Airborne Sun Photometer PLASMA: Concept, Measurements

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Airborne sun photometer PLASMA: concept, measurements, Open Access comparison of aerosol extinction vertical proﬁle with lidarClimate Climate of the Past 1,2 1 1 1 1 1 1 of the1 Past Y. Karol , D. Tanre´ , P. Goloub , C. Vervaerde , J. Y. Balois , L. Blarel , T. Podvin , A. Mortier , and Discussions A. Chaikovsky2 1 Open Access Laboratoire d’Optique Atmospherique´ (LOA), UMR8518, CNRS – Universite´ des Sciences et Technologies de Lille, Open Access 59655 Villeneuve d’Ascq, France Earth System 2B. I. Stepanov Institute of Physics National Academy of Sciences of Belarus, 68, NezavisimostiEarth av., System 220072 Minsk, Belarus Dynamics Dynamics Correspondence to: Y. Karol ([email protected]) Discussions

Received: 1 August 2012 – Published in Atmos. Meas. Tech. Discuss.: 19 September 2012 Open Access Revised: 5 August 2013 – Accepted: 5 August 2013 – Published: 16 September 2013 Geoscientific Geoscientific Open Access Instrumentation Instrumentation Abstract. A 15-channel airborne sun-tracking photometer measuring vertical profiles ofMethods aerosols on and the satellite track; Methods and has been developed. The instrument provides aerosol opti- however, elastic backscatterData lidars Systems have limitations since the Data Systems cal depths over a wide spectral range (0.34–2.25 µm) with an lidar equation cannot be solved without an additional con- Discussions Open Access Open Access accuracy (1AOD) of approximately 0.01. Taking measure- straint such as independent optical depth measurement. Geoscientific ments at different altitudes allow us to derive the aerosol ex- An airborne sun-trackingGeoscientific photometer named PLASMA tinction vertical profile. Thanks to the wide spectral range of (for Photometre` Leger´ Aeroport´ e´ pour la Surveillance des Model Development Model Development the instrument, information on the aerosol size distribution Masses d’Air) has been developed. Aerosol optical depths Discussions along the vertical is also available. (AOD) at several wavelengths are derived from measurements of the extinction of solar radiation by molecular and Open Access Open Access aerosol scattering and absorptionHydrology processes. and Aerosol size dis- Hydrology and tribution information can be retrieved from the AOD spectral 1 Introduction dependence when the spectralEarth range System is large enough (King Earth System et al., 1978). Naturally, flying atSciences different altitudes provides Sciences Atmospheric aerosols play a role in the earth radiative bud- the information along the vertical. get (see, for example, Hansen et al., 1997; Ramanathan Discussions Open Access

Deriving photometric AODs is quite obviousOpen Access as long as et al., 2001 or Kaufman et al., 2002). Due to their inter- the instrument is well calibrated. There is no analog to the re- action with solar and thermal radiation, aerosols first cool strictive satellite constraint of signal contamination by highly Ocean Science the atmosphere–surface system (aerosol direct effect) and Ocean Science reflective pixels and there is comparatively little dependence Discussions by absorbing sunlight in the atmosphere, they further cool on particle properties of the aerosols. Indeed sun photometric the surface but warm the atmosphere. They also modify the measurements such as those made by AERONET (Holben temperature and humidity profiles which creates a more sta- Open Access et al., 1998) are the principal means of validatingOpen Access satellite- ble temperature profile that results in less cumulus cloud based AOD retrievals (Remer et al., 2005; Kahn et al., 2010; cover (semi-direct effect) (Hansen et al., 1997; Koren et al., Solid Earth Tanre´ et al., 2011). With an airborneSolid versionEarth like PLASMA, 2004). Aerosols also impact the cloud properties by act- we can easily sample different locations within a few min- Discussions ing as cloud condensation nuclei and ice nuclei (indirect utes. It can also be used to validate extinction vertical pro- effects). To investigate aerosol–cloud interactions, it is im- files obtained from ground-based or space-borne lidars such portant to determine the 3-D distribution of aerosol proper- as CALIOP on CALIPSO (Winker et al., 2010).Open Access Open Access ties. There are several satellite sensors (imagers or scanners) Similar airborne sun photometers from last decades were that provide a 2-D distribution but the aerosol vertical repar- successfully developedThe (Matsumoto Cryosphere et al., 1987; Schmid The Cryosphere tition is not sampled. Satellite missions that include lidars et al., 2003; Asseng et al., 2004). Compared to AATS-14 Discussions such as CALIPSO (Winker et al., 2010) are useful tools for

Published by Copernicus Publications on behalf of the European Geosciences Union. 2 Y. Karol et al.: Airborne sunphotometer PLASMA

