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Remote Sensing of Nitrogen Dioxide Profiles With the RIVM Mobile Lidar H.Volten, D.P.J. Swart, A.J.C. Berkhout, G.R. Van der Hoff, J.B. Bergwerff National Institute for Public Health and the Environment (RIVM), P.O. Box 1, 3720 BA Bilthoven, The Netherlands,
[email protected]
ABSTRACT At RIVM we built a mobile lidar system that is capable of measuring NO2 profiles in the troposphere, up to about 4 km, with high vertical resolution in the boundary layer. The purpose of the instrument is to provide tropospheric NO2 profiles for the interpretation and validation of satellite data. The instrument participated in the CINDI campaign, which ran from 8 June to 26 July 2009. During this campaign, we compared our lidar measurements with measurements of NO2 monitors with photolytic converters. The results show an excellent agreement between both instruments. Further, we obtained a great number of NO2 profile measurements to monitor the development of the NO2 profiles during the day.
During the DANDELIONS campaign in 2006 we observed with the mobile lidar that differences of 20% to 30% frequently occurred between air parcels about 2 km apart. Also, temporal variations of more than 50% occurred on the time scale of a few minutes (within an area of moving air of roughly 1 × 1 km). These variations are relevant for satellite retrieval; they occur well within a typical satellite pixel. For the CINDI campaign we adapted the lidar instrument to be able to measure NO2 profiles on a virtually continuous basis, so that small-scale temporal variations in the profiles may be monitored. This required significant adaptations to the lidar instrument, which are briefly described in this abstract. In addition we show preliminary results of measured NO2 profiles series.
1. INTRODUCTION
2.
Satellite instruments are efficient detectors of air pollutants such as NO2 . However, one of the major sources of uncertainty in satellite NO2 tropospheric column retrieval is the assumed vertical distribution of NO2 . At RIVM we built a mobile lidar system capable of the remote sensing of NO2 profiles in the troposphere, up to about 4 km at maximum, with high vertical resolution in the boundary layer [1]. The instrument provides tropospheric NO2 profiles for the interpretation and validation of satellite data. It participated successfully in the DANDELIONS campaign held in September 2006 [2]. This campaign was aimed at validation of space-borne measurements of NO2 by the OMI and SCIAMACHY instruments. During the DANDELIONS campaign, we used the NO2 concentration values of chemiluminescence monitors with molybdenum converters placed at ground level and at 200 m in an attempt to validate the NO2 lidar results. Unfortunately, these NO2 monitors were sensitive to NOy components and therefore overestimated the amount of NO2 in the atmosphere [e.g. 3]. For the CINDI campaign (Cabauw Intercomparison of Nitrogen Dioxide Instruments; see hhtp://www.knmi.nl/samenw/cindi) chemiluminescence monitors with photolytic converters [3] were used to validate the lidar results. These monitors are insensitive for interference by NOy . In this abstract we show a preliminary comparison between NO2 -monitor and lidar data.
The RIVM mobile lidar instrument is extensively described in [1]. Below we briefly describe the design of the instrument, including the main improvements made for the CINDI campaign, and we explain how the instrument is operated to obtain NO2 vertical profiles.
2.1.
EXPERIMENTAL METHOD
Design
The RIVM mobile lidar (light detection and ranging) system uses the DIAL (DIfferential Absorption Lidar) technique to measure trace gas concentrations in the atmosphere [4]. The lidar system described in this paper has a flexible design to make it suitable for air quality or climaterelated process studies and the validation of satellite instruments. For the latter application the lidar is designed to be able to measure vertical NO2 concentration profiles. The lidar instrument uses a pulsed Nd:YAG laser to pump a dye laser. Formerly, we used the dye Coumarin 2 (also known as Coumarin 450). Currently, a more stable laser dye (Exalite 411) is used, that lasts several days instead of several hours. We use a piezoelectric actuator to rapidly alternate between the two wavelengths required by the DIAL technique, in our case 413.5 nm and 414.1 nm, generating on-off pairs at 15 Hz. The former wavelength is absorbed more strongly by NO2 than the latter. The on and off signals are thus measured virtually simultaneously. We emit the laser beam to the atmosphere through a set of
© Proceedings of the 8th International Symposium on Tropospheric Profiling, ISBN 978-90-6960-233-2 Delft, The Netherlands, October 2009. Editors, A. Apituley, H.W.J. Russchenberg, W.A.A. Monna
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mirrors and prisms. The beam can be pointed at any direction: in the vertical plane from the horizon to the zenith, in the horizontal plane almost 360◦ around. The backscattered signal is detected by a telescope, with an interference filter, and a photomultiplier tube. Most of the daylight is blocked by the interference filter. A wavemeter measures the instrument-specific differential cross section during a DIAL measurement on a shot-byshot basis; if it changes, the wavelength is re-tuned. The entire system is housed in a custom built mobile laboratory, 8 m long, 2.5 m wide and 2.3 m high, mounted on a vehicle.
