The effects of smoke and dust aerosols on UV-B radiation in Australia from ground-based and satellite measurements.

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1 The effects of smoke and dust aerosols on UV-B radiation in Australia from ground-based and satellite measurements. Olga V. Kalashnikova a, Franklin P. Mills b, Annmarie Eldering a, Don Anderson c and Ross Mitchell d a Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA; b Australian National University, Canberra, ACT, Australia; c Bureau of Meteorology, Melbourne, VIC, Australia; d CSIRO Earth Observation Centre, Canberra, ACT, Australia ABSTRACT An understanding of the effect of aerosols on biologically- and photochemically-active UV radiation reaching the Earth s surface is important for many ongoing climate, biophysical, and air pollution studies. In particular, estimates of the UV characteristics of the most common Australian aerosols will be valuable inputs to UV Index forecasts, air quality studies, and assessments of the impact of regional environmental changes. Based on MODIS fire maps and MISR aerosol property retrievals, we analyzed the climatological distributions of Australian dust and smoke particles and identified two sites where ground-based UV-B spectra were available during episodes of relatively high aerosol activity. Since mid-2003, surface UV spectra ( nm) have been measured routinely at Darwin and Alice Springs in Australia by the Australian Bureau of Meteorology (BoM). Using collocated AERONET sunphotometer measurements at Darwin and collocated BoM sunphotometer measurements at Darwin and Alice Springs, we identified several episodes of relatively high aerosol activity that could be used to study the effects of dust and smoke on the UV-B solar irradiance at the Earth s surface. To assess smoke effects we compared the measured UV irradiances at Darwin with irradiances simulated with the libradtran radiative transfer model for aerosol-free conditions. We found that for otherwise similar atmospheric conditions, aerosols reduced the UVB irradiance by 50% near the fire source and up to 15% downwind. We also found the effect of smoke particles to be 5 to 10% larger in the UV-B than in the UV-A. For the selected period at Darwin, changes in the aerosol loading gave larger variations in the surface UV irradiances than previously reported changes seen in the ozone column. We are continuing similar investigations for the Alice Springs site to assess spectral differences between smoke and dust aerosols. Keywords: Australian aerosols, UV surface irradiances, ground-based UV-B and ozone measurements 1. INTRODUCTION Australia has high levels of UV radiation, particularly during the spring and summer months, for multiple reasons: Northern Australia is close to the equator, the southern atmosphere generally has low pollution and low aerosol loading, the Earth reaches perihelion at southern summer solstice, and ozone depletion over Antarctica in southern spring can reduce ozone levels over Australia. For example, the annual mean (noontime) UV Index at Alice Spring and Darwin, as determined by the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA), is 9.0 and 10.1, respectively, compared to an annual mean 2atmostUKsites. 1 Public health officials raise concerns over exposure to both UV-A ( nm) and UV-B ( nm) in Australia because the combination of a predominately fair-skin population, an outdoor lifestyle, and high levels of UV radiation have produced skin-cancer rates among the highest in the world. 2 An increase in the surface UV irradiances Further author information: (Send correspondence to Olga V. Kalashnikova) Olga V. Kalashnikova: olgak@jord.jpl.nasa.gov, Telephone: Franklin P. Mills: Frank.Mills@anu.edu.au Annmarie Eldering: Annmarie.Eldering@jpl.nasa.gov Don Anderson: d.Anderson@bom.gov.au Ross Mitchell: Ross.Mitchell@csiro.au Ultraviolet Ground- and Space-based Measurements, Models, and Effects V, edited by G. Bernhard, J. R. Slusser, J. R. Herman, W. Gao, Proc. of SPIE Vol (SPIE, Bellingham, WA, 2005) X/05/$15 doi: / Proc. of SPIE 58860R-1