70 In this paper we first present the technical characteris- is disconnected when the aircraft is making a turn or due tics, the calibration procedure and a discussion on the recent 95 to the presence of clouds, then the GPS system takes over. evolutions of the instrument. Preliminary results of ground- The filters are located in two filter wheels which are rotated based, airborne and automobile measurements are discussed by a direct drive stepping motor. For a complete sequence, and a comparison with lidar profiles is finally provided. it takes 1.8 s for visible and near-infrared channels and 1.2 s for the middle infrared channels. For common aircraft speeds 100 ∼ 200 km/h, a complete filter sequence corresponds to a dis- 75 2 Description of the Instrument tance of 100 m and 66 m respectively. This instrument has two collimators both with approxi- The voltage of both detectors is digitized by mean of a delta-sigma ADC. The instrument is operated with a PC mately a 1.5◦ full angle of field of view (FOV) and a four- that records digital count for the visible and middle infrared quadrant detector with a 6◦ angle of FOV. The first detec- 105 channels, latitude, longitude and altitude, speed and other tor (Si) covers visible and near-infrared ranges (0.343 µm, 2 Y. Karol et al.: Airborne sunphotometer PLASMA flight parameters given by the GPS unit. 80 . µ , . µ , . µ , . µ , . µ , 0 380 m 0 441 m 0 499 m 0 553 m 0 677 m ◦ . µ , . µ , . µ ) and the second detector (In- The 1.5 full angle field of view of PLASMA is compa- 0 869 m 0 940 m 1 023 m ◦ 70 In this paper we first present the technical characteris- GaAs)is disconnected covers middle when infrared the aircraft (1.14 isµm making, 1.24 aµm turn, 1.60 orµ duem, rable to the 1.2 of the CIMEL instrument. This relatively tics, the calibration procedure and a discussion on the recent 95 1to.646 theµ presencem, 2.25 µ ofm). clouds, PLASMA then the interference GPS system filters takes are over. sim- small FOV ensures that the instrument is less sensitive to evolutions of the instrument. Preliminary results of ground- ilarThe to filters AERONET are located CIMEL in two sunphotometer filter wheels which filters are (Holben rotated 110 atmospheric scattering into the FOV than instruments with based, airborne and automobile measurements are discussed by a direct drive stepping motor. For a complete sequence,◦ broader FOV. The effect of atmospheric forward scattering 85 et al., 1998). The head can move in elevation (0–90 ) and and a comparison with lidar profiles is finally provided. it takes . for visible and near-infrared channels and . azimuth1 (0–3608 s ◦), and rotation in azimuth can be continuous1 2 s on direct solar irradiance measurements increases with par- for the middle infrared channels. For common aircraft speeds thanks to a ring power connector. Hereinafter, we will limit ticle size, aerosol optical depth and instrument FOV, it has 100 ∼ 200 km/h, a complete filter sequence corresponds to a dis- our study to the channels that are in atmospheric windows been shown that the impact is negligible (less than 1% of 75 2 Description of the Instrument tance of 100 m and 66 m respectively. ◦ (Fig. 2). 115 AOD) for FOV smaller than 2 (Russell et al., 2004). A more This instrument has two collimators both with approxi- The voltage of both detectors is digitized by mean of recent study (Sinyuk et al., 2012) confirms that in most cases a delta-sigma ADC. The instrument is operated with a PC mately a 1.5◦ full angle of field of view (FOV) and a four- for a CIMEL-like instrument, this effect can be neglected ex- that records digital count for the visible and middle infrared quadrant detector with a 6◦ angle of FOV. The first detec- cepted for heavy dust loadings where the relative error may 105 channels, latitude, longitude and altitude, speed and other tor (Si) covers visible and near-infrared ranges (0.343 µm, reach ∼ 1% for an AOD of 3.5. flight parameters given by the GPS unit. 80 . µ , . µ , . µ , . µ , . µ , 120 The InGaAs detector is temperature stabilized while the Si 0 380 m 0 441 m 0 499 m 0 553 m 0 677 m ◦ . µ , . µ , . µ ) and the second detector (In- The 1.5 full angle field of view of PLASMA is compa- detector is not. The 1.023 µm channel has been shown to be 0 869 m 0 940 m 1 023 m ◦ GaAs) covers middle infrared (1.14 µm, 1.24 µm, 1.60 µm, rable to the 1.2 of the CIMEL instrument. This relatively temperature sensitive. A correction derived from our labora- 1.646 µm, 2.25 µm). PLASMA interference filters are sim- small FOV ensures that the instrument is less sensitive to tory measurements is applied to the detector signal using a ilar to AERONET CIMEL sunphotometer filters (Holben 110 atmospheric scattering into the FOV than instruments with coefficient of 0.35%/◦C. ◦ broader FOV. The effect of atmospheric forward scattering 85 et al., 1998). The head can move in elevation (0–90 ) and azimuth (0–360◦), and rotation in azimuth can be continuous on direct solar irradiance measurements increases with particle size, aerosol optical depth and instrument FOV, it has thanks to a ring power connector. Hereinafter, we will limit 125 3 Theoretical Background our study to the channels that are in atmospheric windows been shown that the impact is negligible (less than 1% of Fig. 1. PLASMA scheme. ◦ 2384(Fig. 2). 