scattered flux) +

C (reflection of direct) +

D (reflection of scattered)

(Adapted from Meier et al. [1982])

Factors Affecting Actinic Flux

•solar zenith angle

•observer altitude

•ozone profile/amount

•other absorbers/scatterers (O2, air)

•surface reflectivity (albedo)

•surface altitude

•aerosol morphology/optical properties

•cloud morphology/optical properties (including polar stratospheric clouds)

•atmospheric refraction

Sensitivity: Surface Albedo/Height

[Swartz et al., 1999]

Sensitivity: Ozone Profile

[Swartz et al., 1999]

Determining j-Values

Photolysis Rate Coefficient

Chemical Actinometry

Radiative Transfer Modeling

Radiometry

Irradiance

Actinic Flux

Filter Radiometer

Spectroradiometer

Spectroradiometer

Eppley Radiometer

Why measurements? Why modeling?

(measure chemical change)

(measure solar flux)

(model solar flux)

APL Radiative Transfer Model• developed over 20+ years, for the calculation of j-values in

the stratosphere and troposphere [Anderson and Meier, 1979; Meier et al., 1982; Anderson, 1983; Anderson and Lloyd, 1990; Anderson et al., 1995; DeMajistre et al., 1995; Swartz et al., 1999]

• direct solar deposition and reflection from Lambertian surface calculated in a spherical, refracting atmosphere

• multiple scattering using a plane-parallel approximation

• integral solution to radiative transfer

• parameterization of solar transmission through O2 Schumann–Runge bands (175–204 nm) developed by R. DeMajistre, based on work of K. Minschwaner

• wavelength range: 175–850 nm

• 75 altitude layers, 0–120 km

Objectives

How do various factors affect j-values important to the ozone balance of the troposphere and stratosphere?

How well can we measure/model j-values?

How well can we model j-values with the APL model, over a range of wavelengths, altitudes, and solar zenith angles?

Can we use stellar occultation remote sensing to measure polar stratospheric ozone loss rates?

How can j-value measurement and modeling help elucidate factors influencing photochemical ozone loss within the polar vortex?

1

2

j-Values

Polar Ozone Loss

POLARIS 1997

IPMMI1998

SOLVE 1999/2000

surface; low SZA

lower strat; moderate SZA

lower strat; high

SZA

Is jNO2 Known Accurately Enough? The State of the Art?!

[Lantz et al., 1996]

International Photolysis Frequency Measurement and

Modeling Intercomparison (IPMMI)

NCAR Marshall Field Site, 39°N 105°W, elevation: 1.8 km; June 15–19, 1998

Objectives: j [NO2 NO + O], j [O3 O2 + O(1D)], spectral actinic flux.Measurements by 21 researchers (US, UK, Germany, New Zealand).

Modeling by 18 researchers (US, UK, Canada, Germany, Austria, Netherlands, France, Norway).

1. Measure jNO2 at the surface and compare with other measurements

2. Model jNO2 and jO3 at the surface with APL model

3. Evaluate model by comparing modeled j-values with measurements

4. Evaluate model by comparing modeled j-values with other models

My Objectives

IPMMI: Measurements and Modeling

Photolysis Rate Coefficient

Chemical Actinometry

Radiative Transfer Modeling

Radiometry

Irradiance

Actinic Flux

Filter Radiometer

Spectroradiometer

Spectroradiometer

Eppley Radiometer

(measure chemical change)

(measure solar flux)

(model solar flux)

IPMMI Measurement Site

Photo by Chris Cantrell (NCAR)

UMD jNO2

Actinometer Schematic

NO2 + h NO + O

tj

02NO NO

NO2 ][

Trailer #2UMD Actinometer

UMD Actinometer

inside

on top

quartz photolysis tube

UMD jNO2 Actinometer Data

June 15–19, Overlaid

High day-to-day precision in clear-sky periods.

UMD vs. NCAR Actinometers

June 16 June 19

NCAR actinometer

failed

jNO2 Measurement Comparisonvs. Composite Actinometer

JPL97

JPL97

Harder et al. 97

Harder et al. 97

Larger NO2 absorption cross sections lead to better spectroradiometer–actinometer agreement.

