Ozone Monitoring Instrument satellite UV irradiance ...

Ozone Monitoring Instrument satellite UV irradiance product correction using a global aerosol climatology Antti Arola a, S. Kazadzis*b,l, J. Kujanpää c, A. Lindfors b, A. Bais d, A. Webb e, P. Weihs f, A. di Sarra g, T. Koskela b, M. Janousch h , J. M. Villaplana i , A. M. Sianni j , C. Brogniez k Finnish Meteorological Institute, Kuopio Unit, Finland, bFMI, Climate Change Unit, Finland, cFMI, Earth Observation Unit, Finland, dLaboratory of Atmospheric Physics, A.U.Th, Greece, eUniversity of Manchester, UK, fInstitute of Meteorology, Austria, gENEA-Climate Laboratory, Italy, hCHI, Czech Republic, iINTA, Mazagon, Spain, jSapienza University of Rome, Italy, kLOA-Univ. des Sciences et Technologies de Lille, France, lNational Obesrvatory of Athens, IERSD

a

ABSTRACT Lately a number of studies related with UV irradiance estimates from satellite data based on the Ozone Monitoring Instrument (OMI) have shown a high correlation with ground-based measurements but a positive bias in many locations, the satellite derived UV being higher. One of the key factors that this bias has been attributed to is the boundary layer aerosol absorption not taken into account in the current OMI UV algorithm. In this work we have used a correction procedure based on climatological global aerosol absorption data taken from AeroComm aerosol initiative. This dataset includes aerosol optical depth and aerosol single scattering albedo assembled by combining, ground-based aerosol measurements from AERONET and information from several global aerosol models. The results of this correction were compared with synchronous ground-based measurements from 9 UV monitoring stations. The results generally showed a significantly reduced bias of 7-20%, a lower variability, and an unchanged, high correlation coefficient. Keywords: UV irradiance, Ozone monitoring instrument, aerosol absorption

1. INTRODUCTION Due to the limited availability of long term UV data series, combined with the fact that ground-based (GB) measurements cover a small fraction of the Earth’s surface, satellite sensors estimating UV irradiance reaching the ground from ozone and reflectivity measurements have gained particular attention during the past decades. The development of such satellite UV derivation techniques have been among the most important UV related issues1. Following the Total Ozone Mapping Spectrometer (TOMS) that has been providing global UV irradiance measurements; the Ozone Monitoring Instrument started providing global UV data since September 2004. OMI is a Dutch-Finnish instrument that flies on NASA’s Aura mission as part of the Earth Observation System (EOS) launched in July 2004. Aura is part of a constellation of satellites known as the A-Train. OMI is a contribution of the Netherlands’s Agency for Aerospace Programs (NIVR) in collaboration with the Finnish Meteorological Institute (FMI) to the EOS Aura mission and provides information on various atmospheric parameters2, such as ozone and other trace gasses, aerosols, clouds, and surface UV irradiance. OMI is a wide swath, nadir viewing, near-UV and visible spectrograph that measures reflected and backscattered solar ultraviolet and visible radiation in the range 270-500 nm 2. The spatial resolution of the measurements is 13 X 24 km2 in nadir and larger towards the edges of the swath. One of the goals of OMI is to investigate changes in global surface UV irradiance. Surface UV estimates based on satellite data have been used extensively in the last decade to establish global UV climatologies and to examine possible

Ultraviolet and Visible Ground- and Space-based Measurements, Trace Gases, Aerosols and Effects VI, edited by Jay R. Herman, Wei Gao, Proc. of SPIE Vol. 7462, 74620D · © 2009 SPIE CCC code: 0277-786X/09/$18 · doi: 10.1117/12.825600 Proc. of SPIE Vol. 7462 74620D-1