115 AOD) Y. for Karol FOV et smaller al.: Airborne than 2 (Russell sun photometer et al., 2004). PLASMA A more AOD is derived from measurements of the atmospheric spec- recent study (Sinyuk et al., 2012) confirms that in most cases tral transmission. Due to Bouguer-Lambert-Beer law the sun for a CIMEL-like instrument, this effect can be neglected ex- Atmosphere PLASMA filters irradiance E(λ,z) at a given wavelength λ and at an alti- cepted for heavy dust loadings where the relative error may tude z above sea level is expressed as (Bohren and Huffman, reach100∼ 1% for an AOD of 3.5. 130 1998): 120 The InGaAs detector is temperature stabilized while the Si detector is not. The 1.023 µm channel has been shown to be −τ(λ,z)m E(λ,z) = tg(λ,z)E0(λ)e (1) temperature50 sensitive. A correction derived from our labora-

tory measurements is applied to the detector signal using a where E0(λ) is the extraterrestrial sun irradiance; tg(λ,z) coefficient of 0.35%/◦C. Transmission (%) the gaseous transmission; m the airmass given by 1/cos(θs) 0 500 1000 1500 2000 when refraction and atmospheric curvature are neglected; θs Wavelength (nm) 135 the solar zenith angle and τ(λ,z) the total optical depth of the Fig. 1. PLASMA scheme. 125 3 Theoretical Background atmosphere that is the sum of aerosol extinction and molecu- Fig. 1. PLASMA scheme. Fig. 2. PLASMA filter’s transmission. lar (Rayleigh) optical depths: Fig.AOD 2. isPLASMA derived filter’sfrom measurements transmission. of the atmospheric spec- (Ames Airborne Tracking Sun photometer) and FUBISS- tral transmission. Due to Bouguer-Lambert-Beer law the sun τ(λ,z) = τ a (λ,z)+τ m (λ,z) (2) ASA2 (Free University Berlin Integrated Spectrographic ext ext 90 andirradianceA 1.2microprocessor s forE( theλ,z middle-infrared) at computes a given wavelengththe channels. positionλ For ofand the common Sunat an based alti- air- Atmosphere PLASMA filters − System – Aureole and Sun Adapter), the spectral range is crafttude z speedsabove∼ sea200 level km ish expressed1, a complete as (Bohren filter sequence and Huffman, corre- 100 on time, latitude, and longitude, provided by a GPS system. The digital signal (DN(λ,z)) measured by the instrument similar. The main advantage of PLASMA is its small size sponds to a distance of 100 m and 66 m. 130 It1998): directs the sensor head to the Sun at which point the four- 140 is proportional to sun irradiance E(λ,z) (the relative Earth- and lightness. The weight of the optical head (mobile part) quadrantThe voltage detector of precisely both detectors tracks is the digitized Sun. If by the means tracking of Sun distance is taken to be imbedded in the extraterrestrial −τ(λ,z)m is 3.5 kg and the weight of the electronic modules is around aE delta-sigma(λ,z) = tg(λ,z ADC.)E0( Theλ)e instrument is operated with a PC(1) 4 kg. The50 optical head has been designed to be easily set up that records digital count for the visible and middle-infrared on any mobile platform like a small aircraft or an automobile. channels,where E0( latitude,λ) is the longitude extraterrestrial and altitude, sun irradiance; speed andtg( otherλ,z) It can be used for sampling, in a few minutes, aerosol plumes Transmission (%) flightthe gaseous parameters transmission; given bym thethe GPS airmass unit. given by 1/cos(θs) that are0 not horizontally uniform or for precisely retrieving ◦ 500 1000 1500 2000 whenThe refraction 1.5 full angleand atmospheric field of view curvature of PLASMA are neglected; is compa-θs ◦ aerosol vertical profile. Wavelength (nm) 135 rablethe solar to the zenith 1.2 angleof the and CIMELτ(λ,z) the instrument. total optical This depth relatively of the In this paper we first present the technical characteris- smallatmosphere FOV thatensures is the that sum the of instrument aerosol extinction is less and sensitive molecu- to tics, the calibration procedure and a discussion on the recent atmosphericlar (Rayleigh) scattering optical depths: into the FOV than instruments with evolutionsFig. 