2 could also lead to damage to terrestrial and oceanic vegetation, degradation to man-made objects, changes in the chemistry of the lower atmosphere, e.g. photochemical smog formation, and alteration of plant biochemical cycles. UV irradiances that reach the surface are affected by ozone, solar zenith angle, surface cover, aerosols and clouds. While the relation between ozone and surface UV irradiance is well established 3 and the changes in the surface UV irradiance due to changes in the solar zenith angle are predictable, less is known about the effects of surface type, aerosols, and clouds. The magnitude of the aerosol effect can be large (over certain parts of the Earth, aerosols can reduce the UV flux at the surface by more than 50% 4 ) and is highly variable, depending on the number of particles and their physical and chemical properties. Aerosol particles tend to reduce the surface UV irradiance; however, scattering by non-absorbing particles (background aerosols) can actually increase the UV exposure on non-horizontal surfaces due to additional scattering from low angles. 5 Lui et al. 6 estimated that anthropogenic sulfate aerosols have decreased surface UV-B irradiances by 5 to 18%; Kylling et al. 7 found that surface irradiances measured at two sites at Greece under non-cloudy conditions were reduced with respect to aerosol-free conditions by 5 to 35% depending on aerosol optical depth and single scattering albedo. Very few studies have been done on the effect of different types of aerosols on UV radiation and particle properties, such as single scattering albedo, are poorly constrained in the UV. Balis et al. 8 have shown from collocated Raman lidar system and spectral UV-B irradiance measurements that for the same aerosol optical depth and for the same ozone values the UV-B irradiances at the Earth surface can show differences of up to 10%, which are attributable to differences in aerosol type. In addition, some modeling studies 9 have suggested that aerosol vertical (height) distribution can also affect UV surface irradiances by 2 5% for optical depth on the order of 0.5 at visible wavelengths. Due to the combined involvement of these different aerosol parameters in controlling UV levels, it is difficult to determine accurately the role of each parameter. Australia is an ideal place to study the effects of absorbing aerosols on UV radiation because of its generally clear-sky atmospheric conditions and its very low loading of urban aerosols (pollution). The northern part of Australia is mostly affected by smoke from controlled (planned) fires in September through November; whereas central to southern Australia is mostly affected by dust outbreaks in December through March. 10 In this paper we study the spectrally-resolved signature of one type of absorbing aerosol affecting the Australian continent: smoke. We also investigate the correlation between aerosol optical thickness (AOT) at visible wavelengths and the aerosol s UV spectral effect and examine whether the aerosol s UV spectral effect changes downwind from the source. The latter may occur if particle properties change during transport and aging. 11 We have used daily average total column ozone data from the BoM Dobson spectrophotometer, AOT data from AERONET and BoM sunphotometers, and UV spectrally resolved surface irradiance data from the BoM spectroradiometer at Darwin to characterize the atmospheric conditions and the aerosol s UV spectral effect. Days with minimal, moderate, and high loading of smoke over Darwin were selected based on the sunphotometer measurements and the MODIS Rapid Response System fire maps. Our modeling approach and the BoM UV spectrometer are summarized in Section 2. We then conclude with a discussion of our current results and planned future work. 2. MODELING APPROACH AND INSTRUMENT DESCRIPTION Our approach compares simulated cloudless and aerosol-free spectra against measured spectra for clear-sky and smoke-loaded atmospheres to investigate the UV effects of smoke over Darwin (12.28 S, E). Darwin is located in Northern Australia, has very low background aerosol loadings, and is affected by smoke during the local fire season (Sep-Nov). Total overhead column ozone and spectra of the global surface UV ( nm) have been measured routinely by the Australian Bureau of Meteorology (BoM) at Darwin since