Aerosol optical depth in the UVB and visible wavelength range from Brewer spectrophotometer direct irradiance measurements:

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2003jd004409, 2004 Aerosol optical depth in the UVB and visible wavelength range from Brewer spectrophotometer direct irradiance measurements: J. Gröbner Institute for Health and Consumer Protection, Joint Research Centre, European Commission, Ispra, Italy C. Meleti Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, Thessaloniki, Greece Received 2 December 2003; revised 13 February 2004; accepted 25 February 2004; published 12 May [1] Direct irradiance measurements from Brewer spectrophotometer #66 at Ispra, Italy, were reanalyzed for the period Two calibration methodologies, one adapted to the ultraviolet (UV) and one adapted to the visible wavelength range, have been developed to retrieve the aerosol optical depth (AOD) from these measurements. The measurements in the UV were calibrated in absolute units using laboratory lamp calibrations of the global entrance port of the same instrument, while in the visible wavelength range a zero air mass Langley extrapolation methodology was implemented. The retrieved aerosol optical depths were validated with a colocated CIMEL Sun photometer operating within the Aerosol Robotic Network (AERONET) since AOD values at Ispra show a strong short-term variability between 0.05 and 2 at 320 nm as well as a pronounced seasonal variability. The highest monthly mean values of around 0.6 (at 320 nm) are usually found in spring and summer while the lowest values, of about 0.3, are found in the winter months. An aerosol characterization by means of the Ångström power law yields a mean wavelength exponent of 1.6. The influence of Föhn events on the aerosol climatology at Ispra was investigated. The AOD measurements at Ispra show the strong influence of the Mount Pinatubo eruption in June 1991 and the following decay to background levels in late INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution urban and regional (0305); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0394 Atmospheric Composition and Structure: Instruments and techniques; KEYWORDS: aerosol optical depth, direct solar irradiance, ultraviolet Citation: Gröbner, J., and C. Meleti (2004), Aerosol optical depth in the UVB and visible wavelength range from Brewer spectrophotometer direct irradiance measurements: , J. Geophys. Res., 109,, doi: /2003jd Introduction [2] In recent years, atmospheric aerosols, their radiative properties, and their variability over time have become of major importance because of their impact on global climate [Intergovernmental Panel on Climate Change (IPCC), 1996, 2002; Kaufman et al., 2002]. Their interaction with solar radiation is a crucial element in radiative transfer modeling of the solar radiation as it passes through the atmosphere [Kato et al., 1997; Wild, 1999]. Because of their temporal and spatial variability, long-term measurements at a large number of individual measuring sites are required. [3] Previous studies have listed and discussed the history of aerosol measurements within past or existing networks [Holben et al., 2001; Ingold et al., 2001]. These records show that the longest networks still in operation initiated Copyright 2004 by the American Geophysical Union /04/2003JD measurements in the early 1990s, and only sporadic measurements are available before. From that point of view, the possibility of extending the continuous coverage in time and space by a further network of instruments is tantalizing. In the following study, we will present a methodology that allows the retrieval of aerosol optical depth (AOD) from existing Brewer spectrophotometer measurements with low uncertainties. [4] The Brewer spectrophotometer is a standard instrument developed for high-accuracy measurements of total column ozone [Kerr et al., 1981]. Since its development in the early 1980s, more than 100 instruments have been deployed worldwide and have routinely measured total column ozone from direct solar irradiance measurements. Several publications have discussed methodologies that can be used to retrieve the AOD from these measurements [Kerr, 1997; Carvalho and Henriques, 2000; Marenco et al., 2002; Cheymol and De Backer, 2003; Jarosawski et al., 2003]. All have in common that they use the zero-air-mass 1of12

2 Langley extrapolation to retrieve the reference parameters (extraterrestrial constants) needed for the determination of the aerosol optical depth. However, it is well known that this methodology requires stable atmospheric conditions over at least half a day and for several days. These conditions are usually found on high-mountain stations above the boundary layer, where the atmosphere is nearly free of aerosols and where the assumption of atmospheric stability is best fulfilled. In the case of the Brewer spectrophotometer, an additional difficulty resides in the wavelength range in which the aerosols are observed; the measurements are obtained between 306 and 320 nm and are strongly attenuated by the ozone present in the atmosphere. Thus only a small fraction of the attenuation is due to the aerosols. Therefore, during the zero-air-mass extrapolation, the atmospheric ozone must remain stable also. These two constraints, atmospheric stability with respect to ozone and to aerosols, are only found at highmountain sites close to the equator. This implies that instruments need to be moved from their usual measuring location to these specific sites, calibrated for a certain length of time and then brought back. Considerable cost and effort are involved in such an activity, and only very few instruments have reliable calibration records of this kind [Kerr, 2002]. [5] Another possibility is to use a traveling standard instrument that is calibrated using the procedure outlined previously, which transfers the calibration to other instruments as is described by Gröbner et al. [2001]. This methodology by itself does not allow the reanalysis of old data because of the lack of information on the stability of the instrument over time. [6] In the first part of the manuscript, we will present a novel methodology that does not use the zero-air-mass extrapolation to retrieve the parameters required for the aerosol retrieval in the UV. Instead, the methodology relies on accurate calibration records by use of laboratory calibrations using stable reference lamps traceable to primary standards held at national standards laboratories. [7] In the second part, a methodology is described that retrieves the aerosol optical depth in the visible part of the solar spectrum using measurements from the same Brewer spectrophotometer used with a different grating order. This wavelength range is affected by NO 2, but because of its low amount in the atmosphere compared to ozone, zero-air-mass extrapolation is used and validated with colocated measurements from a CIMEL spectrophotometer used within the Aerosol Robotic Network (AERONET). [8] The retrieved aerosol optical depth in the UV and visible wavelength range at Ispra, Italy, from 1991 to 2002, forms one of the longest records of aerosol optical depth in the world. 