long-term changes in surface UV3. The OMI surface UV algorithm consists of a calculation for clear sky cases extended with corrections in the case of cloudy pixels (or ones containing non-absorbing aerosols) 4. The validation of satellite-derived UV products against GB measurements is an essential task in order to assess their accuracy. Several validation studies5, 6, 7, 8 revealed a positive bias in many locations, with the satellite derived UV being higher. It was also observed that satellite UV agrees much better with the GB measurements at pristine sites (Lauder, in New Zealand9, or Canadian West coast10 than in more polluted European and North-American sites7. Therefore, it was suggested that at least part of the bias could be attributed to the boundary layer aerosol absorption, not accounted for in the current satellite UV algorithm1,10,11 In this study we present an aerosol correction procedure for OMI UV data, exploiting the newly developed aerosol climatology of AeroComm12. We also evaluate the results of this correction, by comparing spectral OMI UV products with synchronous GB measurements from various European UV monitoring sites having different aerosol characteristics. Results showed a significantly reduced bias. We now plan to implement this correction into the next version of OMI UV data.

2. DATA AND METHODOLOGY 2.1 Aerosol absorption data 12

The global monthly aerosol climatology of the AeroComm initiative provides aerosol optical properties, such as Aerosol Optical Depth (AOD) and Single Scattering Albedo (SSA), at 1x1 degree spatial resolution. This climatology uses ground based aerosol data from the AERONET network, and global modelling results. AOD was converted to the UV wavelengths by using the Ångström parameter. This parameter was also used in SSA spectral dependence (and thus to obtain SSA at UV wavelengths), by estimating fine and coarse mode contributions to AOD. The coarse mode fraction of AOD was attributed to sea-salt, dust or a mixture of both, whose typical spectral dependence was used to get SSA at UV from the measurements at mid-visible. For the fine mode contribution, spectrally independent SSA was assumed. In this work, we used this global monthly aerosol climatology for Aerosol Absorption Optical Depth (AAOD) AAOD = AOD * (1-SSA) 1,11

at 315nm and applied the parameterization suggested

(1)

:

CF = 1 / (1 + 3 * AAOD)

(2)

where CF is the post-correction factor to multiply the OMI UV estimate, to account for absorbing aerosols. Fig. 1 shows an example of this global correction factor as an annual mean over all months. For instance, the strongest reduction of surface UV irradiance, due to the absorbing aerosols, is about 22% in Equatorial West Africa and East Asia, about 15% in Eastern Europe and 5-10% in the East Coast of the United States.

Fig. 1. Mean correction factor for absorbing aerosols at 315nm

Proc. of SPIE Vol. 7462 74620D-2

2.2 Ground based data We included nine European stations that provide spectral surface UV measurements, to evaluate the improvements that can be obtained in OMI-UV by this correction for absorbing aerosols for the 2005-2006 period. The spectral irradiance GB data were corrected for possible wavelength shifts and standardized to 0.55 nm spectral resolution (equal to the OMI slit function). Information of the included sites is given in the following table. Table 1. Monitoring station information Location Lampedusa, Italy El Arenosillo, Spain Thessaloniki, Greece Rome, Italy Sonnblick, Austria H. Kralove, Czech Rep. Vineuve d’ Ascq, France Reading, UK Jokioinen, Finland

Lat (N)/ Lon (W) 35.5 / 12.6 37.1 / 6.7 40.6 / 22.9 41.9 / 12.5 47.0 / 12.9 50.1 / 15.8 50.7 / 3.0 51.2 / -0.9 60.8 / 23.4

Alt (m)

instrument

# of days

50 40 60 60 3106 285 60 66 104

Brewer MKIII Brewer MKIII Brewer MKIII Brewer MKIV Bentham Brewer MKIV Jobin Yvon Bentham Brewer MKIII

238 285 319 282 257 401 388 405 381

Ground-based data were averaged within a one-hour time window centered on OMI overpass time. In the following analysis ratios of OMI vs GB measurements are presented.