2. PLASMA of the filter’s instrument. transmission. Preliminary results of ground- broader FOV. The effect of atmospheric forward scattering a m based, airborne and automobile measurements are discussed onτ(λ,z direct) = solarτext(λ,z irradiance)+τext( measurementsλ,z) increases with par-(2) 90 andA a microprocessor comparison with computes lidar profiles the position is finally of provided. the Sun based ticle size, aerosol optical depth and instrument FOV; it has on time, latitude, and longitude, provided by a GPS system. beenThe shown digital that signal the (DN impact(λ,z is)) negligible measured by(less the than instrument 1 % of ◦ It directs the sensor head to the Sun at which point the four- 140 AOD)is proportional for FOV smaller to sun irradiance than 2 (RussellE(λ,z) et(the al., 2004 relative). A Earth- more 2quadrant Description detector of precisely the instrument tracks the Sun. If the tracking recentSun distance study (Sinyuk is taken et to al. be, 2012 imbedded) confirms in the that extraterrestrial in most cases for a CIMEL-like instrument, this effect can be neglected ex- This instrument has two collimators both with approx- cepted for heavy dust loadings where the relative error may imately a 1.5◦ full angle of field of view (FOV) reach ∼ 1 % for an AOD of 3.5. and a four-quadrant detector with a 6◦ angle of FOV. The InGaAs detector is temperature stabilized while the Si The first detector (Si) covers visible and near-infrared detector is not. The 1.023 µm channel has been shown to be ranges (0.343 µm, 0.380 µm, 0.441 µm, 0.499 µm, 0.553 µm, temperature sensitive. A correction derived from our labora- 0.677 µm, 0.869 µm, 0.940 µm, 1.023 µm) and the second de- tory measurements is applied to the detector signal using a tector (InGaAs) covers middle infrared (1.14 µm, 1.24 µm, coefficient of 0.35 % ◦C−1. 1.60 µm, 1.646 µm, 2.25 µm). PLASMA interference filters are similar to AERONET CIMEL sun photometer filters (Holben et al., 1998). The head can move in elevation (0– 3 Theoretical background 90◦) and azimuth (0–360◦), and rotation in azimuth can be continuous thanks to a ring power connector (Fig. 1). Here- AOD is derived from measurements of the atmospheric spec- inafter, we will limit our study to the channels that are in tral transmission. Due to Bouguer–Lambert–Beer law the sun atmospheric windows (Fig. 2). irradiance E(λ,z) at a given wavelength λ and at an altitude z A microprocessor computes the position of the Sun based above sea level is expressed as (Bohren and Huffman, 1998): on time, latitude, and longitude, provided by a GPS system. It directs the sensor head to the Sun at which point the four- −τ(λ,z)m quadrant detector precisely tracks the Sun. If the tracking E(λ,z) = tg(λ,z)E0(λ)e , (1) is disconnected when the aircraft is making a turn or due to the presence of clouds, then the GPS system takes over. where E0(λ) is the extraterrestrial sun irradiance; tg(λ,z) The filters are located in two filter wheels which are ro- the gaseous transmission; m the air mass given by 1/cos(θs) tated by a direct drive stepping motor. For a complete se- when refraction and atmospheric curvature are neglected; θs quence, it takes 1.8 s for visible and near-infrared channels the solar zenith angle and τ(λ,z) the total optical depth of the

Atmos. Meas. Tech., 6, 2383–2389, 2013 www.atmos-meas-tech.net/6/2383/2013/ Y. Karol et al.: Airborne sun photometer PLASMA 2385 atmosphere that is the sum of aerosol extinction and molecu- More advanced retrieval (King et al., 1978) using spectral lar (Rayleigh) optical depths: information have been developed and shown able to derive information on the aerosol size distribution n(r,z) by invert- a m τ(λ,z) = τext(λ,z) + τext(λ,z). (2) ing the following equation:

The digital signal (DN(λ,z)) measured by the instrument ∞ rmax is proportional to sun irradiance E(λ,z) (the relative Earth– Z Z 2πr τ m (λ,z) = dz0 πr2Q (m(λ), )n(r,z0)dr, (8) Sun distance is taken to be imbedded in the extraterrestrial ext ext λ DN value in order to simplify the nomenclature) one can z rmin write: where r is particle radius; n(r,z) particle’s size distribution;

−τ(λ,z)/cos(θs) m(λ) complex refractive index; Qext extinction efficiency DN(λ,z) = tg(λ,z)DN0(λ)e . (3) factor. Equation (8) assumes that the aerosol type does not From Eqs. (3) and (2) and after gaseous absorption correc- depend on the altitude. tion, we can obtain aerosol extinction optical depth: 1 4 Calibration and ground-based measurements τ a (λ,z) = [ln(DN(λ)/t (λ,z)) − ln(DN (λ))] ext m g 0 For PLASMA calibration we used Langley method and inter- −τ m (λ,z). (4) ext calibration with master sun photometer CIMEL CE-318 that To calculate gaseous absorption, i.e. absorption by oxy- is used in Aerosol Robotic Network (AERONET) (Holben gen, ozone, water vapor and other gases, we use spectral ab- et al., 1998). sorption lines provided in the 5S code and the mid-latitude Langley plots are made to determine the spectral extrater- summer atmospheric model (Tanre´ et al., 1990). restrial voltage. The site has to be located at high altitude and Molecular scattering optical depth at the altitude z is given in an area with a very stable aerosol regime. The Langley plot by is a log of the DN against the optical air mass during the day (from Eq. 3) for the