Aerosol impact on the surface UV radiation from the ground-based measurements taken at Belsk, Poland,

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. D13, PAGES 16,175-16,181, JULY 20, 1998 Aerosol impact on the surface UV radiation from the ground-based measurements taken at Belsk, Poland, Janusz W. Krzy cin and Sylwester Puchalski Institute of Geophysics, Polish Academy of Sciences, Warsaw Abstract. The aerosol optical depth (AOD) at 550 nm, inferred from the solar radiation measurements taken at Belsk, Poland (52øN, 21øE), with a Sonntag pyrheliometer in the period , has been analyzed for detection of its long-term characteristics. The overall mean AOD equals to about 0.27 q (la). The AOD varies in the range , with a maximum (0.33) in April-May and a minimum (0.21) in November-December. The long-term decrease of AOD (-7.4% per decade) yields an increase of the surface UV dose (erythemally weighted) of,. 1% per decade. The influence of different parameters on the transmission of UV through the atmosphere on a day-to-day timescale has been analyzed on the basis of the multiple regression model that relates the daily UV erythemal dose at the ground level with total ozone, global solar radiation (surrogate for cloud properties variations), and AOD in the period It is found that a 10% increase (decrease) of AOD at 550 nm relative to its reference value manifests as 1.5% decrease (increase) of the UV erythemal daily dose at the ground level. The changes of UV radiation due to the AOD variations are, in the mean, comparable with those due to total ozone changes. The mean cloud signal in the UV data has been found to be a few times larger than the mean ozone signal. The statistical model suggests that the extreme values of AOD in the Belsk record (that is, 0.1 and 0.7) would be associated with 20-30% changes of the erythemally weighted UV daily dose. This estimate is corroborated by radiative transfer model calculations. 1. Introduction Aerosol can affect ground level UV irradiances both directly and indirectly. Direct influences manifest themselves as absorption and scattering of radiation. These effects provide a decrease of the solar UV direct radiation and, if aerosols are not highly absorbing, an increase of the diffuse component reaching the surface. In- direct influences are related to cloud formation (aerosols act as cloud condensationuclei) and the impact of clouds on transmission of UV. Because sulfate aerosols are involved in the ozone chemistry [e.g., World Mete- orological Organization (WMO), 1995 ], they can modulate UV irradiance at the ground level through the changes in the ozone vertical profile. The importance of aerosols on the surface UV irradiance was recognized by Liu et al. [1991]. They estimated that the surface solar irradiance in the range nm (biologically active solar radiation, UV-B) had decreased over nonurban areas in the eastern United States and Europe by about 5-18% since the industrial Copyright 1998 by the American Geophysical Union. Paper number 98JD /98/98 JD ,175 revolution owing to increasing aerosoloading of the atmosphere. This change in aerosols had been deduced from the observed reduction in horizontal visibility, i.e., from natural background visual range (near 550 nm) of 95 km in the preindustrial era down to 15 km in the 1980s. The physical-mathematical principles of aerosol scattering and absorption are reasonably well known. However, in practice, many of the aerosol optical characteristics (e.g., particle size distribution, single scatter albedo, asymmetry factor) that are necessary as input parameters to a radiative transfer model (RTM), are not routinely measured. RTM calculations by DeLuisi [1997] showed that with constant aerosol optical depth (AOD) it was still possible to have about 10% variations in the surface irradiance caused by changes in the single scatter albedo or asymmetry factor. The UV observations by the Brewer spectrophotometers revealed that the reduction of erythemally weighted UV irradiances at the ground level can reach 10-30% for extreme changes of AOD [Zerefos, 1997; Kerr, 1997]. In this work, we focus on the long-term ( ) seasonal pattern and trend of AOD (section 2). Manifestation of AOD changes in the measured erythemal UV doses at the Earth's surface and the comparison with other factors that impact UV radiation are discussed in section 3. The variation of UV irradiance due