mid Global UV irradiance spectra have been measured by the BoM at Darwin since June The BoM spectrometer is a double monochromator with holographic grating (3600 lines/mm) from Bentham. The diffuser and entrance optics were built by Richard McKenzie s group at the New Zealand National Institute of Water and Atmospheric Research (NIWA). The bandwidth (full width at half maximum, or FWHM ) is 0.6 nm, and each scan covers the wavelength range from 285 to 450 nm with steps of 0.2 nm. The instrument is configured to perform daily scans in 5 degree steps of Solar Zenith Angle (SZA) from 95 degrees (95, 90, 85,...) until 11:30 local time, after which it scans every 15 minutes until 14:00. The instrument then returns to scanning Proc. of SPIE 58860R-2

3 Figure 1. An example of BoM measured surface irradiance spectra: surface irradiances measured on April 15, 2004 at SZA=30 o, 40 o, 50 o, 60 o, 70 o, and 80 o (largest irradiances correspond to the smallest SZA). every 5 degrees of SZA to 95 degrees. A complete scan takes about 4.5 minutes. The instrument temperature is stabilized within 0.5 K of a nominal 30 C value. Wavelength calibration is done weekly using a mercury lamp and flux calibration is done weekly using a transfer standard lamp and semiannually using a suite of four transfer standard lamps. The overall uncertainty of the calibration is estimated to be ± 5%. Figure 1 shows measured surface irradiance spectra for six solar zenith angles on April 15, 2004, at the Darwin site. As expected, irradiances increase as the solar zenith angle approaches the local minimum (local noon). Irradiances at UV-B wavelengths, which are efficiently absorbed by ozone, are several orders of magnitude smaller than UV-A but have the most detrimental effects on organic life. We simulated the clear-sky spectra using the libradtran 12, 13 model, which allows specification of the atmospheric, instrumental, and measurement characteristics as is necessary for systematic comparisons with the UV spectra. The model has been designed for use at nm with resolution from nm. The libradtran package includes three different radiative transfer solvers. We used the general purpose discrete ordinate algorithm DISORT (8 streams) for our calculations, and assumed a vertically inhomogeneous, non-isothermal plane-parallel atmosphere. The most important parameters for modeling cloudless and aerosol-free sky irradiances are the total ozone column and the surface albedo. Daily average ozone values measured at Darwin by the BoM using a Dobson spectrometer were used as input for our model calculations. Experimental determination of the surface albedo is somewhat complicated and the UV surface albedo has not been measured at Darwin where the measurement site is surrounded by grass, so we used the libradtran standard model for grass which varies with wavelength from 0.15 to For the exoatmospheric irradiance spectrum we used the standard libradtran solar spectrum, which was measured by the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) during the ATLAS 3 mission in A standard tropical atmospheric profile 14 was used with the ozone profile scaled so that the total ozone column matched that measured by the BoM. The global irradiance spectra were calculated at 0.15 nm intervals and then averaged using a triangle filter with FWHM of 0.6 nm to simulate the slit function of the spectrometer. 3. RESULTS, DISCUSSIONS AND FUTURE WORK Our initial investigation has concentrated on the effects of smoke for selected days at the Darwin site. We selected three types of cases for comparisons: clear sky, low aerosol loading, and high aerosol loading based on MODIS fire maps and daily-averaged AERONET sunphotometer AOT values. For each selected day, we examined the complete AERONET AOT time series, identified the range of SZA with minimum AOT variation, and chose SZAs that were within a several hour period of nearly constant AOT. Figure 2 shows the relatively stable times Proc. of SPIE 58860R-3