IPMMI June 19 Model Specifications

AOD0

eIIaerosol optical depth:

aerosol single-scattering albedo: fraction of photons scattered

aerosol asymmetry factor: 1 = completely forward-scattering, 0 = isotropic scattering, -1 = completely backward-scattering

aerosol Ångström parameter: AOD dependence

(APL*) (APL)

Model vs. Measurement:Effects of Aerosol Optical Depth

Though optically thin, aerosols did have a measurable impact on jNO2.

jNO2 Model Comparison (June 19)

Excellent overall agreement with TUV and model consensus. Larger NO2 absorption cross sections lead to better model–actinometer agreement.

(ACDTUV)

Good high-SZA behavior.

“composite” actinometer

+

jO3 Model Comparison (June 19)

Excellent overall agreement with TUV and model consensus, when IPMMI aerosol specification and ATLAS extraterrestrial solar flux are used.

IPMMI: Summary & Conclusions•first “blind,” international intercomparison of many j-value

measurement and modeling techniques•UMD chemical actinometer measured jNO2 with excellent

precision, and in good agreement with NCAR actinometer•APL model calculated jNO2 in excellent agreement with

spectroradiometers (<1–2% on average)•APL model calculated jO3 in excellent agreement with

actinometer and spectroradiometers (<1–2% on average)•spectroradiometers and models underestimated actinometer jNO2

by a significant amount (APL model –14%; though within combined uncertainties)

•larger NO2 absorption cross sections (e.g., Harder et al. [1997]) lead to better agreement—We need to re-evaluate laboratory measurements!

•aerosol parameters must be accurately determined in order to reach model–measurement agreements of <~5%

•ATLAS extraterrestrial irradiance gives best j-value agreement (esp. jO3)

Arctic Ozone Depletion

[Newman et al., 1997]

Summertime Arctic Ozone Loss

[Lloyd et al., 1999]

Photochemistry of Ozone Loss in the Arctic Region in Summer

(POLARIS)

Based in Fairbanks, Alaska, 65°N 148°W; April–September 1997

Objectives: evaluate (measure and model) naturally occurring summertime ozone loss at high northern latitudes, to determine

contributions from chemical loss cycles and transport. NASA ER-2 high-altitude aircraft, balloons, ground-based, and

space-based observations.

1. Model j-value sensitivity in the lower stratosphere

2. Model jNO2 and jO3 along ER-2 flight tracks (20 km) with APL model

3. Evaluate model by comparing modeled j-values with measurements, particularly in light of modeled sensitivities

New Challenges:

Characterizing aircraft geophysical environment

My Objectives

Sensitivity: Ozone Profile

[Swartz et al., 1999]

POLARIS:Measurements and Modeling

Photolysis Rate Coefficient

Chemical Actinometry

Radiative Transfer Modeling

Radiometry

Irradiance

Actinic Flux

Filter Radiometer

Spectroradiometer

Spectroradiometer

Eppley Radiometer

(measure chemical change)

(measure solar flux)

(model solar flux)

CPFM

CPFM Spectroradiometer(Environment Canada)

•surface albedo

•overhead ozone column

• j-values

jNO2 alongJune 29,

1997 Flight Track

[Swartz et al., 1999]

APLCPFM, APLTOMS

vs. CPFM

POLARIS: Summary & Conclusions

•modeled sensitivity of jNO2 and jO3 to surface albedo, surface altitude, total ozone, ozone and temperature profiles, and refraction, in the context of the POLARIS mission

• jNO2: albedo > surface altitude » total ozone (at 20 km)

• jO3: total ozone » albedo > surface altitude (at 20 km)

• jNO2: APLCPFM > CPFM by 6%; APLTOMS > CPFM by 9% (average)

• jO3: APLCPFM > CPFM by 7%; APLTOMS > CPFM by 1% (average)

•model–measurement agreement has improved to the point where variability along flight tracks can be attributed to geophysical variability

SAGE III Ozone Loss and Validation Experiment (SOLVE)

Based in Kiruna, Sweden, 68°N 20°E; November 1999–March 2000

Objectives: study the development of the polar vortex and PSCs, quantify chlorine activation, and measure and model ozone loss.

NASA ER-2 high-altitude and DC-8 aircraft, balloons, ground-based, and space-based observations.