2 16,176 KRZY CIN AND PUCHALSKI: AEROSOL IMPACT ON THE SURFACE UV RADIATION to the observed range of AOD at Belsk, Poland, is derived by means of a detailed RTM (LOWTRAN 7) in section 4. Section 5 gives a summary and conclusions. 2. Long-Term Characteristics of the Aerosol Optical Depth The uniform measurements of AOD have been carried at Belsk, Poland (21øE, 52øN, altitude 188 m), since June The site is rural and probably well isolated from local urban sources of air pollution. Pyrheliometric measurements of transmitted direct solar radiation flux by a Sonntag pyrheliometer with a set of cut-off filters (OG1 and OG3 Schott filters) have been used to determine AOD around 550 nm [Michatowska-Smak, 1985]. It should be noted that every year, the pyrheliometer and filters underwent calibration procedures to ensure a consistent data set. Figure 1 shows the time series of the local noon values of AOD. If there was no measurement around local noon, the observation from the time closest to local noon was chosen. Because of the observation's preconditions, i.e., almost clear sky and no clouds passing the Sun during observations, there were only 1120 days with AOD measurements during almost 17 years. Since January 1993, AOD observations have been much more frequent than they were earlier. This is related to increasing interest in such observations rather than to improving weather conditions in last few years. It is seen that the AOD varies significantly through the year. The overall mean AOD in the period of observations is 0.27 * (la). There were several days with AOD as low as 0.1 or as high as 0.7. The long-term seasonal pattern of AOD is estimated as a superposition of few sinusolds that are fitted to the daily AOD data. The seasonal pattern shows a maximum (0.33)in April-May, a minimum (0.21)in November-December, and a secondary maximum (0.29) in mid-september. The statistical significance of the appearance of the double maximum in the yearly pattern of AOD has been corroborated by the examination of differences between the half monthly long-term ( ) means of AOD (a standard t test of the difference between the mean values was used). It is found that the appearance of double maximum in the AOD time series by chance has a probability of < 0.1. It should be noted that the seasonal course of AOD as obtained by the superposition of the sinusoids follows almost ex- actly the pattern of the half-monthly long-term means of AOD. For each day with AOD observations, the normal- ized AOD deviation, i.e., the AOD value expressed in percent of its long-term reference, is calculated. The straight line fit (by an ordinary least squares method) to all the data points is shown in Figure 2. The slope of this line is-7.4% :k 6.0% (2a) per decade. Thus the atmosphere has become more transparento solar radiation during the period of observations. 3. Observed Aerosol Impact on the Day-to-Day Variations of UV Radiance The value of the Belsk AOD data is increased by the availability of accompanying data including total ozone (from Dobson spectrophotometer), global (Sun+sky) solar radiation in the whole ( nm) spectral range (by Kipp and Zonnen CM 11 pyranometer), and the UV irradiance spectrum in the range nm (by the Brewer spectrophotometer). We select the period, when all these measurements have been routinely performed. A time series of these variables is shown in Figure 3. It should be noted that using Brewer spectra and the Brewer software extension for the range nm, it is possible to estimate the erythemally weighted UV daily dose. In Figure 3 the solid line represents a pattern of the reference values obtained as a superposition of sinusolds. 9O 8O 7O 6O 5O 4O 3O 2O 10 0 I I I : I, I ], I, I, I,, I Year Figure 1. Time series of aerosol optical depths at nm from pyrheliometric measurements taken at Belsk, Poland, June 1980 to December The solid line represents the long-term seasonal cycle as described in text.

