# Presentation: 835514

## Satellite-derived estimates of radiative forcing due to tropospheric ozone and nitrogen dioxide

- Alexander Vasilkov1, Joanna Joiner2, Mark Schoeberl2, James Gleason2, Lazaros Oreopoulos3, Steve Platnick2, Pepijn Veefkind4, Edward Celarier5, Nathaniel Livesey6
- 1. Science Systems and Applications, Inc.
- 2. NASA/Goddard Space Flight Center
- 3. JCET, University of Maryland Baltimore County
- 4. Royal Netherlands Meteorological Institute (KNMI)
- 5. SGT, Inc.
- 5. NASA/Jet Propulsion Laboratory

## How important are O3 and NO2 radiatively?

- IPCC ozone radiative forcing estimate is 0.35 W/m2 (LW+SW) based on modeled anthropogenic tropospheric ozone, 3rd most important climate gas
- NO2 is not a global player but its net heating is significant locally for polluted regions (e.g., *Solomon et al*., 1999 estimates of highly localized forcing in the range of 5-20 W/m2). NO2 is a weak absorber (mostly in 350-530 nm) with optical thickness less than 0.01 even for polluted areas with NO2 column of 0.5 DU.

## Radiative calculations

- Chou-Suarez code to compute radiative fluxes
- The code has been applied to various atmospheric models at Goddard.
- O3 absorption in 9.6 m band, Hartley-Higgins and Chappuis bands
- Parameterization of NO2 absorption in 320-400 nm & 400-700 nm bands
- Diurnal averaging assumes a model of frozen cloudiness
- LW: 1:30/13:30 orbits average
- SW: diurnal average using data computed every 2 hours
- Radiative forcing (RF) due O3 is defined as the change in net irradiance at the tropopause.
- Definition of the tropopause
- Determines the amount of ozone assigned to the troposphere
- Determines how much of the atmosphere is included in the radiative calculation
- Standard lapse rate definition (where the lapse rate decreases to 20/km or less).

## Radiative calculations: data used

- Satellite data used (gridded to 1 x 1 degree resolution)
- MODIS L3 daily (cloud optical depth, effective radius, fraction, phase). The sensitivity of O3 forcing to *r**e* and phase is relatively small.
- OMI daily optical centroid cloud pressure (OCCP), assume 100 hPa geometrical thickness. OCCP approximates an average level reached by backscattered photons. It is more appropriate than cloud-top pressure for SW (*Vasilkov et al*. 2008). Sensitivity study: using OCCP results in much less sensitivity to PDF of cloud  and *r**e*
- MODIS albedo over land, ocean surface model (no sea ice), IR emissivity data base
- Tropospheric daily column-mean ozone mixing ratio from OMI/MLS trajectory-based residual (*Schoeberl et al*., 2008)
- OMI NO2 monthly-mean columns and GEOS-Chem climatological profiles
- Other data
- GEOS-5 daily temperatures, humidity, stratospheric ozone

## O3/NO2 effects in short-wave

- Clear sky
- O3/NO2 absorption reduces irradiance at the surface (dimming effect)
- O3/NO2 absorption increases atmospheric heating in the troposphere
- Cloudy sky
- Clouds reduce O3/NO2 absorption below
- Clouds significantly increase O3/NO2 absorption above and within the clouds due to increased photon path-length
- NO2 heating significantly depends on NO2 and cloud vertical structure

## Tropo. O3 effects in long-wave

- Clear sky
- Tropospheric O3 reduces outgoing long-wave radiation contribution from warm surface
-  Tropospheric O3 causes heating at the surface
- Cloudy sky
- Clouds reduce tropospheric O3 surface heating
- Clouds reduce O forcing because they emit at cooler temperature than the surface

## OMI/MLS column-mean tropospheric O3 mixing ratio

- January 2005

- July 2005

- Daily

- Monthly mean

- Significant spatial
- variations on both
- daily and seasonal
- time scales

## Total-sky SW tropospheric O3 forcing and cloud impact

- July 2005

- January 2005

- RF

- Total-sky minus
- clear-sky RF

- Determined
- by O3 amount,
- surface albedo,
- SZA, and clouds

- Persistent low
- clouds 
- increased effect
- in S.E. Asia

## Total-sky LW tropospheric O3 forcing and cloud impact

- Total-sky LW tropospheric O3 forcing and cloud impact

- July 2005

- January 2005

- RF

- Total-sky minus clear-sky RF

- Clouds reduce RF
- where warmer
- surface T, can
- increase where
- low clouds
- & T inversions

- Lowest RF in
- the presence of
- high clouds.
- Highest RF in areas
- with high surface T.

## Tropospheric O3 radiative forcing: summary

|  | January 2005 |  |  | July 2005 |  |  |
| --- | --- | --- | --- | --- | --- | --- |
| Spec. range | Global mean (W/m2) | Cloud forcing (W/m2) | Cloud forcing (%) | Global mean (W/m2) | Cloud forcing (W/m2) | Cloud forcing (%) |
| SW | 0.194 | 0.090 | 46.4 | 0.244 | 0.112 | 46.0 |
| LW | 0.997 | -0.241 | -24.2 | 1.272 | -0.280 | 22.0 |
| net | 1.191 | -0.151 | -12.7 | 1.526 | -0.168 | -11.0 |

- Note: IPPC estimate of 0.35 W/m2 is for anthropogenic ozone only

## NO2 column amounts and total-sky heating rates

- January 2005

- July 2005

- Column amounts
-  (mol/cm2)

- Net heating rates
-  (W/m2)

- Highest values in
- winter because of
- longer lifetime

- Highest values in
- summer because of
- lower SZA

## Regional total-sky NO2 net heating rates

- January 2005

- July 2005

- N. America winter:
- higher NO2
- but lower RF. Highest values
- in L.A., Vancouver, N.Y.

- S.E. Asia

- No dramatic
- differences
- between winter
- & summer

## Conclusions

- We have used A-train satellite data to very accurately compute the radiative forcing due to tropospheric ozone in all sky conditions (its spatial and temporal variability)
- Clouds affect O3 radiative forcing:
- SW (46%); LW(-23%)
- Net effect is about 12%
- Net cloud effect from models is somewhat underestimated
- Trop. NO2 contributes to local dimming and increases heating rate in lower troposphere.
- Although Solomon et al. 1999 saw high amounts of heating due to NO2 (5-20 W/m2 in vicinity of thunderstorms), on the scale of OMI measurements, NO2 effect is much smaller (1-2 W/m2 in polluted regions).