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Lower Tropospheric Ozone Measurements by Light Aircraft Equipped with Chemiluminescent Sonde. McKendry, Ian G. 2011

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136 VOLUME 15J O U R N A L O F A T M O S P H E R I C A N D O C E A N I C T E C H N O L O G Y q 1998 American Meteorological Society Lower Tropospheric Ozone Measurements by Light Aircraft Equipped with Chemiluminescent Sonde I. G. MCKENDRY AND D. G. STEYN Atmospheric Science Programme, Department of Geography, University of British Columbia, Vancouver, British Columbia, Canada S. O’KANE Levelton Associates, Richmond, British Columbia, Canada P. ZAWAR-REZA Department of Geography, University of Canterbury, Christchurch, New Zealand D. HEUFF Department of Mathematics, University of Canterbury, Christchurch, New Zealand (Manuscript received 11 October 1996, in final form 30 May 1997) ABSTRACT Novel use of a commercial, battery-powered, chemiluminescent ozonesonde on a light aircraft is described. This fast-response instrument, originally designed for balloon deployment into the stratosphere, is light, inex- pensive, robust (reuseable), reliable, and accurate. Integration with other lightweight components (data logger, global positioning system, pressure, temperature, and humidity sensors) renders the system suitable for use in a light (rental) aircraft with no modification of the aircraft required. The system is well suited to routine reconnaissance and vertical profiling in regions of complex terrain, and with well-designed field studies, mass budget analyses are feasible. The application and validation of the system is described for the Lower Fraser Valley, British Columbia, a region of complex coastal terrain where photochemical smog is a significant problem in the summer months. 1. Introduction Full understanding of the air pollution meteorology of any region can only be based on consideration of the entire lower troposphere where the processes of trans- port and dispersion take place. This has been highlighted in intensive field campaigns in regions where photo- chemical air pollution is a problem. For example, studies in northeastern North America (e.g., Clarke and Ching 1983), the Swiss Alps (Neu et al. 1994), the Los Angeles Basin (McElroy and Smith 1993), and Vancouver, Brit- ish Columbia (McKendry et al. 1997), all describe en- vironments characterized by horizontal transport of pre- cursors and photochemical oxidants to remote/rural sites and vertical down-mixing of pollutants from residual or elevated layers. In such circumstances, data from stan- dard surface monitoring networks may misrepresent the Corresponding author address: Ian G. McKendry, Department of Geography, University of British Columbia, #217-1984 West Mall, Vancouver, BC V6T 1Z2, Canada. E-mail: ian@geog.ubc.ca true vertical and spatial distribution of pollutants in a region and, furthermore, are of limited use in developing and validating photochemical models used to develop abatement strategies. Simple methods of spatial survey and vertical profiling are therefore of utmost importance in defining the dimensions of regional photochemical pollution problems (perhaps as a prelude to surface net- work design) as well as providing data for both devel- opment of parameterizations of vertical mixing pro- cesses and model validation. This paper describes application of a commercial, battery-powered, chemiluminescent sensor for aircraft measurements of ozone (O3) in the lower troposphere. This fast-response instrument, originally designed for balloon deployment into the stratosphere, is light, cheap, robust (reuseable), reliable, and accurate. Data acquisition is by standard data logger and notebook computer. These qualities render the system suitable for use in a light (rental) aircraft with no modification of the aircraft required. Consequently, the system of- fers distinct advantages over other airborne systems that are often expensive and require large modified FEBRUARY 1998 137M c K E N D R Y E T A L . aircraft. Hence, it is particularly attractive for both rou- tine reconnaissance and intensive research campaigns. In addition to describing the instrumentation, its re- sponse characteristics, installation on the aircraft, and postflight corrections, validation of the method by comparison with surface monitors and tethered ozo- nesondes will be presented. Finally, application of the system will be demonstrated in the context of the Low- er Fraser Valley (hereafter referred to as LFV), British Columbia, a region of complex coastal terrain where photochemical smog is a significant problem in the summer months. 