3 KRZY CIN AND PUCHALSKI: AEROSOL IMPACT ON THE SURFACE UV RADIATION 16, <%) 150 elo o el, e'..' :.'''...e e,! 1, e!_,le..e, # '_. '....,:,... ;._..... _'..,;.%'.,,,. : ;..; ',.' %;!.:.?, i.._'._..',... 'i, [:.*..Z'- ',.. -'.'- i _:Y,. -.- ]¾.k.x'.". 9',f,e,," _.,.r., -.:t],;,."..r..(;.,?k:_-' &:., ', :;,:.,.,,,..,.. '.,'....,..:,.. -1 O0 I, I i t, a, a, I, a, I, I, Yeat Figure 2. Fractional deviations of aerosol optical depths from the reference values in the period The straight line represents the linear fit (by an ordinary least squares method) to the time series shown. The following model is proposed to explain much of the observed variations of the UV daily dose, U - UV? Oa - 0, Cl - =a+b ' +c UVi* 0;, i Cl? +d Di - D? + ei (1) D? where UV/ is the UV erythemal dose on day i; 03,i is the total ozone amount; Cli is the surface global solar radiation normalized to the mean aerosoloading of the atmosphere (used as proxy of the cloud impact on the UV radiation, see next paragraph for more details); Di is the noon value of the vertical aerosol optical depth around 550 nm; (.)? means the reference value on day i for variable (.), a, b, c, and d are the model's constants to be determined by an ordinary least squares fit; and ei is the noise term. It should be noted that the predictor variables of the model are chosen to account for total ozone, cloud, and aerosol impact on the surface UV. The aerosols effects could not be described precisely if the measured global solar radiation (daily sum of direct and diffuse irradi- and Gl?moa is the modeled clear sky value of global radiatioh if all input parameters are equal to their reference values on day i. Two regression models are examined. The first one, model la, is defined exactly by the expression (1) and the regression coefficients are calculated using 256 data points (i.e., about 20% of all possible days in the period ). The other model, model lb, has no explicitly defined aerosol term: UV} - UVi* _ a/+ b/ Oa i -- O,i -{- C/ Gli - Gl? + ei This model is constructed to compare the ozone impact on the UV dose with that due to combined cloud and aerosol forcing. In the case of model lb (expression (1')), the total number of points used is 1075 (i.e., about 70% of all possible days). Models are run separately for the cold (November- April) and warm (May-October) seasons. The regression coefficients are shown in Table 1 together with an estimate of P, i.e., the mean range of X factor disturbances in the erythemal UV dose. P is defined as the standard deviation of X multiplied by the absoances) were used as the predictor variable in the model lute value of the regression coefficient pertaining to this because global solar radiation at the Earth's surface de- factor. This means that almost in 70% of cases, the pends on characteristics of both clouds and aerosols. changes in UV dose due to the X factor are in the range Thus, to parameterize the clouds effect on the UV ra- 4-P. The percent of the total variance explained by diation, a new variable Cli, is constructed, model (R 2) and the standard error of noise (Res) are also shown in Table 1. eli -- (GliGl?,mod)/Gli,mod The results presented in Table 1 show that all regreswhere Gli is the measured global radiation on day i, sion coefficients are statistically significant at a level Gli,mod is the modeled clear sky value of global radi- higher than 2 r. For each season, the models give almost ation calculated for aerosols with optical depth equal the same values (within the error of estimate) of b and c. to Di, other model input parameters (total ozone, sur- Moreover, these values correspond with those obtained face air, and water vapor pressure; for model's details, by Kvzydcin, [1996] using the UV dose monthly means see Krzygcin and Jarostawski [1997]) are taken from the from the broadband meter measurements [see Kvzydcin, daily Dobson ozone and meteorological observations, 1996, Table 4].