4 Figure 2. Examples of AERONET level 2 (cloud-screened) AOT time series showing minimum aerosol variability over the range of solar zenith angles. (a) clear sky and (b) low aerosol loadings - downwind fires (c) high aerosol loadings - flaming fires series for the days that we selected for comparison calculations. We compared modeled and measured spectra for low SZAs because the uncertainties are smaller for SZAs close to local noon. First, to test for disagreements between the model and measurements, we chose several days at the Darwin site when AERONET-measured optical depth was very low (less than 0.1) and used these as benchmark cases for clear sky conditions. Table 1 lists the selected days together with the AERONET-measured AOT and the BoM reported total column ozone. Next, to study the differences between the spectral signatures of smoke from proximate and distant open brush fires, we chose two sets of smoky days from the MODIS Rapid Response System fire maps: Low aerosol loading - distant fires High aerosol loadings - flaming fires near the site Tables 2 3 summarize the selected days for each set, together with the reported AOTs at the selected SZAs and the measured total column ozone values, which were input to the model calculations. As shown in Figure 3, we found good agreement between the model and measurements for all selected clear sky days. (Comparable agreement was found for the other days in Table 1 as well.) We found model/measurement Proc. of SPIE 58860R-4

5 Table 1. Selected aerosol-free benchmark cases Date Time of day Selected SZAs AOT Ozone April 15, 2004 Afternoon 22.3, 22.7, 23.6, April 16, 2004 Morning 23.9, 25.4, 27.3, April 17, 2004 Morning 23.3, 24.2, 27.5, April 18, 2004 Afternoon 23.6, 24.5, 25.8, April 19, 2004 Morning 24.9, 26.2, 28.0, Figure 3. Examples of relations between measured and modeled irradiances on clear sky days (AERONET-reported AOT< 0.04: (a) April 15, 2004; (b) April 17, 2004 differences to be less than 5% throughout the 295 to 400 nm range except near the deep calcium solar absorption lines at 393 and 397nm. The residual differences between model and measurement are within the uncertainties in both the measurements (e.g., calibration uncertainties) and the model calculations (e.g., uncertainties in model parameters such as the shape of the ozone profile). The residual scatter of a few percent may be due to a small remaining difference between the modeled and actual instrumental slit function. The comparisons between modeled clear sky and measured spectra are quite dissimilar for the aerosol-loaded cases, as shown in Figures 4 5. The overall spectral signature seems to be similar for both the proximate and distant source fires, which implies their UV radiative properties have not changed significantly over the time required to transport the smoke. The net reduction in global UV-B irradiances in all analyzed cases is about 10% larger than the reduction in global UV-A irradiances. The differences between the measurement/model ratios for the range of SZAs shown may be due to small variations in AOTs over the duration of the 4.5 minute instrument scan, movement of a small cloud, and/or small variations in aerosol loadings. The total smoke effect increases with an increase in the optical depth as expected; the reduction in UV-B surface irradiance in the presence of smoke aerosols changes from about 10 15% for AOT (at 500nm) of 0.2 to up to 50% for AOT (at 500nm) of 0.6 as demonstrated in Figure 6. The most interesting conclusions may come from comparing the UV effects of dust and smoke aerosols. Our next step (work in progress) will be to select dust events at Alice Springs with the same measured AOTs as we found for smoky days at Darwin and to investigate dust UV spectral signatures. The goal is to determine the importance of the single scattering albedo (or aerosol type) is for interpretation of observed UV changes. This knowledge is important for an interpretation of measurements in regions with highly variable aerosol conditions. Proc. of SPIE 58860R-5