2. Instrumentation [9] The instrument used in this study is a commercially available Brewer spectrophotometer MKIV #66. It has a grating of 1200 lines per millimeter, which is used in the third order. By rotating a filter placed in front of the photomultiplier, the instrument is also able to use the grating in a second-order configuration and so be sensitive to radiation around 440 nm. The instrument is located at the Table 1. Neutral Density Filter Attenuations Used During the Direct Irradiance (Ozone) Measurements of Brewer #66 a Exit Slit ND Filter b a Filter ND 1.5 was replaced in July 1993 with a new filter (denoted as ND 1.5b in this table). Joint Research Centre of the European Commission in Ispra, Italy. The instrument was installed in the summer of 1991 and has measured total column ozone, NO 2, and spectral global UV irradiance since then [Cappellani and Koechler, 1999; Casale et al., 2000]. Between 30 and 60 ozone measurements per day are taken from sunrise to sunset, depending on the season. The total column ozone calibration is traceable to the Brewer reference triad based in Toronto, Canada, via a traveling Brewer spectrophotometer. These visits have been performed with an annual or biannual schedule. [10] The average number of NO 2 observations per day were 7 for the period 1991 to 1999, while a more frequent monitoring was started in 2000 with between 10 and 30 measurements per day. The NO 2 calibration of Brewer #66 took place at the factory in 1991, and no additional calibration was performed in later years Measurement Procedure [11] The standard ozone measuring procedure of the Brewer spectrophotometer records raw photon counts of the photomultiplier at the five nominal wavelengths 306.3, 310.1, 313.5, 316.8, and nm using a blocking slit mask, which opens successively one of five exit slits. The five exit slits are scanned twice within 1.6 s, and this is repeated 20 times. The whole procedure is repeated 5 times for a total of about 3 min. Because of the rapid scanning of the individual wavelengths they can be considered as having been measured essentially simultaneously. Thus the five sets of measurements can also be used to assess the variability of the atmosphere during the measurement and to discriminate situations contaminated by clouds [Kerr, 2002]. The total column ozone is obtained from a judicious combination of measurements at 310.1, 313.5, 316.8, and nm weighted with a predefined set of constants chosen to minimize the influence of SO 2 and linearly varying absorption features such as from clouds or aerosols. [12] The measurement procedure for total NO 2 column is essentially the same as the one used for ozone measurements, with the slight difference of using a different filter combination so as to be sensitive to the wavelength range around 440 nm. The wavelengths sampled through the 5 exit slits are then 431.4, 437.3, 442.8, 448.1, and nm with a resolution of about 1 nm Neutral Density Filter Transmission [13] Three filters with nominal attenuations of (ND 0.5) 10 1 (ND 1), and (ND 1.5), are required during direct solar measurements to attenuate the solar beam to acceptable levels. The attenuations of these neutral density (ND) filters are listed in Table 1 and have been determined 2of12

3 weekly since 2000 using the internal lamp of the Brewer. Before that, they were measured once in 1991 by the manufacturer prior to the delivery of the instrument to the Ispra site. The stability of the ND filters over time is demonstrated by a comparison between these two measurement sets, which differ by less than 0.5%. Since the ND 1.5 filter was exchanged in July 1993, two different attenuations are used for each time period. The attenuation measured in the factory in 1991 is the one used before July 1993, and the ND measurements after 2000 are used for the second time period. The long-term stability of the ND 1.5 filter is inferred from its measured stability over the past 3 years as well as from the long-term stability of the two other ND filters. 3. Aerosol Optical Depth Retrieval From Brewer Direct Sun Measurements [14] First the direct irradiance measurements F are linearity corrected and temperature compensated and the filter attenuation is taken into account, before being converted to absolute irradiance units (see following section). Then, the derivation of the aerosol optical depth at a wavelength l is achieved using the following equation, which is derived from Beer s law: t a ¼ log ð F 0=F Þ t R m R P n i¼1 t im i m a where the wavelength l has been suppressed for clarity. In this equation, F 0 represents the solar irradiance that would be measured by the instrument outside the Earth s atmosphere (extraterrestrial constant), t R, t a, and t i represent the optical depth of Rayleigh scattering, aerosol and absorbing gases i, respectively, and m R, m a, and m i represent the relative slant paths through each species in the atmosphere. The optical depth of molecular scattering t R, normalized to the standard station pressure of 988 mbar, can be calculated using the formula given by Nicolet [1984], and the relative slant paths m R, m a, and m i using Kasten s approximation [Kasten and Young, 1989]. The optical depths t i of the absorbing gases are easily estimated when the corresponding concentrations and absorption coefficients are known. Thus the only variable to be determined in order to retrieve the AOD t a from equation (1) is the extraterrestrial constant F 0 at each measuring wavelength l. [15] In the following two sections, the methodologies for retrieving the AOD in the UV and visible wavelength range will be presented in more detail Retrieval of UV Aerosol Optical Depth [16] For the reasons discussed previously (atmospheric variability at a midlatitude site), we will use the solar spectrum measured during