4 . 16,178 KRZY CIN AND PUCHALSKI: AEROSOL IMPACT ON THE SURFACE UV RADIATION ! O 3O 2O (M D). o ) (a) I ø,, :o oø : eee - t.. e.. - '"e'... (MJ/m2) q,,, ele d', ' 4' q,, (c) The regression coefficient pertaining to the aerosol forcing is about 10 times smaller than that for total ozone. However, the aerosol in Belsk is much more variable than total ozone. The relative standard deviation of the aerosol is about 45% (independent of season), whereas that for total ozone is only about 6% and 12% in the warm and cold period of the year, respectively. Thus the relative contribution of the UV variability from the ozone and aerosols variations may be comparable. The ratio between the mean range of the UV disturbances due to the total ozone variability and that due to aerosols, Po3/PAo0, is 1.1 and 1.8 in the warm and cold season, respectively. The range of the UV dose disturbances due to changes in the atmospheric transparency (combined effects of the cloud and aerosol changes) is about 3-5 times larger than those due to total ozone (see Table 1- modelb). Model la does not account for all possible changes in the cloud and aerosol characteristics (i.e., values of Pet and PAOO are probably underestimated) because the aerosol observations are taken when the sky is almost clear and there are no clouds passing the Sun during observations near local noon. It should be noted that similar values (depending on season) of the range of UV disturbance due to the total ozone variability are found in both models. This tends to support the regression models used. In Figure 4 the fractional deviations of daily UV doses after removal of the ozone and cloud signal from the UV data are shown for the warm period. Comparing Table 1. Regression Coefficients a, b, c, and d and Their Standard Errors by the Multiple Regression Models Used 10 Cold Season Warm Season Model 1 a ,, 100a b c d Po3,% Pro, % PAOD, % R 2, % Res, % Model lb Figure 3. The time series used as input to the multiple regression model. (a) UV erythemal dose at the ground level under all-sky conditions. (b) Total ozone from Dobson spectrophotometer. (c) Global solar radiation. (d) Aerosol optical depth at 550 nm. The Also presented are the range of the UV disturbances due solid line represents the reference time series obtained to total ozone (Po ), global radiation (%,), cloud (Pcz), from the least squares fit to the time series shown. The and aerosol (Paoo) variations; percent of variancexplained data points (daily values) are shown if all variables were by the regression model (R2); and variance of the model measured. 100a' b' c' Po,% PG, % R 2, % Res, % noise (Res)

5 KRZY CIN AND PUCHALSKI: AEROSOL IMPACT ON THE SURFACE UV RADIATION 16, } 10 > 0 g -5 tn -10! AOD D viations (%) Figure 4. The fractional deviation of the UV daily dose afteremoval of the total ozone and cloud-related variations versus that of aerosol optical depth. The solid and dotted lines represent ordinary and locally weighted linear least squares fits, respectively. the ordinary straight line fit with the locally weighted The UV irradiance response to variation over this range linear fit, it seems that the sensitivity of UV daily dose will be calculated by means of the LOWTRAN 7 model to the aerosol changes is almost constant over whole [Kneizys e! el., 1988]. One of the input parameters to range of the AOD variability. From Figure 4 it can be this model is the observed visual range (OVR). Using estimated that the UV dose change associated with the the standard (U.S. Standard Atmosphere, 1976) aerosol full range of observed aerosol loadings (AOD changes profiles and the Koschmieder formula,/ V - from 0.7 to 0.1) is about 20-30% of reference value. (which relates the extinction coefficient at 550 nm,/35 0, in the terrestrial boundary layer, with the observed vi- 4. Results of the Radiative Transfer sual range V), it is found that the range of AOD corresponds to a range of about km for OVR. Model In Figure 5 the daily course of the erythemal UV In this section we estimate the maximum variations of irradiance at the ground for total ozone equal to 330 the erythemally weighted UV irradiance at the ground DU (Dobson units) and different OVRs (12.5, 27 and due to changes in AOD at Belsk. Figure 3d shows that 80 km) are shown for June 21. An OVRof27 kmis the AOD varies approximately between 0.1 and 0.7. estimated from the AOD reference value for this day g I, I [ I I I I I I, Hour (UT) Figure 5. The modeled (LOWTRAN 7) profile of the UV erythemally weighted irradiance on June 21 for aerosol loading in the boundary layer, parameterized by the visual range of 12.5 km, 27 kin, and 80 km (left-hand axis, i MED -210Jm-e), and the normalized increase of the UV irradiance due to possible extreme changes in Belsk's aerosol (right-hand axis).