optical air mass range between 5 and 2. m m P (z) The intercept is the calibration coefficient, and the slope the τext(λ,z) = τext(λ,z0) , (5) P (z0) optical thickness. The deviation of the intercept is a measure of the precision of the technique. If the aerosol loading is not m where τext(λ,z0) is the molecular optical depth at the surface constant, we may observe deviation from the linear regres- level z0 and P (z0) the associated pressure. sion line but such cases can be easily excluded at Mauna Loa In the UV spectral range, the Rayleigh AOD is signifi- and Izana˜ when Langley plots are performed. The Langley cant and has to be known with good accuracy to be prop- calibration is suitable for all spectral channels but the proce- erly corrected for. Since it depends on the atmospheric pres- dure is not straightforward since there are only a few sites sure, a pressure gauge has been included in the PLASMA that meet the requirements (Shaw, 1983). instrument. Unfortunately, the measurements were not avail- Intercalibration between instruments is easier and of an able at the time of our experiment and a crude estimate was accuracy comparable to Langley plots and can be done performed using the equation: P (z) = P (z0)exp(−z/8.5), with near-simultaneous measurements of sun irradiance by where z is the altitude of the aircraft expressed in km. PLASMA and CIMEL sun photometers at the same location. Since the aerosol extinction coefficient is the altitude Spectral extraterrestrial voltage can be found from the rela- derivative of AOD tion: dτ a PLASMA PLASMA a = ext DN0 (λ) DN (λ) σext , (6) = . dz CIMEL CIMEL (9) DN0 (λ) DN (λ) we can compare aerosol extinction coefficient derived by The accuracy of AOD depends on the accuracy of calibra- PLASMA and inferred from lidar backscatter profiles to val- tion coefficients. A 1 % error in calibration coefficient leads idate lidar retrieval procedure. to inaccuracy 1AOD ≈ 0.01 at m = 1, with the error de- In addition to the aerosol content, PLASMA can provide creasing by a factor of 1/m as air mass increases. Usually the ˚ information on the aerosol size through the Angstrom¨ expo- profile flights last around 30 min from 11:00 to 13:00 UTC; nent. Based on spectral AODs, angstrom exponent is defined during this period the change of air mass is less than 5 %. by Eq. (7) and is sensitive to the aerosol size (Schuster et al., Since a miscalibration results in a bias in AOD and the air 2006). mass is constant, it does not impact the value of the extinction coefficient that is the derivative of AOD (see Eqs. 4 and 6). λ −α The first calibration campaign was organized at the Atmo- τ(λ) = τ(λ0) (7) spheric Observatory in Izana˜ (28.3◦ N, 16.5◦ W; alt. 2391 m) λ0 www.atmos-meas-tech.net/6/2383/2013/ Atmos. Meas. Tech., 6, 2383–2389, 2013 4 Y. Karol et al.: Airborne sunphotometer PLASMA 2386 Y. Karol et al.: Airborne sun photometer PLASMA are present on both PLASMA and CIMEL instruments. in October 2009 for Langley approach and in Carpentras 343 nm 441 nm 667 nm 1024 nm 1646 nm ◦ ◦ Of course, PLASMA covers a larger spectral range with (44.1 N, 5.1 E; alt. 100 m) in March 2009 for intercalibra- 0.6 270 a 2.25 µm channel that is very important for AOD inver- tion. sion. Nevertheless at this stage we do not consider this wave- The calibration coefficients calculated with Izana˜ and Car- 0.5 length since we cannot validate calibration coefficient with pentras data in 2009 were applied to all data collected in ◦ ◦ any other instrument. Lille (50.6 N, 3.1 E; alt. 60 m) until March 2010 to obtain 0.4 aerosol optical depth (AOD). The comparison with CIMEL measurements at common wavelengths (0.34 µm, 0.38 µm, 0.3 AOD 5 Airborne Measurements 0.44 µm, 0.67 µm, 0.86 µm, 1.02 µm, 1.64 µm) showed a good agreement between the two instruments with with rms AOD 0.2 275 In addition to ground-based measurements, several flights differences being between 0.01 and 0.02. were performed in Lille: two technical flights in 2009, 6 After modifications of some mechanical elements, a new 0.1 flights in 2010 and 3 flights in 2011. We provide here- calibration campaign was organized at Izana˜ in March 2010. inafter results for flights performed after first improvements, Several measurements were then performed in LOA to sam- 0 i.e. since October 2010 only. ple different atmospheric conditions during 12 days in April– 7.00 9.00 11.00 13.00 15.00 17.00 280 Figure 4 shows profiles of 4 flights in 2010–2011: June 2010 and a better agreement with CIMEL master instru- UTC ment was obtained with rms AOD differences ∼ 0.01. The 12/10/2010, 28/09/2011, 29/09/2011 and 15/10/2011 from PLASMA calibration is not too far from the calibration of Fig. 3. Comparison of simultaneous ground-based measurements the top to the bottom. The first column of graphs on the AERONET reference instruments (0.002 < 1AOD < 0.009) Fig.of PLASMA 3. Comparison (colored of simultaneouslines) and CIMEL ground-based CE-318 measurements (black lines) left presents the vertical profile of AOD at different wave- (Eck et al., 1999). of18 PLASMA April 2011. (colored lines) and CIMEL CE-318 (black lines) lengths; the second is the profile of extinction coefficient; Once the technical development has been achieved, 18/04/2011. 