3. RESULTS For each of the 9 GB monitoring stations OMI vs. GB UV irradiance mean ratios (R) together with their standard deviation at 324nm are presented before and after applying the aerosol absorption correction. In addition, the OMI and GB UV irradiance correlation coefficients are shown in the following table. Table 2. Statistics of the ratio OMI/GB at 324 nm for the validation sites. Location Mean ratio - 324nm Correl. Coeff. Corr. / Uncorr. Corr. / Uncorr. El Arenosillo 1.16 / 1.20 0.97 / 0.97 H. Kralove 1.07 / 1.26 0.96 / 0.96 Jokioinen 0.99 / 1.07 0.92 / 0.91 Lampedusa 1.03 / 1.13 0.94 / 0.95 Reading 1.27 / 1.33 0.96 / 0.96 Rome 1.19 / 1.30 0.97 / 0.97 Sonnblick 0.81 / 0.91 0.77 / 0.77 Thessaloniki 1.10 / 1.21 0.95 / 0.95 Vineuve d’ Ascq 1.12 / 1.19 0.96 / 0.96

Standard deviation Corr. / Uncorr. 0.15 / 0.16 0.26 / 0.30 0.26 / 0.29 0.16 / 0.17 0.23 / 0.24 0.19 / 0.19 0.27 / 0.31 0.20 / 0.21 0.22 / 0.24

In the analysis of the above statistics we have excluded a small number of points of R > 2 and R < 0.5 which are most likely due to the rapidly changing cloudiness or occasional technical problems in the ground-based instruments. Post correction using the AeroComm aerosol climatology reduces the OMI bias from 19% to 7% for 324nm and from 30% to 22% for 305nm irradiances (not shown here). Also, the standard deviation is reduced by 7 % and 2% respectively comparing 2 years of daily OMI overpass data over 9 European UV monitoring stations. The correlation coefficients remain practically unchanged. In figure 2 we present an overall picture of possible improvements concerning the statistics of OMI and GB UV irradiance deviations before and after the correction.


OMI / GB at 324 nm (SD) 40

Uncorrected 1.19 (0.27) Corrected

1.07 (0.20)

% of cases

30

20

10

0 > 25%

10% to 25% -10 to 10% -25% to -10%

OMI vs GB diference at 324 nm

0.9 There is a 11% improvement on mean R324 differences but there is a remaining +12% OMI UV irradiance overestimation. Most of the scatter in R values is related with cloudy conditions. In these cases (when CMF < 0.8) the standard deviation of the ratio is in the order of 33% and 31% before and after the correction respectively.


4. CONCLUSIONS Overestimation in satellite-based UV data, when compared to the GB UV measurements, has been documented in several studies. It is believed that this is mostly caused by the absorbing aerosols that have not been accounted for properly in the satellite-UV algorithms. Post correction using the AeroComm aerosol climatology reduces by 12% the OMI bias for 324nm and by 8% for 305nm Irradiances. Also, the standard deviation is reduced by 7 % and 2% respectively comparing 2 years of daily OMI overpass data over 9 European UV monitoring stations. The correlation coefficients remain practically unchanged. In general, part of the bias is corrected using this aerosol absorption approach but there is still a remaining bias that shows a wavelength dependence (being higher at lower wavelengths). In the particular correction in case of cloudy conditions we assume that the aerosol absorption effect is similar to the cloudless ones and we have not taken into account any other sources of errors due to cloud effects such as their spatial variability, enhanced scattering effects or any other corrections related with the way that OMI algorithm is taking clouds into account. We found no obvious solar zenith angle dependent error in the OMI/GB UBV irradiance ratios. Moreover, maximum OMI overestimation is mostly due to cloudy conditions. The standard deviation of the OMI / GB ratios is increasing with increasing cloudiness for the 9 sites. Especially cases of low UV irradiance (low CMF’s and/or high solar zenith angle measurements) have overly strong influence in the presented statistics. In all the sites, except for Sonnblick, a reduced bias is achieved. Sites like Sonnblick (3000 m altitude) are very challenging for satellite UV estimates, also due to the other reasons than aerosols. Satellite algorithms require assumptions to distinguish the backscattered radiance from snow and clouds for any type of surface, but at high altitudes there are cases where this can be further complicated especially when clouds appear below the altitude of the monitoring station.

ACKNOWLEDGMENTS S. Kazadzis would like to acknowledge the Marie Curie Intra European fellowship “Validation of Aerosol Optical Properties and surface Irradiance measured from Ozone Monitoring Instrument on board of AURA satellite” VAP-OMI, AOR A/119693 – PIEF-GA-2008-219908

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