6 16,180 KRZY CIN AND PUCHALSKI: AEROSOL IMPACT ON THE SURFACE UV RADIATION The rural aerosol model is used to described the aerosol properties. The difference between the irradiances for the extreme aerosol loadings in percent of the mean irradiance (OVR--27 km) is also shown in Figure 5. It is seen that an increase in the UV irradiance of ~25% (relative to its long-term reference) may occur around local noon as a result of extreme changes in the aerosol loading. The increase seems to be higher for larger solar zenith angles (SZA). However, the increase of UV daily dose (daily sum of the UV irradiances) is found to be only a few percent larger than the noon increase of the UV irradiance. It should be noted that the difference between the irradiances for the extreme aerosoloadings is sensitive to other characteristics of aerosol. For example, assuming that the aerosol is less absorbing (singlescattering albedo equal to 0.94 regardless of wavelength; this assumption was used by Liu ½t al. [1991] for the U.S. nonurban aerosol), the increase in the UV daily dose is reduced to about 20% for July 21. The extreme change in the UV daily dose estimated by means of the regression model (Figure 4) corresponds spheric ozone depletion. The aerosol data used by Liu with that calculated using the radiative transfer model. The statistical model also provides a larger response of the UV daily dose to the aerosol change in the cold season (lower Sun elevations) than in the warm season (see the cold and warm season aerosol regression coefficients et al. [1991] came from the observations taken up to the early 1980s. Evidences for the continuation of the northern hemisphere midlatitudinal total ozone depletion in the 1980s and 1990s has been reported [e.g., WMO, 1995; Harris et al., 1997]. The total ozone trend in Table 1). This behavior is in accord with the increase in Belsk also shows the same tendency [e.g., Degdrska of the UV sensitivity to the aerosol changes for larger SZA as seen in Figure 5. et al., 1992]. The long-term changes in Belsk's aerosols during the 1980s and 1990s seem to have little impact Zerefos [1997] estimated solar UV irradiance over the on the long-term UV changes (i.e., increase of the UV Greek Aegean islands (39.6øN), at different extreme dose by about 1% per decade) because a 1% change in aerosol loadings (SZA=17 ø, total ozone-330 DU, tur- AOD forces a change of only ~0.15% in the UV dose. bidity at 355 nm equal to 0.2 and 0.8) showing a mean change of about 10% in irradiance over the UV-B range Thus in central Europe the aerosol compensation of the UV increase due to total ozone is not present during two (see their Figure 4). Kerr [1997] using the UV Brewer decades of large ozone depletion. Zerefos e! al. [1997] observations taken at Toronto in the period found that an increase of AOD at 315 nm of i (this value corresponds to the extreme changes of AOD at 550 nm found at Belsk) manifests as about exp(-0.2) to exp(-0.3) reduction of the UV global irradiances at 50 o pointed out a large increase of the UV-B irradiance over Europe in the 1990s depending on local environmental conditions and the period of the observations. An estimation of future changes in the UV-B radiation at the ground level based only on total ozone prediction should SZA. Thus these estimates are in reasonable agreement be treated with our results based on the regression and radiative with caution. transfer model results. Acknowledgments. The research was supported in 5. Summary and Conclusions The intensity of solar UV radiation incident on the Earth's surface depends on many atmospheric variables. Applying a multiple regression model to the data set that contains the daily erythemal UV dose (from the Brewer spectra), Dobson total ozone, global (Sun+sky) solar radiation over whole spectral range (corrected or uncorrected for the aerosols effects), and aerosol optical depth at 550 nm measured at Belsk, it becomes possible to estimate a range of the UV dose variations caused by the variations of any selected factor. Using this estimate, the UV forcing factors used here can be ordered as follows: clouds, total ozone, and aerosols. The mean range of the UV daily dose changes due to aerosol is about 4-5% and 4-7% relative to the climatological UV level in the warm and cold period of the year, respectively. In the warm period, when the normal level of UV radiation is much higher than in the cold season, the manifestation of the aerosol changes in the UV daily doses is as strong as that related to the total ozone changes. Moreover, large day-to-day fluctuations of AOD are possible in both seasons; thus in the extreme case, 20-30% increases of the UV daily dose can be attributed to the aerosol loading changes. This value corresponds to the UV increase caused by the extreme total ozone departures (-20%) from its normalevel found in the Belsk's total ozone data during the warm period. Thus, especially in the warm period of the year, the aerosol fluctuations are essential to explain or predict the level of the surface UV radiance. Liu et al. [1991] suggested that the observed reduc- tions in visibility over nonurban areas in the eastern U.S. and Europe since the preindustrial era would have offset much of the UV-B increases related to the strato- part by the Commission of the European Communities project UVRAPPF ERB-IC20-CT References Deg6rska, M., B. Rajewska-Wiqch, and R.B. Rybka, Total ozone over Belsk; Updating of trends through 1991 and comparison with those from TOMS in , Publ. Inst. Geophys. Pol. 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