the ATLAS-3 SUSIM Space Shuttle mission [VanHoosier, 1996], which was subsequently validated by ground-based measurements at the observatory of Mauna Loa, Hawaii [Gröbner and Kerr, 2001]. The F 0 (l) are obtained from this extraterrestrial spectrum by convolving the 0.15-nm-resolution solar spectrum with the slit function of each exit slit of the Brewer spectrophotometer (full width at half maximum (FWHM) of approximately 0.6 nm). ð1þ Figure 1. Responsivity history of Brewer #66 at 320 nm from 1991 to The responsivities were determined by 50 W lamp measurements (crosses) prior to 2000 and by 1000 W calibrated lamps (circles) afterward. The traceability of all calibrations to the reference standards used since 2000 is achieved through an overlapping chain of 50 W lamp measurements. [17] The measured direct irradiances F(l), in absolute units of W m 2 nm 1, are obtained by using the history of responsivity calibrations of the global entrance port of Brewer #66. This methodology is described in the next section Direct Irradiance Calibration Procedure [18] The responsivity change over time of Brewer #66 at one selected wavelength is shown in Figure 1. The absolute calibration of the global entrance port was obtained after January 2000 through monthly laboratory measurements of 1000 W DXW-type lamps which are traceable to the primary standard held at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany. The expanded uncertainty of the calibration record in this period is 3.3% and was estimated according to Gröbner et al. [2002] and Bernhard and Seckmeyer [1999]. Prior to that, the calibrations were based on a rotating set of between two and four 50 W lamps. Fortunately, the 1000 W calibrations done after January 2000 can be traced back to 1991 through an overlapping chain of 50 W lamp calibrations. The relative uncertainty of 50 W lamp calibrations was investigated using a period where both 50 W and 1000 W lamp measurements were available (after January 2000). The results showed that the calibrations based on individual 50 W lamps vary differently for each lamp with a maximum peak-to-peak variability for some lamps of up to 1.7%. Similar results were also reported by Kjeldstad et al. [1997]. This uncertainty can be substantially reduced by using several 50 W lamps for each calibration, such as was done at Ispra in the period As can be seen in Figure 1, beginning in 1996, lamp calibrations were done in weekly intervals, while prior to 1996, calibrations were done more or less annually. For the period beginning in 1996, calibrations were usually performed with at least three 50 W lamps, resulting in a combined expanded uncertainty 3of12

4 of 3.8%, taking into account the uncertainty from variations between successive calibrations of up to 1.8%. Similarly, the overall uncertainty in the calibration record for the period prior to 1996 is estimated at 7.5% on the basis of the changes observed between successive calibrations. One should note that in this period, , at least one calibration per year was performed, which reduces the uncertainty close to these calibration times to 3.8%, as determined for the period [19] Three discrete changes in responsivity occur in the middle of 1993, in the spring of 1996, and in June The large drop in sensitivity after July 1993 is due to the use of a neutral density filter (ND 0.5) during lamp calibrations and for the global irradiance measurements. At the same time, the ND 1.5 neutral density filter was exchanged with a new one whose transmission was nearly twice that of the old one. The responsivity changes in 1996 and 2002 are related to the exchange of the N i S o4 filter mounted in front of the photomultiplier, which was degrading and needed replacement. [20] No other substantial changes have been made to the instrument, apart from these modifications. The remaining drift in sensitivity of about 10% between 1996 and late 1998 is likely to be due to a slow aging of the photomultiplier, which is also seen as a slow drift of the dead time of the photomultiplier during the same time period from, initially, 40 ns to 32 ns. After 1998 the instrument seems to have stabilized, and changes in the sensitivity are smaller than the variabilities seen during successive lamp calibrations Relative Sensitivity of Direct and Global Entrance Port [21] In the Brewer spectrophotometer, the global and direct entrance ports are selected using a rotating Quartz prism. Measurements of the direct irradiance are done with the Quartz prism pointing directly toward the Sun with only a tilted Quartz window between the prism and the solar radiation. On the other hand, the global irradiance is sampled through a flat Teflon diffuser covered by a Quartz dome. The main difference in terms of transmission properties of the two entrance ports is the Teflon diffuser, which absorbs and scatters the incoming radiation. [22] The calibration information obtained on the global entrance port was transferred to the direct port by a variant of the procedure described by Bais [1997] and Gröbner and Kerr [2001]. A measurement procedure is started during stable clear-sky weather conditions, where successive measurements of the direct, diffuse, and global solar irradiance are taken using a modified ozone measurement procedure. The diffuse solar irradiance is measured through the global entrance port using a shadow disk to block only the unscattered solar radiation (direct irradiance). Similarly, the global irradiance is measured by rotating the Brewer spectrophotometer by a few degrees in the azimuth so as to expose the Teflon diffuser to the direct and diffuse irradiance, while still blocking a small part of the circumsolar radiation. To first order, this methodology should therefore not be sensitive to the circumsolar radiation because only the difference between the global and diffuse irradiance is used in the subsequent analysis. [23] Some further information is required before the relationship between the direct and global entrance port Figure 2. Direct to global entrance port sensitivity ratios measured in 2001 (circles), 2002 (crosses), and 2003 (squares). Shown are the mean measurements from each year. The 5 exit slit positions in wavelength space are situated at 306.3, 310.1, 313.5, 316.8, and nm. The sensitivity of the direct entrance port is about 15 times higher than the sensitivity of the global entrance port because of Teflon, which is used as diffusing material. can be obtained: First, the directional response of the global