285 the third column on the right presents the comparison of ex- PLASMA was refurbished in March 2011. The stepping az- tinction at 0.553 µm with lidar retrievals at 0.532 µm. We imuth and zenith motors were replaced for better pointing 5 Airborne measurements use the Cloud and Aerosol Micro-LIDAR (CAML) CE 3702 capability and signal processing steps taken to improve the Several measurements were then performed in LOA to sam- manufactured by CIMEL. The instrument has been already signal to noise ratio. We also replace all the filters, which re- pleIn differentaddition atmosphericto ground-based conditions measurements, during 12 several days in April–flights described in Pelon et al. (2008) and Leon´ et al. (2009). The quired a new calibration of the instrument. After calibration Junewere 2010 performed and a in better Lille: agreement two technical with CIMEL flights in master 2009, in- 6 290 aerosol extinction profile as well as the effective extinction- the difference of AOD retrieved by PLASMA and CIMEL strumentflights in was 2010 obtained and 3 flights with in rms 2011.AOD Wedifferences provide hereinafter∼ 0.01. to-backscatter ratio are retrieved using combination of lidar master instrument is less than 0.005 for all channels except240 Theresults PLASMA for flights calibration performed is not after too first far from improvements, the calibration i.e. data and sun-photometer AOD (Leon´ et al., 2009). 0.34 µm channel (see Fig. 3). ofsince AERONET October 2010 reference only. instruments (0.002 < ∆AOD < To get extinction profile we decided to remove all noisy The differences observed before 09:00 UTC and after 0.009Figure) (Eck4 shows et al., 1999).profiles of 4 flights in 2010–2011: 12 Oc- data due to the presence of clouds and then to average over 15:00 UTC may come from filter out-of-band leakage result- toberOnce 2010, the 28 technical September development 2011, 29 September has been 2011 achieved, and 295 10 measurements. It means that we assume that the state ing from incomplete blocking of solar energy from outside PLASMA15 October was 2011 refurbished from the in top March to the 2011. bottom. The The stepping first col- az- of the atmosphere was stable over 30–60 s, along 50–100 m the filter bandpass as seen in some AERONET CIMEL in-245 imuthumn of and graphs zenith on the motors left presents were replaced the vertical for profilebetter pointingof AOD in vertical direction and 2–4 km in horizontal direction. Al- struments. In the laboratory measurements of filters trans- capabilityat different and wavelengths; signal processing the second steps is taken the profile to improve of extinc- the titudes below 500 m could not be observed by lidar. Lidar mission, such leakage was not observed over ±100 nm from signaltion coefficient; to noise ratio. the We third also column replace on all the the right filters, presents which the re- profiles presented on figure 4 are extended to the ground level the central wavelength. Differences may also come from a quiredcomparison a new of calibration extinction of at the 0.553 instrument. µm with After lidar calibration retrievals 300 by using the correction function based on independent mea- miscalibration of this channel. Knowing the calibration co- theat 0.532 difference µm. We of useAOD the retrieved Cloud and by PLASMAAerosol Micro-LIDAR and CIMEL surements of AOD. Direct measurements of PLASMA can efficient within 1 % error only results in errors in the AOD250 master(CAML) instrument CE 3702is manufactured less than 0.005 by CIMEL. for all channels The instrument except be used for validation of this function. The extinction pro- of 0.025 for an air mass of 2.4 as anticipated around 07:30 0has.34 beenµm channel already (see described Fig. 3). in Pelon et al. (2008) and Leon´ files retrieved from lidar measurements are broadly consis- in Lille in April. This effect would lead to a symmetrical be- etThe al. ( differences2009). The observed aerosol extinction before 9UTC profile and as after well 15UTC as the tent with PLASMA results, showing coarsely similar verti- havior in the afternoon as observed in Fig. 3. mayeffective come extinction-to-backscatter from filter out-of-band ratio leakage are retrieved resulting using from 305 cal attributes. The best agreement between aerosol extinc- Since PLASMA has the same interference filters as incompletecombination blocking of lidar ofdata solar and energy sun-photometer from outside AOD the (Leon´ fil- tion coefficient profiles retrieved from PLASMA and lidar CIMEL, we