6 Table 2. Selected downwind low-aerosol-loading cases Date Time of the day Selected SZAs AOT Ozone August 28, 2004 Morning 27.0, 29.3, August 30, 2004 Afternoon 25.7, 28.0, 30.0, September 1, 2004 Afternoon 25.2, 27.5, 30, September 2, 2004 Afternoon 21.5, 23.0, 25.0, September 5, 2004 Afternoon 24.2, 26.7, 30.0, September 6, 2004 Morning 23.9, 26.4, 30, September 7, 2004 Morning 21.6, 26.1, 30, September 8, 2004 Afternoon 23.5, 26.1, 30, September 9, 2004 Afternoon 23.3, 25.9, 30, September 12, 2004 Afternoon 21.9, 24.5, 30.0, Table 3. Selected flaming fires high-aerosol-loading cases Date Time of the day Selected SZAs AOT Ozone October 28, 2004 Afternoon 21.9, 25, 30, October 29, 2004 Morning 20, 25, 30, October 30, 2004 Morning 35, November 1, 2004 Morning 30, 35, 40, November 2, 2004 Morning 20, 25, 30, November 14, 2004 Afternoon 18.8, 22.3, 25, November 17, 2004 Morning 18.9, 22.3, 25, Figure 4. Examples of relations between measured and modeled (clear-sky) irradiances on low aerosol-loadings (downwind fires) days (AERONET-reported AOT 0.2): (a)august 28, 2004; (b)september 1, 2004 Proc. of SPIE 58860R-6

7 Figure 5. Examples of relations between measured and modeled (clear-sky) irradiances on high aerosol-loadings (close to fire sources) days (AERONET-reported AOT 0.6): (a)october 29, 2004; (b)october 30, 2004 Figure 6. Model/measurement absolute percentage differences for the different aerosol loadings. Model is clear-sky: (a) SZA=25 0, (b) SZA=30 0 Proc. of SPIE 58860R-7

8 ACKNOWLEDGMENTS The work of O. Kalashnikova was supported by the NASA New Investigator Program in Earth Science under M-Y Wei. Part of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. Travel funding for F. Mills from The Ian Potter Foundation, the International Society of Biometeorology Tromp Travel Fund, and the Australian Research Council is gratefully acknowledged. REFERENCES 1. P. Gies, C. Roy, J. Javorniczky, S. Handerson, L. Lemus-Deschamps, and C. Driscoll, Global Solar UV Index: Australian Measurements, Forecasts and Comparison with the UK, Photochemestry and Photobiology 79, pp , T. Slevin, J. Clarkson, and D. English, Skin cancer control in Western Australia: is it working and what have we learned?, Radiat. Prot. Dosim. 91, pp , S. Madronich, R. McKenzie, L. Bjorn, and M. Caldwell, Changes in biologically active ultraviolet radiation reaching the Earth s surface, Photochemestry and Photobiology 46, pp. 5 19, N. A. Krotkov, P. K. Bhartia, J. R. Herman, V. Fioletov, and J. Kerr, Satellite estimation of spectral surface UV irradiance in the presence of tropospheric aerosols 1. Cloud-free case, J. Geoph. Res. 103, p , R. Dickerson, S.Kondragunta, G. Stenchikov, K. Civerolo, B. Doddridge, and B. Holben, The impact of aerosols on solar ultraviolet radiation and photochemical smog, Science 278, pp , S. Lui, S. McKeen, and S. Mandronich, Effect of anthropogenic aerosols on biologically active ultraviolet radiation, Geoph. Res. Lett. 18, pp , A. Kylling, A. Bais, M. Blumthaler, J. Schreder, C. Zerefos, and E. Kosmidis, Effect of aerosols on solar UV irradiancies during the Photochemical Activity and Solar Ultraviolet Radiation campaign, J. Geoph. Res. 103, pp , D. Balis, V. Amiridis, C. Zerefos, A. Kazantzidis, S. Kazadzis, A. Bais, C. Meleti, E. Gerasopoulos, A. Papayannis, V. Matthias, H. Dier, and M. Andreae, Study of the effect of different type of aerosols on UV-B radiation from measurements during EARLINET, Atmos. Chem. Phys. 4, pp , J. P. Diaz, F. J. Exposito, C. J. Torres, V. Carrena, and A. Redondas, Simulations of the mineral dust effect on the UV radiation level, J. Geophys. Res. 105, pp , J. M. Prospero, P. Ginoux, O. Torres, S. E. Nicholson, and T. E. Gill, Enviromental characterization of global sourses of atmospheric soil dust identified with the NIMBUS 7 Total Ozone Mapping Spectrometer (TOMS) absorbing aerosol product, Rev. Geophys. 40(1), p. doi: /2000rg000095, O. Torres, P. K. Bhartia, J. R. Hermann, Z. Ahmad, and J. Gleason, Derivation of aerosol properties from satellite measirements of backscattering ultraviolet radiation: Theoretical basis, J. Geophys. Res. 103, pp , B. Mayer and A. Kylling, Technical note: The LibRadtran software package for radiative transfer calculations - description and examples of use, Atmos. Chem. Phys. 5, pp , B. Mayer, G. Seckmeyer, and A.Kylling, Systematic long-term comparison of spectral UV measurements and UVSPEC modeling results, J. Geoph. Res. 102, pp , National Geophysical Data Center, U.S. Standard Atmosphere Supplements, 1966, U.S. Government Printing Office, Washington, D.C., Proc. of SPIE 58860R-8