entrance port is needed to determine the direct irradiance flux through a horizontal surface (global irradiance port) in relation to the direct irradiance flux through a surface normal to the direct solar beam; this information is obtained from measurements in the laboratory. A description of the setup is described by Gröbner [2003]. Second, the transmissions of the first three neutral density filters (ND 0.5, 1, 1.5) that are used during the ozone measurements need to be determined; the procedure was described in section 2.2. [24] The relationship between the direct and global entrance port was determined 7 times between 2001 and 2003 for different solar zenith angles (SZA) and turbidity conditions and is shown in Figure 2. The measurements through the five exit slits are within 1% of the mean for SZA between 22 and 56 and aerosol optical depths between 0.05 and The latter are interesting because they allow a quantitative estimate of the effect of the circumsolar radiation on this procedure. The measured data show that the effect of the circumsolar radiation is less than 1% and can be neglected for our purposes. While there is no plausible reason to expect any changes in this relationship over time, additional measurements will be performed in the future to continue investigating its stability over longer time periods Slit-to-Scan Correction [25] A further element needs to be taken into account before the lamp calibrations done on the global entrance optic can be applied to the direct entrance port. The lamp calibrations are made in the traditional wavelength scanning mode, which rotates the grating, and the radiation is sampled through the first exit slit. The ozone measurement, on the other hand, is measured with a fixed grating 4of12

5 and through the five exit slits. Differences between these two measurements arise mainly because the radiation is sampled on a different area of the photosensitive surface of the photomultiplier and because the resolution of each exit slit is slightly different. The small corrections that need to be applied to the exit slits are 0.96, 0.99, 0.94, and 0.94 for exit slits 2 to 5, respectively. Obviously, no correction is needed for slit 1, which is used in both measurements. [26] The absolute direct solar irradiance at the nominal ozone measuring wavelengths is obtained by combining all steps described previously. When they are combined with the lamp calibration history, the direct irradiance can be retrieved for the entire measurement period, and from that the aerosol optical depth can be calculated using equation (1) Retrieval of Visible Aerosol Optical Depth [27] As mentioned at the beginning of this manuscript, the Brewer spectrophotometer operating at Ispra also measures total atmospheric NO 2 column in the wavelength range nm. It is unquestionable that a homogeneous AOD data set measured by the same instrument in the UVB and in the visible wavelength range is very valuable to retrieve not only the AOD at each wavelength but also some information on the aerosol size distribution from the spectral distribution of the AOD. Furthermore, the added knowledge of the NO 2 content in the atmosphere allows an AOD retrieval in this wavelength range with less uncertainty than can be obtained from Sun photometric data alone. [28] However, a different calibration methodology was needed because no lamp calibrations are available in this wavelength range. We have therefore opted for a hybrid methodology, where we combined zero-air-mass extrapolations of Brewer measurements with a validation through the CIMEL Sun photometer in the period According to this method, the relative extraterrestrial constants (ETC) F 0 (l) were estimated using the uncalibrated direct irradiance measurements. [29] Candidate days for the zero-air-mass extrapolation were selected on the basis of the following criteria: The internal variability of five successive NO 2 measurements should be below 10%, the minimum air mass should be below 1.4, and the sampled air mass range should be above 1.3. These requirements were partly imposed by the data set itself, which had very few measurements per day in some of the years. Then, the retrieved AOD was used to reject the day if the AOD was above 0.11 and if it showed too much variability during the day. Finally, the yearly mean relative ETCs were determined for each year. Figure 3 shows the selected ETCs using the above criteria and the yearly mean ETCs used for the AOD calculation. In 1991 and 1996, no ETCs that satisfied the criteria could be found. Finally, the ETC for each day was determined by a linear interpolation between two successive years. This approach was justified because of the small interannual variation of the instrument. One should note that the replacement of the N i S o4 filter in 1993 and 1996 did not affect the NO 2 measurements because this filter is not used during NO 2 measurements. [30] After the estimation of the relative extraterrestrial constants F 0 (l), the AODs at the five wavelengths 431.4, Figure 3. Time series of the extraterrestrial constant F 0 at nm determined from zero air mass Langley extrapolations. The small circles represent all ETCs that satisfied the criteria outlined in the text. The yearly mean values (solid circles) are linearly interpolated to determine the relevant F 0 for each day. The sample standard deviation of each calculated yearly mean is also shown and varies between 0.5 and 2% , 442.8, 448.1, and nm were obtained from equation (1) considering NO 2 as an absorbing gas. The NO 2 absorption coefficients were taken from Vandaele et al. [1998]. 4. Cloud-Screening Algorithm [31] Because of the different data sets and the different retrieval methodologies, two slightly different cloudscreening algorithms were used for the UV and visible wavelength range UV Wavelength Range [32] To detect measurements that are contaminated by clouds, a two-stage process is applied to each group of five measurements, and a decision is made to accept the whole group or not. The first stage accepts a whole measurement set only if the standard deviation of the five ozone values retrieved from it is below 2.5 DU. This criterion effectively removes a large majority of cloud-contaminated conditions and insures that the ozone value needed for the subsequent retrieval of the AOD is valid. The second stage checks the variability of the AOD itself and accepts a measurement set only if the standard deviation of the five AOD values is below The value of 0.02 is an empirical choice and was a compromise so as not to reject too many measurements, especially at high turbidity. Incidentally, this value is equal to the 0.02 or 0.03 value used in the AERONET cloud-screening algorithm [Smirnov et al., 2000]. Further, measurements at air masses above 4 are also rejected. [33] The most reliable quality control of automated aerosol optical depth measurements remains operator-assisted data discrimination. This visual data inspection is important to remove disturbed measurements that were not detected during the automated procedure, especially measurements 5of12