follow the AERONET calibration policy and the255 teret al. bandpass, 2009). as seen in some AERONET CIMEL instru- data was observed 15 October 2011 when the atmosphere instrument is going to be calibrated every year. ments.To get In the extinction laboratory profile measurements we decided of to filters remove transmission all noisy was stable enough as seen from AOD profile. Currently, we analyze only results in spectral bands that suchdata leakagedue to the wasn’t presence observed of clouds over and±100 then nm tofrom average the over cen- For other days, differences might be explained by time and are present on both PLASMA and CIMEL instruments. tral10 measurements. wavelength. Differences It means that may we also assume come that from the a state miscal- of 310 space variability of aerosol and clouds. On 29 September Of course, PLASMA covers a larger spectral range with ibrationthe atmosphere of this channel. was stable Knowing over 30–60 the calibration s, along 50–100 coefficient m in 2011 clouds were present in the vicinity and the turbidity a 2.25 µm channel that is very important for AOD inver-260 withinvertical 1% direction error only and results 2–4 km in in errors horizontal in the direction. AOD of 0.025 Alti- was rather high and variable, between 0.15 - 0.20 at 0.553 µm sion. Nevertheless at this stage we do not consider this wave- fortudes an below airmass 500 of m 2.4 could as anticipated not be observed around by lidar. 7.30 Lidarin Lille pro- in around noon; as a result the data are noisier and a 50 % dif- length since we cannot validate calibration coefficient with April.files presented This effect on would Fig. 4 leadare extended to a symmetrical to the ground behavior level in ference of extinction coefficient at an altitude around 400 m any other instrument. theby afternoonusing the correction as observed function in Fig. based 3. on independent mea- 315 is observed. Discrepancies can also result from spatial vari- surementsSince PLASMA of AOD. Direct has the measurements same interference of PLASMA filters can as ability of the aerosol field; distance between both instruments 265 CIMELbe used we for follow validation the AERONET of this function. calibration The extinction policy and pro- the was around 10 km when the airplane was on the ground, and instrumentfiles retrieved is going from to lidar be calibrated measurements every are year. broadly consis- more than 50 km when the plane was flying at the altitude of tentCurrently, with PLASMA we analyze results, only showing results coarsely in spectral similar bands vertical that 3000 m. Moreover, the profiles take around 20 min and are

Atmos. Meas. Tech., 6, 2383–2389, 2013 www.atmos-meas-tech.net/6/2383/2013/ Y. KarolY. Karol et al.: et Airborne al.: Airborne sun sunphotometer photometer PLASMA PLASMA 5 2387

Fig. 4. PLASMAFigure AOD 3: (left), PLASMA aerosol AOD extinction (left), coefficient aerosol extinction (center) at coe7 channelsfficient and (center) aerosol at extinction 7 channels coefficient and aerosol compared extinction with lidar (right) coefficient compared with lidar (right) as a function of the altitude acquired near Lille region the 12th as a functionFig. 4. ofPLASMA the altitude AOD acquired (left), aerosol near Lille extinction region coefficient the 12 October (center) 2010, at 7 channels 28 September and aerosol 2011, extinction 29 September coefficient 2011 compared and 15 withOctober lidar 2011. (right)October as a function 2010, of the 28th altitude September, acquired near 29th Lille September region the 12/10/2010, and 15th 28/09/2011, October 2011. 29/09/2011 and 15/10/2011. attributes. The best agreement between aerosol extinction co- was rather high and variable, between 0.15–0.20 at 0.553 µm efficient profiles retrieved from PLASMA and lidar data was around noon; as a result the data are noisier and a 50 % dif- observed 15 October 2011 when the atmosphere was stable ference of extinction coefficient at an altitude around 400 m enough as seen from AOD profile. is observed. Discrepancies can also result from spatial vari- For other days, differences might be explained by time ability of the aerosol field; distance between both instruments and space variability of aerosol and clouds. On 29 Septem- was around 10 km when the airplane was on the ground, and ber 2011 clouds were present in the vicinity and the turbidity more than 50 km when the plane was flying at the altitude www.atmos-meas-tech.net/6/2383/2013/ Atmos. Meas. Tech., 6, 2383–2389, 2013 6 Y. Karol et al.: Airborne sunphotometer PLASMA