1. Add new geophysical inputs to the APL model

2. Model jNO2 and jO3 along ER-2 flight tracks (20 km) with APL model

3. Model jNO2 and jO3 along DC-8 flight tracks (11 km) with APL model

4. Evaluate model by comparing modeled j-values with measurements

New Challenges:

Twilight conditions (wintertime); fewer direct ancillary measurements

My Objectives

APL Model Input Data

Mode Albedo Ozone

APLclim climatology climatology

APLTOMS TOMS TOMS (total ozone)

APLPOAM TOMS POAM III (O3–PV reconstruction)

APLCPFM CPFM CPFM (overhead ozone, TOMS total)

APLclim*, APLTOMS*, APLPOAM*, and APLCPFM* also use in situ ozone.

SOLVE: Measurements and Modeling

Photolysis Rate Coefficient

Chemical Actinometry

Radiative Transfer Modeling

Radiometry

Irradiance

Actinic Flux

Filter Radiometer

Spectroradiometer

Spectroradiometer

Eppley Radiometer

(measure chemical change)

(measure solar flux)

(model solar flux)

SAFS Spectroradiometer (DC-8)(NCAR)

(downwelling)

(upwelling)

Model–SAFS Agreement (DC-8)

jNO2 jO3

TOMS albedo and POAM III O3 reconstructions, as well as in situ O3, lead to the best agreements with SAFS.

Attenuation of (Measured) jNO2

Outlying points (from PSC flights) indicate attenuated actinic flux, relative to clear-sky

model calculations.

Polar Stratospheric Clouds (PSCs)

SOLVE: Summary & Conclusions

•unique set of measured j-values at high SZAs in the wintertime Arctic

•new temperature/pressure/ozone/albedo climatologies, POAM III O3–PV reconstructions, and in situ O3 constraints added to model

•measured O3 (POAM, in situ) and albedo (TOMS) were superior to climatologies for calculating j-values in nearly all cases

• jNO2: model–SAFS agreement: 2–4% (<85°), 4–6% (>85°) (average)

• jO3: model–SAFS agreement: 0–13% (<85°), 3–15% (>85°) (average)

•attenuation of jNO2 up to 75% (attributed to PSCs)

Objectives (revisited)

How do various factors affect j-values important to the ozone balance of the troposphere and stratosphere?

How well can we measure/model j-values?

How well can we model j-values with the APL model, over a range of wavelengths, altitudes, and solar zenith angles?

Can we use stellar occultation remote sensing to measure polar stratospheric ozone loss rates?

How can j-value measurement and modeling help elucidate factors influencing photochemical ozone loss within the polar vortex?

1

2

j-Values

Polar Ozone Loss

Photochemical Ozone Loss using MSX/UVISI Stellar Occultation (during SOLVE)

Krypton lamp

UVISI spectrographic

imagers (5)

Space-based visible (SBV)

instrument

UVISI WFOV and NFOV

imager - UV

Space infrared imaging

telescope (SPIRIT III)

UVISI WFOV and NFOV imager - visible

Xenon lamp

Mass Spectrometer

MSX

UVISI

Extinction:

Refraction:

Observed Stellar Spectra

Min

imu

m R

ay

He

igh

t (k

m)

Wavelength (nm)

Sta

r E

qu

iva

len

t B

righ

tne

ss (

R/n

m)

Sampling of the Polar Regionduring SOLVE

25 in-vortex occultations,Jan 23–Mar 4

MSX–POAM III Ozone Comparison

POAM III ozone based on ozone–PV reconstruction.[Swartz et al., 2002]

Air Parcel Trajectories Jan 15–Mar 31

Diabatic forward and back trajectories of air parcels sampled with the January 23 occultation.

Ozone Change since Jan 23

[Swartz et al., 2002]

Ozone Loss usingIndividual

Trajectories

Average ozone loss rates on 3 surfaces derived from occultation measurements and related by individual diabatic trajectories.

[Swartz et al., 2002]

SOLVE Ozone Loss Profiles

[Swartz et al., 2002]

Stellar OccultationSummary & Conclusions

•first science application of space-based stellar occultation

•25 profiles within the polar vortex during SOLVE

•good temperature agreement with UKMO analysis

•good ozone agreement with POAM III ozone–PV reconstructions

•analysis using diabatic descent trajectory calculations to derive photochemical ozone loss rates in the Arctic during SOLVE: up to ~24 ppbv/day (average) at 400–500 K over 1/23/2000 to 3/4/2000, or about 1 ppmv, consistent with other analyses

•demonstrates the utility of extinctive–refractive stellar occultation for ozone monitoring, having several advantages over other techniques

Objectives (revisited)

How do various factors affect j-values important to the ozone balance of the troposphere and stratosphere?