6 Figure 4. Histogram of the AOD differences obtained from the automated and manual cloud-screening procedure for the year The bin width is From a total of 245 daily AOD values that passed the automated cloud screening, 34 were rejected by the subsequent manual procedure. There is a slight asymmetry toward lower daily mean values after the manual cloud-screening intervention, which is expected from removing cloud-contaminated AOD values from the daily mean value. disturbed by instrumental problems. To limit the subjective nature of this approach, additional ancillary data were included in the decision process. One-minute mean total irradiance data measured since 1995 by a colocated TSP- 700 total solar pyranometer from Yankee Environment Systems were found extremely helpful for that. For the years before 1995, the global irradiance measurements of the Brewer #66 itself were used. [34] A comparison between the performance of the automated and the manually assisted cloud-cleaning algorithm for all the data of 2000 is shown in Figure 4. The average difference between the two data sets is less than 0.01 and does not show any seasonal dependence. The manualscreening procedure rejected 34 days from a total of 245 days that passed the automated screening. From these, 19 have less than 5 valid measurements per day. Thus the automated procedure could be slightly improved by adding a criterion taking into account the fraction of accepted measurements per day relative to the total. [35] Differences between the automated and manual methods can reach daily mean differences of up to 0.15 even though the average difference is usually below One day in particular, 17 September, passed the automatic screening procedure even though two measurements around noon were clearly affected by an instrument error (unrealistically large differences between individual wavelengths and AOD above 2) Visible Wavelength Range [36] A set of five NO 2 measurements is accepted when the standard deviation of the calculated gas column is less than 0.4 DU and the slant path is less than 5 (SZA smaller than approximately 78 ). These restrictions were applied to the sets of direct Sun measurements used, and many distorted data were rejected. A set of direct irradiances taken at high solar zenith angles may present quite large variation, which results from the rapid changes of the zenith angle and not from the atmospheric variability. For this reason, linear fits between the observation time and the relative irradiance at each exit slit of every set were applied, and the coefficients of variation (CV) of the corresponding deviations were calculated. The coefficient of variation indicates the uniform atmospheric conditions during the measurement procedure (stable cloudy or noncloudy measurements but no distorted). Another restriction related to the CV value was imposed, and the final accepted sets should have CV less than at all wavelengths. The limit of is empirical and resulted from checking a large statistical sample. The aerosol optical depths derived from the accepted sets of the visible direct Sun observations were checked with the cloud-screened UV AODs and the CIMEL measurements. Examination of the AOD time series produced showed that after the application of the above methodology the cloud screening was successful with the exception of very few observations, which were removed manually. 5. Uncertainty Budget [37] The uncertainties associated with the AOD values retrieved in the UV and visible wavelength range from past Brewer ozone and NO 2 measurements are summarized in Table 2. The uncertainties of the AOD are calculated for an air mass of 1.5, which represents the average air mass of a summer day at Ispra. Thus the uncertainties are representative for the daily mean AOD in summer and slightly overestimate the uncertainties during the rest of the year. [38] The uncertainty contribution from total column ozone is limited to the UV wavelength range, while the one from NO 2 is restricted to the visible range. The uncertainty in the NO 2 optical depth is estimated at 50% because no NO 2 calibration was done since the installation of the instrument at Ispra in On the other hand, the ozone calibration was performed annually or biannually, and its uncertainty is 1% or less [Kerr et al., 1985]. The uncertainty contribution from each species to the AOD uncertainty depends then on the amount of each species in Table 2. Uncertainty Budget of the Aerosol Optical Depth in the UV and Visible Wavelength Range for an Air Mass of 1.5 a Uncertainty, AOD Uncertainty Item % nm nm UV calibration UV calibration UV calibration Direct to global transfer Total column O Total column NO UV ET constants Visible ET constants Total Total < Total < a The quoted uncertainties are expanded uncertainties with a coverage factor of 2. 6of12

7 Figure 5. Scatterplot of instantaneous CIMEL and Brewer AOD data for the years 2001 and The CIMEL data were measured with the 340 nm channel, which was installed in February 2001, while the Brewer data were measured at 320 nm. The data were selected to be within a 5 min window, and 5432 data points fulfilled the criterion. The correlation coefficient between the two data sets is the atmosphere multiplied by the absorption cross section of each species. [39] The uncertainty in AOD resulting from the calibration of the direct irradiance is a combination of a variety of factors discussed in the previous sections and reported below. For the UV range the uncertainties of the extraterrestrial constants based on the ATLAS solar spectrum were estimated by Gröbner and Kerr [2001]. The uncertainty of the absolute direct UV irradiance calibration is described in section 3.2. The uncertainty is split into a period before 1996 with higher uncertainties due to the few calibrations performed in this period, a second period after 1996 with a much higher calibration frequency, and finally, a third period after 2000 when 1000 W lamp calibrations have been used. The uncertainty introduced by the nonlinearity correction can be neglected since the uncertainty in the determination of the dead time is about 1 ns, which affects the corrected irradiances by less than 0.2%. [40] The uncertainty of the extraterrestrial constants used to retrieve the visible AOD is based on the variability and number of zero-air-mass extrapolations done each year. Uncertainties estimated for individual years range from 1% to 4% (k = 2), while the mean uncertainty for the measurement period would be 2.3%. The uncertainty of 4.0% reported in Table 2 is a conservative estimate valid for the whole measurement period from 1991 to [41] When all contributions are combined, the expanded uncertainty in AOD retrieval with the Brewer #66 is between 0.03 and in the UV wavelength range and 0.03 in the visible wavelength range. 