320 compared to the nearest in time lidar profile (∆t ≈ 15 min). 7 Conclusions Despite these differences, the aerosol layers are located at the same altitude and we can say that both vertical profiles The new sunphotometer that we have developed has been are coarsely consistent. fully described. Following the calibration procedure recom- 350 mended by AERONET, PLASMA provides AOD measurements with an accuracy of 0.005 < ∆AOD < 0.01 over a wide spectral range. 6 Automobile Measurements Its capability to follow the sun when it moves is very at- tractive. Installed on an aircraft, vertical profiles of AOD 325 In addition to airborne measurements it is also possible to 355 and aerosol extinction coefficient can be so derived and com- set up PLASMA on the roof of an automobile in order to pared to lidar retrievals. Overall, PLASMA is a user-friendly obtain horizontal transects of AOD. In mountain areas, we instrument, easy to install on moving platforms and perform- can also get vertical profiles using measurements performed ing well compared to other instruments. at2388 different altitudes. Y. Karol et al.: Airborne sun photometer PLASMA Acknowledgements. This study has been funded by CNES (Cen- ´ 360 7tre Conclusions National d’Etudes Spatiales), CNRS (Centre National de la Recherche Scientifique), the University of Lille, and Region´ Nord- ThePas-de-Calais. new sun photometer Y. Karol that is supported we have developed by a fellowship has been from fullyCNES described. and CNRS. Following PLASMA the calibration calibration was procedure performed recom- at Izana˜ observatory using AERONET-EUROPE calibration center, sup- mended by AERONET, PLASMA provides AOD measure- 365 ported by ACTRIS from European Union Seventh Framework ments with an accuracy of 0.005 < 1AOD < 0.01 over a Program (FP7/2007-2013) under grant agreement no. 262254. wideEmilio spectral Cuevas-Agullo range. from the Meteorological State Agency of SpainIts capability (AEMET) to is follow acknowledged the sun for when his ithelp moves during is thevery experi- at- tractive.ment inTenerife. Installed Theon an authors aircraft, are vertical very thankful profiles to of AERONET AOD and for 370 aerosolestablishing extinction and maintaining coefficient the can sites be usedso derived in this and work. compared to lidar retrievals. Overall, PLASMA is a user-friendly instrument, easy to install on moving platforms and performing well compared to other instruments.

Acknowledgements. This study has been funded by CNES (Centre ´ INSU National d’EtudesInstitut Spatiales), national des CNRS sciences (Centre de l'Univers National de la Recherche Scientifique), the University of Lille, and Region´ Nord-Pas-de-Calais. Y. Karol is supported by a fellowship from Fig. 5. Vertical profiles of AOD during automobile experiment CNESThe publication and CNRS. of PLASMA this article calibration is financed was by CNRS-INSU. performed at Izana˜ Fig.13 January 5. Vertical 2011 onprofiles Tenerife of island,AOD during Spain. automobile experiment observatory using AERONET-EUROPE calibration center, sup- 13/01/2011 on Tenerife island, Spain. ported by ACTRIS from European Union Seventh Framework ProgramReferences (FP7/2007-2013) under grant agreement no. 262254. Emilio Cuevas-Agullo from the Meteorological State Agency of of 3000 m. Moreover, the profiles take around 20 min and 375 SpainAsseng, (AEMET) H., Ruhtz, is acknowledgedT., and Fischer, for J.: his Sun help and during Aureole the Spectrome- experi- 330 In January 2011 an experiment was successfully carried are compared to the nearest in the time lidar profile (1t ≈ out on the island of Tenerife, Spain. The measurements mentter in for Tenerife. Airborne The Measurements authors are very to Derive thankful Aerosol to AERONET Optical Prop-for 15 min). Despite these differences, the aerosol layers are lo- establishingerties, Appl. and maintaining Opt., 43, 2146–2155, the sites used doi:10.1364/AO.43.002146, in this work. were made from sea level to an altitude of 2400 m in one cated at the same altitude and we can say that both vertical 2004. hour.profiles Horizontal are coarsely coverage consistent. area was about 30 km and we EditedBohren, by: C. O. and Dubovik Huffman, D.: Absorption and Scattering of Light by had the opportunity to compare the PLASMA AOD with the 380 Small Particles (Wiley science paperback series), Wiley-VCH, 335 Izana˜ AERONET site (alt. 2391 m) very close to the road- 1998. way. There are also two additional AERONET sites, La La- Eck, T. F., Holben, B. N., Reid, J. S., Dubovik, O., Smirnov, guna (alt. 590 m) and Santa Cruz (alt. 54 m), that were lo- A., O’Neill, N. T.,The Slutsker, publication I., and of Kinne,this article S.: Wavelength de- 6 Automobile measurements cated at a distance of 40 km from our measurements. pendence of the opticalis financed depth by of CNRS-INSU. biomass burning, urban, and The vertical profiles of AOD at 0.380 µm and 0.677 µm is 385 desert dust aerosols, J. Geophys. Res., 104, 31 333–31 349, doi: In addition to airborne measurements it is also possible to 10.1029/1999JD900923, 1999. 340 compared to the three AERONET stations in Fig. 5. 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