Surface UV from TOMS/OMI measurements N. Krotkov1, J. Herman2, P.K. Bhartia2 , A. Tanskanen3 , A. Arola3 Goddard Earth Sciences and Technology (GEST) Center, UMBC, Baltimore, MD Laboratory for Atmospheres, NASA GSFC, Greenbelt, MD Finnish Meteorological Institute , Helsinki, Finland Collaboration with UMCP group on validation provisional OMI SO2 data

OMI Science questions: Is the ozone layer recovering ? What are sources and distributions of aerosols and trace gases that affect global air quality? What are the roles of tropospheric ozone and aerosols in climate change? What are the causes of surface UV-B change ? Levelt et al “ Science objectives of Ozone Monitoring Instrument “ in IEEE- TGRS AURA special issue

UV products: noon irradiance + Daily CIE dose (305nm, 310nm, 324nm, 380nm, CIE - UV index) Erythemal – UV index

Current Applications of TOMS/OMI UV data 1. Sun burn and skin cancer PHS, NIH, WHO 2. Eye cataracts PHS, NIH, WHO 3. Plant damage - Crop yields USDA 4. Food chain - Land – Oceans USDA, NOAA 5. Effect on insect population NIH, PHS, WHO

Fc ~ FO3(1 – R) TOMS/OMI UV algorithm Sun: Fo OMI Ozone Sun: Fo Clouds Clouds and non-absorbing aerosols are well corrected Fc ~ FO3(1 – R) OMI shares TOMS UV algorithm Absorbing aerosols are still a major problem Aerosols

Cloud correction algorithm (CT): ISCCP TOMS daily Satellite CT estimate 10 day OMI surface UV represents snapshot average over footprint area 13 by 24km.This is best validated by comparing to the spatial average of ground instruments over 10 day to monthly time periods. Williams et al compared 2 satellite algorithms of cloud correction ( ISCCP and TOMS/OMI) with ground data averaged over 2 by 2 grid boxes. The comparison indicates that TOMS cloud correction provides the most reliable estimate of UV for snow/ice free periods Similar conclusion was drawn in Dye et al [GRL 1995] for PAR irradiance monthly UV- Williams et al GRL 2004 (1:1) (1:1) PAR - Dye et al GRL 1995 Ground CT measurement

Clouds over snow correction: Aapo, please explain your current correction

The TOMS-Brewer difference for erythemally weighted UV irradiance and for UV irradiance at 305, 310, and 324 nm. Summer noon values for mostly clear sky conditions (TOMS reflectivity <0.2) an interim OMI SO2 algorithm (the Band Residual Difference or BRD algorithm) uses calibrated residuals at SO2 absorption band centers produced by the NASA operational ozone algorithm (OMTO3). The SO2 vertical distribution should be known (or assumed) a-priori. This is most critical for tropospheric SO2. Currently uniform mixing in assumed in the planetary boundary layer below 700mb. Data are provisional: released for validation Pis and science team collaborators This needs validation with aircraft data.

Long-range ABSORBING aerosol transport in free troposphere is uniquely tracked by OMI/TOMS Aerosol Index (AI) US AI cannot detect boundary layer UV absorbing aerosols resulting in overestimates of UV irradiance that are frequently 10% and sometimes 20%. India Southeast Asia TOMS AI examples of high-density smoke aerosols that affect various coastal regions in the US, India, and Southeast Asia. Lesser amounts of smoke, dust, and carbonaceous aerosols frequently cause overestimations of UV irradiance if ignored.

UV reduction due to absorbing aerosols in free troposphere (dust, smoke) is corrected using positive AI >0.5 data Industrial aerosols close to the ground are not seen in AI data (AI <0), so they are treated as thin clouds, which leads to positive UV bias It was also shown that aerosols at low altitudes (below 2 km) tend to produce small values of AI even for strongly absorbing aerosols.

The TOMS-Brewer difference for erythemally weighted summer noon UV irradiance for mostly clear sky conditions (TOMS reflectivity <0.2) Urban locations

Ground AEROSOL absorption measurements UV Multifilter Rotating Shadowband Radiometer AERONET CIMEL sun-sky radiometers Brewer spectrometer ozone, SO2, NO2 [Cede and Herman] Since 2002, the NASA TOMS, AERONET and USDA UVB programs have shared equipment, personnel and analysis tools to quantify aerosol UV-VIS absorption using a combination of ground based radiation measurements. Since 2002 there was a comprehensive effort at GSFC to make definitive measurements of the atmospheric optical components that affect penetration of UV irradiance to the ground. 2) The effort consists of combining simultaneous measurements of UV radiation and aerosol properties from a unique assembly of co-located instruments: 1) a modified double Brewer, 2) a modified UV Multifilter Rotating Shadowband Radiometer (UV-MFRSR), 3) an AERONET Cimel sunphotometer, 3) With the data derived from these instruments, we have been able remove the large (10 to 20%) discrepancy between measured and radiative transfer calculated UV irradiances

TOMS UV Correction for Absorbing Aerosols in Greenbelt, USA 1 TOMS/Ground UV ABS at 325nm ABS at 325nm Absorbing aerosols are currently corrected off-line with ground measurements of aerosol absorption optical thickness Time series of aerosol absorption optical thickness tabs at 368nm, derived from 17 months UV-MFRSR operation at NASA GSFC site in Maryland, US. The data are for cloud-free and snow-free conditions and tabs(440)>0.1. Individual tabs(368) values were averaged over 1-hour period of time within ±60min of the AERONET inversion. The ratio between satellite estimated (by TOMS UV algorithm7-10) and measured (by UVMFRSR) total (direct plus diffuse) surface UV irradiance at 325nm versus aerosol absorption optical thickness at 325nm inferred from combined UV-MFRSR and AERONET measurements at NASA/GSFC site. The line shows theoretical relationship derived from radiative transfer modeling10. 1Krotkov et al. Opt. Engineering 2005

TOMS UV Correction for Absorbing Aerosols in 2 urban sites 1 SZA=20-30 SZA=40-50 Absorbing aerosols are currently corrected off-line using ground measurements of aerosol absorption optical thickness. We need network of aerosol absorption measurements: Global AERONET sun-sky photometers network provides aerosol absorption data in the visible. These need to be complemented with aerosol measurements in UV, similar to UV-MFRSR network. Combination of co-located AERONET and UV-MFRSR instruments offers inexpensive (relative to UV spectrometers) and powerful combination of measuring aerosol properties in UV-VIS spectral range. SZA=60-70 The ratio of TOMS to Brewer irradiance at 324nm against aerosol absorption optical thickness in Thessaloniki, Greece The ratio of TOMS to Brewer irradiance at 324nm against aerosol absorption optical thickness at Ispra, Italy 1Arola et al. accepted JGR 2005

Future Applications of OMI data 1. Global mapping of PAR (400-700nm) 2. Actinic flux and J-rates for photochemistry models 3. Global mapping of underwater UV-PAR irradiance with Chlorophill estimate from OMI RR algorithm [Vasilkov et al 2000] 4. Global primary production estimates 5. Global carbon cycle models

O3 R360 Erythemal Irradiance Trend 1980 to 2002 Problems with UV trends are filling gaps between N7 and EP TOMS and OMI

305 nm Irradiance Trend 1980 to 2002 O3 R360