6. Validation [42] The previous discussion has shown that several assumptions were required to retrieve the AOD from past ozone and NO 2 measurements. The main assumptions in the first case were the constancy of the relationship between the direct and global entrance port and the validity of the retrieved global irradiance calibration chain. In the second case the zero air mass extrapolations at a low-lying middle-latitude site might have introduced a systematic bias in the retrieved AOD. Even though the associated uncertainties were carefully estimated, a proper validation of the calculated AOD data using measurements from an independent instrument would enhance the credibility of the retrieved AOD data set. [43] In 1997 a CIMEL Sun photometer was installed at Ispra on the same measuring platform as Brewer #66. Since this Sun photometer is part of AERONET, the quality of the AOD products obtained from this instrument has been investigated with meticulous care. The uncertainty of the AOD measured by a Sun photometer from the AERONET network is estimated to be 0.01 at wavelengths longer than 440 nm and 0.02 at shorter wavelengths [Holben et al., 1998]. This implies good maintenance by the site operator and regular calibrations by comparing it with a reference instrument traceable to a Langley calibration at Mauna Loa in Hawaii. These prerequisites are fulfilled by the CIMEL installed at Ispra Validation of UV Aerosol Optical Depth [44] In February 2001 the CIMEL Sun photometer was calibrated, and a measurement channel at 340 nm was added, which is the one used in this validation exercise. [45] Figure 5 shows 5432 AOD measurements from Brewer #66 and the CIMEL Sun photometer for the years 2001 and 2002 (coincidence ±5 min). The sampled AOD ranges from values close to with most measurements between 0.05 and 1.3. Even though the measurement wavelengths are slightly different (320 nm for Brewer #66 and 340 nm for the CIMEL) the correlation of 0.99 demonstrates the consistency between the measurements of the two instruments. The differences between the two data sets are shown in the histogram in Figure 6. Histogram of the differences between the Brewer AOD at 320 nm and the CIMEL AOD measured at 340 nm. A normal distribution fitted to this data set has a mean of and a standard deviation of of12

8 Figure 7. Scatterplot of instantaneous CIMEL and Brewer AOD data for the years The CIMEL data were measured with the 440 nm channel, while the Brewer data shown here were measured at nm. The data were selected to be within a 5 min window, and 4967 data points fulfilled this criterion. The correlation coefficient between the two data sets is Figure 6. They are quite well represented by a normal distribution (random scatter) with a mean offset of and a half width of The slightly higher values of the Brewer relative to the CIMEL 340 nm channel can be explained by the wavelength difference between the CIMEL and Brewer #66. Also noteworthy is the quasitotal absence of outliers, which validates reciprocally the quality control procedures applied to each data set Validation of Visible Aerosol Optical Depth [46] The retrieved AOD values from the Brewer NO 2 measurements were compared to those obtained with the 440 nm channel of the CIMEL Sun photometer (FWHM 10 nm). Data were selected if the measurement times coincided to within 5 min. Since the AOD determined by AERONET is not NO 2 corrected, we have also used uncorrected AOD from the Brewer for the following validation. [47] A total of 4967 measurements in the period satisfied this condition. Figure 7 shows the AOD values of the Brewer at nm and the 440 nm channel of the CIMEL. Measurements at the other wavelengths give similar results, but because of the gradual difference in wavelength from the CIMEL channel, they are not shown. [48] The extremely high correlation of between the measurements of the CIMEL and those of the Brewer suggests that the methodology used for the AOD retrieval from the Brewer NO 2 measurements works very well. The differences between the two data sets are shown in the histogram in Figure 8. They are quite well represented by a normal distribution (random scatter) with a mean offset of 0.01 and a half width of Since the uncertainty of an AERONET field instrument is estimated to be between 0.01 and 0.02, the observed mean difference between the two instruments is not significant. The variability of 0.03 between the two data sets can be partly explained by the slightly different measurement times, which were selected to lie within a 5 min window, and by the different NO 2 absorption cross sections between the two wavelengths Measurement Precision [49] The precision of the AOD measurements of Brewer #66 is discussed using an 8-day period of low AOD from 5 to 12 September Figure 9 shows the AOD measurements of Brewer #66 and of the CIMEL Sun photometer at 320 nm and 340 nm, respectively (left panel), and at nm and 440 nm, respectively (right panel). The figure shows that the AOD variations during this period are tracked very well by both instruments. The AOD determined by the CIMEL Sun photometer at 440 nm is typically higher than the AOD from the Brewer by a constant offset of In the shorter-wavelength range, the differences are usually below 0.01 with a notable exception on the first day. Indeed, AOD measurements on the morning of 5 September show a larger difference than usual with the CIMEL measurements higher by about relative to the corresponding measurements from the Brewer. The reason for it is unexplained, but since the days before were rainy, some residual