2.2. Measurement Method of NO2 Profiles The lidar instrument measures backscattered signals, Ion and Ioff , at the on and the off wavelengths. From these signals a DIAL curve is derived as follows [4]: dial = ln
Ion Ioff
(1)
The DIAL curve has a range resolution of 3.75 m. The slope of the DIAL curve is a measure for the NO2 concentrations in the atmosphere as follows: [N O2 ] =
−slope · 5 · 109 c
(2)
where c is the differential cross section, which in our case was 2745.9 cm2 /g. The slope of the DIAL curve changes as a function of range, depending on the way the NO2 gas is distributed over the atmosphere. To obtain data points, we select range intervals of the DIAL curve that have a constant slope, i.e., a constant NO2 concentration. Generally, one or two data points are obtained per DIAL curve. The noise on the DIAL curve and the length of the selected range intervals determine the precision of the concentration data points. In principle, a vertical concentration profile can be made by pointing at the zenith. However, the lidar only yields results when the laser beam is fully in the field of view of the telescope, which occurs in our system beyond a distance of 300 m. When measurements closer to the surface are required, the emitter and telescope are tilted under various angles and we build profiles from a number of data points, obtained from DIAL curves each measured at a different elevation angle. This also enhances the vertical resolution and increases the precision. During the DANDELIONS campaign a measurement sequence consisted of measurements at six elevations, in the following order: 0.75◦ , 3◦ , 12◦ , 24◦ , 6◦ , and 1.5◦ . The sequence of elevation angles was chosen to minimize the time needed for changes in elevation angle during the measurements (Fig. 1). A measurement at 90◦ was added afterward, because it was deemed irresponsible to move the live laser beam over such a long distance in the sequence. However, since the NO2 concentrations in the atmosphere may change on the time scale of minutes [1], for the CINDI campaign the 90◦ position was included in the sequence. Adaptations were made to the
Figure 1. An NO2 profile is obtained from a sequence of measurements at different elevations. Shown here are the elevations for the DANDELIONS campaign in 2006. A different, but similar sequence was used during the CINDI campaign. A sequence is repeated several times to obtain a profile.
instrument so that for safety reasons the laser beam is switched off during its move to and from the 90◦ position. Thus, a measurement sequence during the CINDI campaign consisted of measurements at eight elevations, in the following order 0.5◦ , 1◦ , 2◦ , 4◦ , 8◦ , 15◦ , 30◦ , and 90◦ . At each elevation a DIAL measurement was done consisting of 200 laser shots in 100 on-off pairs. A complete sequence took 57 s. A number of sequences may be averaged to increase the precision. Data points obtained from DIAL curve measurements at these elevation angles may overlap in altitude range. Elevations close to the horizontal yield NO2 concentrations at low altitudes but pertaining to a certain horizontal extent away from the instrument (for the slant measurements, typically between about 300 m and 2.5 km away from the instruments), whereas a zenith observation yields NO2 concentrations exactly above the instrument. We obtained profiles consisting of about six or seven values inside the planetary boundary layer, and one or two above this layer. A vertical profile provides data for altitudes ranging from a few meters, for small elevation angles, up to a few kilometers for an elevation angle of 90◦ (see Fig. 1). The vertical resolution of a profile varies, it depends on the projection of the elevation angle, but also on how the NO2 is distributed in the atmosphere. This results in a broad area of possible vertical resolution values, in particular at high altitudes. The choice for a certain vertical resolution is a trade-off with precision in NO2 concentration. This choice is made during the analysis. From the same data, higher resolution profiles can be obtained at the cost of precision. A similar reasoning holds for the temporal resolution.
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lidar 0.47º (15.6±5 m) 3.16º (105±35 m) 6.16º (205±67 m) in-situ monitors 3m 105 m 205 m
-3
Concentration (µg m )
30 25 20 15 10 5 0 04:00
05:00
06:00
07:00 08:00 Time (UTC)
09:00
10:00
11:00
Figure 2. Lidar NO2 concentrations compared to concentrations measured with NO2 monitors with photolytic converters. The in-situ monitors were placed in the tower at 3 m, 105 m and at 205 m. The lidar measured at 3 elevation angles, over a range of about 180 m corresponding to altitude intervals of 15.6 ± 5 m, 105 ± 35 m, and 205 ± 67 m. The measurements were performed on 16 July 2009. Local time was UTC + 2h, sunrise was at 3:40 UTC.