How well can we measure/model j-values?

How well can we model j-values with the APL model, over a range of wavelengths, altitudes, and solar zenith angles?

Can we use stellar occultation remote sensing to measure polar stratospheric ozone loss rates?

How can j-value measurement and modeling help elucidate factors influencing photochemical ozone loss within the polar vortex?

1

2

j-Values

Polar Ozone Loss

What Are the (Optical) Effects of PSCs on Photolysis and Ozone

Loss?

PSCs, over Kiruna, Sweden, January 2000

Photo by Jim Ross (NASA/Dryden)

Identification of PSC Effects

Modeled/measured jNO2 > 1.18 considered PSC-attenuated.

Temperature Dependence

March 8, 2000

all flights:

PSC attenuation coincides with cold temperatures (13–25 km) relative to the saturation point of nitric acid trihydrate (NAT).

j-Value fdirect

Dependence

jNO2

jNO2

jNO2

a

b

c

PSC attenuation as a function of jdirect/jtotal (fdirect).

Slant Path (SZA) Dependence

SZA dependence follows Beer–Lambert relationship as a function of the slant path (through 13–25 km).

se

2-D Fit

PSC effect as functions of j-value direct fraction and slant path only.

ClOOCl Loss Cycle and jClOOCl

sky-arClOOCl,cle

PSCClOOCl,

sky-clear

3

PSC

3 OOj

j

dtd

dtd ][][

Source: P. A. Newman (NASA/Goddard)

PSC-affected vs. clear-sky jClOOCl.

PSC Probability

UKMO meteorological

temperature fields

January 25, 2000; 68.1 mb (18 km)

Diurnal PSC jClOOCl Effect

× PPSC

January 25, 2000

Diurnally Integrated jClOOCl Effect

January 25, 2000; 68.1 mb (18 km)

day PSCClOOCl,

day s-ClOOCl,c

PSCClOOCl,

s-ClOOCl,c

dtj

dtjpolar night

vortex edge

Photolysis affected within the cold vortex, when the Sun is present.

Integrated photolysis:

Vortex-Averaged jClOOCl Effect

SOLVE: Summary & Conclusions

•attenuation of jNO2 up to 75% (attributed to PSCs)

•attenuation correlated with cold temperatures along solar line of sight

•attenuation also related to the slant path through PSC layer

•putative PSCs have up to 10% effect on daily ClOOCl photolysis (ozone loss) within the Arctic polar vortex, during SOLVE

•we are ready and in a unique position to accurately model j-values during SOLVE-2, even in the presence of PSCs….

SOLVE-2 (2003):Modeling is All There Is

Photolysis Rate Coefficient

Chemical Actinometry

Radiative Transfer Modeling

Radiometry

Irradiance

Actinic Flux

Filter Radiometer

Spectroradiometer

Spectroradiometer

Eppley Radiometer

(measure chemical change)

(measure solar flux)

(model solar flux)

Final Remarks•if you want to get modeled j-values right, you need to know:

altitude, solar zenith angle, day of year (Earth–Sun distance), ozone profile, pressure/temperature profile, surface altitude, spectral surface albedo, spectral aerosol properties (optical depth, single-scattering albedo, scattering phase function)…in cloud-free skies

•we need to learn how to better handle clouds, including PSCs

•we need to measure the optical effects of PSCs throughout the stratosphere and model their impact in chemistry–transport models

•we need to consider using stellar occultation as a means of monitoring long-term trends in ozone

AcknowledgmentsCOLLABORATORS:

Measurements: Russ Dickerson, Jeff Stehr, Shobha Kondragunta (UM)

Modeling: Steve Lloyd, Don Anderson, Tom Kusterer (APL)

Data Analysis: Sam Yee, Ron Vervack (APL) Paul Newman (Goddard)

IPMMI: Rick Shetter, Sasha Madronich (NCAR)

POLARIS: Tom McElroy, Clive Midwinter (Environment Canada)

SOLVE: Rick Shetter (NCAR) Karl Hoppel (NRL), Cora Randall (LASP) Stacey Hollandsworth Frith, Gordon Labow (Goddard)

$$$: NASA OES, C4 (NSF), APL, …. T = –30°C

Acknowledgments

ADVISOR:

Russ Dickerson (UM)

MENTORS:

Steve Lloyd (APL) Don Anderson (APLNASA/HQ)

T = –30°C