humidity inside the CIMEL Sun photometer optics might be the cause for its slightly higher values (fog on the entrance optics). [50] It is interesting to note that the amplitude of the diurnal variation of 0.05 of the AOD at 320 nm is at least twice the one observed simultaneously at 440 nm. The reason for this striking difference in behavior is for the moment unknown, and we have not found any reference to such an effect in the literature so far. A similar effect has been observed in Spain, but with a much higher diurnal variation than observed here (V. E. Cachorro Revilla, personal communiation, 2001 and 2004). [51] The reproducibility of both instruments is comparable, with a measurement reproducibility of for an AOD of As pointed out by Kerr [2002], Brewer direct irradiance measurements can reach a precision of 0.1%, Figure 8. Histogram of the differences between the Brewer AOD at nm and the CIMEL AOD measured at 440 nm. A normal distribution fitted to this data set has a mean of 0.01 and a standard deviation of of12

9 Figure 9. AOD measurements (left) at 320 nm and (right) at 440 nm between Brewer #66 and the CIMEL Sun photometer in an 8-day period from 5 to 12 September The circles and diamonds represent measurements of Brewer #66 and of the CIMEL Sun photometer, respectively. limited by the random Poisson noise of the raw photon counts. 7. Results and Discussion [52] The daily mean AOD at 320 nm measured at Ispra from July 1991 to the end of 2002 is shown in Figure 10. A four-month running mean of monthly AOD values has been added to the figure to illustrate the seasonal variability. The visible AOD measurements between 431 and 453 nm are not shown because they have very similar behavior with a few exceptions in the years 1996 and 1997, where the number of NO 2 measurements was very low. [53] The most striking feature that is apparent in the figure is the large spread of values between the lowest AOD values at around 0.05 in the final years to values above 2, which occur regularly during the whole measurement period. These high values at Ispra occur mostly in the spring and summer months and are due to haze in the boundary layer. The boundary layer height is typically around m and can be clearly observed when looking downward from the surrounding mountains. [54] The running mean that is superimposed on the daily AOD values shows for the most part a clear annual variation of the AOD with a minimum in the winter months and maximum in the spring-summer season. Thus the average AOD during the winter months is around 0.3, while the maximum in summer is around 0.6. [55] Surprisingly, no clear maximum in AOD is observed in the summer of The reason for its absence is due to frequent days with very low AOD during this summer. Usually, the average number of days with daily mean AOD values below 0.2 (at 320 nm) was around 14 in the years , while more than twice as many days with such low AOD values can be found in It is likely that this is due to a more than average number of Föhn events, which produced these many days with very clear atmospheres. Indeed, the number of Föhn days between the spring and autumn equinoxes in 2001 was 28, compared to an average of 14 in the years before. Section 7.3 contains a more detailed discussion of the effect of the Föhn on AOD measurements in Ispra Long-Term Trend [56] The long-term behavior of the AOD over Ispra can be grouped into two main periods; the first period which extended from 1991 to the end of 1996 was characterized by a slow decrease of the AOD over time. In the second period, which started in 1997 and still continues, the AOD seemed to have reached an equilibrium with no visible long-term variations. While the decrease can be seen in the monthly running mean curve shown in Figure 10, it is most obvious in the slow decrease of the background Figure 10. Daily mean aerosol optical depth measurements at 320 nm from Brewer #66 at Ispra, Italy. A fourmonth running mean calculated from monthly mean AOD values is overlaid on the individual daily mean values (circles). 9of12

10 Table 3. Mean Monthly AOD and Its Associated Standard Deviation at 320 and nm Calculated From the Daily Mean Values for Every Month of the Period nm nm Month AOD s.d. AOD s.d. January February March April May June July August September October November December AOD values from 1991 to 1997 and the stabilization afterward. Thus minimum values were around 0.2 in 1991 and 1992, which decreased over time to stabilize at a background level of about 0.07 after 1997, which has lasted till now. The reason for these initial high AOD values was the stratospheric aerosols present in the stratosphere because of the Pinatubo eruption in June This is confirmed from a similar study performed on the AOD data set from Davos, Switzerland [Ingold et al., 2001]. In that study, it was found that the maximal aerosol loading at 500 nm occurred about a year later with an enhanced AOD of about 0.2 above the background level, which is in excellent agreement with our measurements. The background AOD levels at Ispra reached pre-pinatubo values in 1997, as is discussed by World Meteorological Organization (WMO) [1999, 2003] Local and Regional Aerosol [57] The period after 1997 can be considered unperturbed and should therefore reflect predominantly the local and regional aerosol changes. The location of Ispra is interesting because of its position at the edge of the industrial Po Valley to the south and the main Alpine ridge to the north. One of its most characteristic features are the Föhn events, which produce very clear atmospheres and correspondingly very low AOD values below 0.07 even in the summer months, where the mean AOD is 0.6. The lowest recorded daily mean AOD values in Ispra of were found in November and December of [58] The monthly mean AOD values can be found in Table 3 for the period On a seasonal scale, aerosols are lowest in winter months with a mean AOD of 0.3. In spring and summer the mean AOD is around 0.5 (March, April) to 0.65 in July. A similar situation is seen at visible wavelengths, where the values are lower overall (see Table 3) Ångström Turbidity Parameters [59] An approximate characterization of the aerosol size distribution can be obtained from the spectral behavior of the AOD. Traditionally, the empirical formula first formulated by Ångström [1929], t = bl a, is used, where the wavelength exponent a is a measure for the aerosol size distribution while b is the aerosol extinction at a wavelength of 1 mm. In