4
3. RESULTS 3.1. Validation of the Lidar System
3.2. NO2 Profiles on a Continuous Basis During the CINDI campaign the lidar instrument proved that the adaptations made to the instrument to be able to
1000 6 4
altitude (m)
To validate the lidar results using NO2 monitors with photolytic converters, these monitors were placed at 3 m, 105 m, and 205 m in the tower at Cabauw. Subsequently, the mobile lidar was stationed 1900 m away from the tower, and it directed its laser beam towards the tower where the monitors were located, so that both sampled the same air as much as possible. The lidar measured at 3 elevation angles, 0.47◦ , 3.16◦ , and 6.16◦ , over a range of about 180 m corresponding to altitude intervals of 15.6 ± 5 m, 105 ± 35 m, and 205 ± 67 m. Preliminary results of the comparison are presented in Fig. 2. In the early morning of 16 July 2009 the boundary layer top was low, and there were significant differences in NO2 concentrations around 15 m, 105 m, and 205 m. Later on the day the boundary layer top has risen and the lowest part of the atmosphere has become well mixed. During this period the concentrations reported by all instruments on all heights are similar, and the agreement between the in-situ measurements and the lidar measurements is excellent. Earlier in the day, the lidar yields slightly higher values than the NO2 monitors, in particular for the altitude of 100 m. Apart from some small calibration issues which may play a role for these preliminary data, the fact that the lidar measures over a height interval may cause a positive bias relative to the point measurements of the NO2 monitors. This may happen when the boundary layer top moves through the measurement interval of the lidar.
17-06-2009 RIVM mobile lidar 05:50:18 - 06:45:46 (UTC)
2
2
100 6 4 2
10 6 4
0
10 20 3 concentration NO2 (µg/m )
30
40
Figure 3. Lidar NO2 profile on a log-scale (red symbols) obtained on 17 June 2009. Horizontal bars indicate ±1σ values for the concentrations. Vertical bars indicate the height intervals over which concentrations have been determined
measure on an almost continuous basis were successful. A great number of NO2 profiles were obtained. An example of such a profile is shown in Fig. 3. The NO2 profile is rather block shaped, indicating a well mixed boundary layer, with high NO2 concentrations in the boundary layer and very low concentrations above [cf. 1]. We obtained NO2 profiles measurements over a time period of several hours on 19 measurement days (Fig. 4) The data presented here are preliminary. A reanalysis of the data is planned so that the measurements will yield NO2 profiles with a higher temporal resolution, and temporal variations may be studied in detail.
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Figure 4. NO2 profile measurements obtained during the CINDI campaign.
4. CONCLUSIONS
REFERENCES
The agreement between the in-situ measurements and the lidar measurements are excellent, in particular for well-mixed boundary layer situations. This implies that the NO2 lidar profiles may be used as the standard for satellite validation effort [5], but also as a reference for other methods to derive NO2 profiles, such as from MAX-DOAS measurements. During the CINDI campaign the lidar instrument proved that the adaptations made to the instrument to be able to measure on an almost continuous basis were successful. This yielded an unprecedented amount of information on the NO2 profile shapes and on temporal variations in the NO2 concentrations in the lower troposphere. The results will be very valuable for further studies of satellite validation.
[1] H. Volten, E. J. Brinksma, A. J. C. Berkhout, J. Hains, J. B. Bergwerff, G. R. Van der Hoff, A. Apituley, R. J. Dirksen, S. Calabretta-Jongen, and D. P. J. Swart NO2 Lidar Profile Measurements for Satellite Interpretation and Validation, J. Geophys. Res., in press, 10.1029/2009jd012441.
ACKNOWLEDGMENTS We would like to thank the organizers of the CINDI campaign, and the people who helped with the lidar measurements, in particular Lou Gast and Ar¨ıen Stolk. Also, we are indebted to Folkard Wittrock (IEP, Bremen, Germany) who kindly supplied us with data from the NO2 monitor with photolytic converter at ground level.
[2] Brinksma, E. J., et al. (2008), The 2005 and 2006 DANDELIONS NO2 and aerosol intercomparison campaigns, J. Geophys. Res., 113, D16S46, 10.1029/2007JD008808. [3] Steinbacher, M., C. Zellweger, B. Schwarzenbach, ˜ S. Bugmann, B. Buchmann, C. Ordo´ nez, A. S. H. Prevot, and C. Hueglin (2007), Nitrogen oxide measurements at rural sites in Switzerland: Bias of conventional measurement techniques, J. of Geophys. Res., 112(D11), D11,307, 10.1029/2006JD007971. [4] Measures, R. M. (1984), Laser remote sensing: Fundamentals and applications. [5] Hains, J., et al. (2009), Testing and Improving OMI DOMINO Tropospheric NO2 Using Observations from the DANDELIONS and INTEX-B Validation Campaigns, J. Geophys. Res., submitted.