that formula, large values of a indicate a predominant contribution to the AOD from small particles relative to larger ones and vice versa. [60] Figure 11 shows the wavelength exponent a versus the AOD at 320 nm. The AOD at 320 nm is plotted on a logarithmic scale to illustrate the correlation that exists between the two parameters in Ispra. A smooth transition seems to exist between low-aod conditions with a predominance of small particles and high-aod conditions with a dominance of large particles. This smooth variation between these two situations also implies that essentially the same type of aerosol is present over Ispra. Indeed, there does not seem to be any significant change over time of the parameter a. There are 78 a values out of a total of 12,649 that are above 4, which implies an error in the AOD on which these values are based. This low fraction of less than 1% shows the good quality of the AOD retrieval. The mean value for a for the whole period is 1.61 with a standard deviation of Föhn Events [61] As mentioned previously, the Föhn events are characteristic situations encountered on the north and south sides of the main Alpine ridge. The meteorological conditions are low ambient humidity, higher than usual temperatures, middle to strong winds, and good visibility. This last point is of interest to us because it allows us to sample the background aerosol conditions over Ispra from 1991 to Figure 12 shows the AOD at 320 nm for all entire Föhn days on record. The figure shows again clearly the Pinatubo effect and the return to background AOD values from 1997 onward. [62] Figure 13 shows a selection of days in which the Föhn started during the day, and one day where it lasted the whole day. As can be seen in the figure, the onset of the Föhn coincides nearly exactly with a sharp decrease of the AOD, which reached its lowest values within a few hours. This rather fast decrease shown here is of the order of Figure 11. Scatterplot of Ångström turbidity parameter a versus the AOD at 320 nm from measurements of Brewer #66 for the period The determination of a was done using the available measurements from nm to 320 nm and from nm to 453 nm. 10 of 12

11 0.2 within 2 3 hours. The minimum values reached during these Föhn events represent background values that are representative for the Ispra site and are not dependent on the conditions encountered on any specific day. The slow buildup after each Föhn event usually takes around one full day and is often accompanied by clouds and bad weather conditions. [63] The wavelength exponent a of these background aerosols can be differentiated into a period influenced by Pinatubo and a post-pinatubo period after In the first period the mean a calculated only on Föhn days was 0.96 ± 0.3, while the second period can be characterized by an a of 1.3 ± These results are in agreement with values found in the literature, which also showed the predominance of larger particles during the Pinatubo-affected period [Ingold et al., 2001]. 8. Conclusion [64] The existing data set of direct irradiance measurements at Ispra, Italy, has been reanalyzed to retrieve the aerosol optical depth at 10 wavelengths in the UV and visible wavelength range. The retrieved data set spans the period July 1991 to the end of 2002 and shows the variability of the aerosols at this location. Measurements since 1997 have been compared to a colocated CIMEL Sun photometer. Differences in the retrieved aerosol optical depth values from both instruments are mostly below 0.03, which is in agreement with the estimated uncertainties of the retrieval methodology. [65] The first years of measurements are influenced by the aerosols ejected during the Pinatubo eruption in June 1991, which produced more than twice as much background aerosol loading as is currently observed. Pre-Pinatubo background levels were reached in 1997 with background AOD values around 0.07 (at 320 nm). Other studies have found a return to pre-pinatubo levels in 1995 or 1996, which is slightly earlier than found in this study [see Barnes and Hofmann, 2001; Ingold et al., 2001; and references therein]. However, if the wavelength at which Figure 13. AOD measurements at 320 nm versus time for a selection of 4 Föhn days. On 30 October 1996 the Föhn lasted from 0300 to 1800 UT (squares), while the other 3 days show the evolution of the AOD before and after the onset of the Föhn. On 6 March 1995 (triangles) and 19 March 1997 (diamonds) the Föhn started at around 1100 UT, while on 16 September 2001 it started around 1400 UT (circles). the AODs were measured is taken into account, the results of the previous studies and this one are consistent. Indeed, the washing out of the Pinatubo aerosols from the stratosphere was found to be faster for larger particles [Kent and Hansen, 1998; Ingold et al., 2001]. Thus AOD measurements at longer wavelengths will show a faster return to background levels than those at shorter wavelengths, as is the case here. [66] The overall day-to-day variability of the AOD at Ispra, Italy, was found to be very high, with minima around and maxima around 2. The minimum values occur usually during Föhn conditions, while high values are characteristic of very hazy conditions with high humidity. [67] This study demonstrates that direct irradiance measurements of Brewer spectrophotometers can be used to retrieve AOD values for long-term climatological studies. [68] Acknowledgments. This study would not have been possible without the long-term commitment of Franco Cappellani and Claudio Koechler. Their calibrations and careful maintenance of Brewer #66 were an indispensable element for this study. Thanks are also due to Sergio Sartori from MeteoSvizzera for a list of Föhn days during Finally, we would like to mention the constructive comments from two anonymous reviewers. Figure 12. Daily mean AOD measurements of Brewer #66 at 320 nm for full day Föhn situations. The outlier on 16 May 1997 is unexplained. References Ångström, A. K. (1929), On the atmospheric transmission of the Sun radiation and on the dust in the air, Geogr. Ann., 11, Bais, A. F. (1997), Absolute spectral measurements of direct solar irradiance with a Brewer spectrophotometer, Appl. Opt., 36, Barnes, J. E., and D. J. Hofmann (2001), Variability in the stratospheric background aerosol over Mauna Loa Observatory, Geophys. Res. Lett., 28, Bernhard, G., and G. Seckmeyer (1999), Uncertainty of measurements of spectral solar UV irradiance, J. Geophys. Res., 104(D12), 14,321 14, of 12