2. Methods a. Measurements of ozone in the lower troposphere Several techniques have been used to measure ozone concentrations above the earth’s surface. These include ground-based and airborne remote sensing using laser technology (Schiff et al. 1994), deployment of bulky commercial UV photometers or chemiluminescent sen- sors (with some modification to account for pressure effects) on dedicated research aircraft (Kondo et al. 1987; Beekman et al. 1995), and vertical profiling using lightweight electrochemical sensors supported by bal- loons (Pisano et al. 1997) and kites (Balsey et al. 1994). Ozone flux measurements have also been made from aircraft over a variety of ecosystems (e.g., Ritter et al. 1994) using chemiluminescent sensors based on the de- sign of Pearson and Stedman (1980). Low-cost, light- weight electrochemical sensors (e.g., the ECC, Brewer– Mast, and Atmospheric Instrumentation Research son- des) have been traditionally used for ozone soundings (Beekman et al. 1995). However, recently, a cheap, lightweight (0.8 kg) chemiluminescent instrument has become available for balloon deployment in the tro- posphere and stratosphere (Schurath et al. 1991; Güsten et al. 1992). It is this commercially available instrument that forms the basis of the airborne system described here. b. Ozone sensor Specifications of the GFAS (Gesellschaft Für Angle- wandte Systemtechnik) OS-B-2 ozonesonde are de- scribed in Schurath et al. (1991) and Güsten et al. (1992). The instrument is based on the principle of sur- face chemiluminescence due to the reaction of ozone with an organic dye (Coumarin 47). The organic dye is incorporated into the outer layer of a replaceable, com- mercially available chemiluminescent target. The inten- sity of chemiluminescence is measured by a side-win- dow photomultiplier tube and converted into an analog output signal. A crucial component of the ozonesonde is a miniaturized fan that ensures sufficient flow rate to maintain the instrument in a regime, whereby chemi- luminescent intensity is independent of flow rate and is proportional only to the absolute concentration of ozone in the sample (the ‘‘flow-independent’’ regime). The instrument has a specified accuracy of 65% and shows good agreement with Brewer–Mast electrochemical sondes in profile intercomparisons extending into the stratosphere (Speuser et al. 1989). Before deployment of the ozonesonde, a ‘‘preozonization’’ procedure en- sures activation of the target and the maximum sensi- tivity of the instrument. During this procedure, the ozo- nesonde is exposed to concentrations of 100–150 ppb of ozone for 1–1.5 h from a custom-designed GFAS ‘‘control unit’’ (ozone generator). Once preozonization is completed, the instrument is calibrated (this requires the availability of a secondary standard ozone monitor) and the sonde is then ready for use. The lifetime of the chemiluminescent targets is approximately 6 months in storage and 2000 ppb h once activated. The instrument itself is extremely robust and therefore, provided that targets are replaced periodically, it may be used re- peatedly. c. Preflight calibration and postflight corrections After preozonization the OS-B-2 is calibrated by mea- suring the output voltage at a known concentration (pro- duced by the ozone generator) and measured by refer- ence monitor. Once calibrated, the sensitivity of the OS-B-2 is de- pendent upon temperature, pressure, and humidity (Schurath et al. 1991). With respect to temperature, the OS-B-2 is equipped with a heater that maintains the temperature of the chemiluminescent target at either 08C (preferable for profiles through the troposphere and stratosphere) or 308C (preferable for boundary layer measurements). Provided that the instrument is cali- brated with heater at the 308C setting, no temperature corrections are required during flights within the lower troposphere. Decreasing ambient pressure will lead to higher sensitivity values for the instrument. The cor- rection factor Fp for the pressure dependence of the sensitivity is given by pmeasF 5 1 2 0.63 log ,p 1 2pcal where pcal is the ambient air pressure during calibration and pmeas is the ambient pressure during subsequent mea- surements. For measurements within the planetary boundary layer (PBL), Fp may be of order 0.97, which represents only a 3% change in sensitivity from cali- bration (usually conducted at the surface). Consequent- ly, it too can be neglected. Finally, the sensitivity of the OS-B-2 changes with gross changes in absolute humid- ity as found between the troposphere and stratosphere. However, for measurements in the PBL this effect is also negligible. In summary, when deployed in the PBL, corrections to the OS-B-2 signal for pressure, temper- ature, and humidity effects are small and can be ne- 138 VOLUME 15J O U R N A L O F A T M O S P H E R I C A N D O C E A N I C T E C H N O L O G Y TABLE 1. Instruments required, their weight (kg), and approximate cost ($CDN). Note that costs are approximate only; an ozone monitor is required for calibration procedures, and temperature and humidity sensors may be added to the system. Instrument Weight (kg) Cost (approx, $CDN) OS-B-2 Ozonesonde—GFAS VM-K-2 control unit (ozone generator)—GFAS Intellisensor II pressure sensor—Model AIR-AB-2A CR10X Data Logger Power supply—12-V gel cell GPS—e.g., Trimble SveeSix Series (including magnetic mount antenna, baud rate converter, interface cable) 0.8 6.0 0.3 0.8 8.0 0.4 2000 5000 500 2000 80 3000 glected. Outside the PBL these effects become signifi- cant and must be corrected for. d. Airborne system configuration In adapting the GFAS OS-B-2 ozonesonde to deploy- ment in a light aircraft (in this case, a single-engine Cess- na 172), the following critical concerns were studied. 1) ENSURE AN UNCONTAMINATED SAMPLE To avoid mounting the system on the aircraft exterior (an advantage when using rental aircraft), air is fun- nelled into the instrument that is clamped to the rear of the cockpit via Teflon intake tubing extended through the open cockpit window. The intake is at- tached to the left-hand side of the cockpit window within the propwash and away from the aircraft exhaust on the right-hand underside of the aircraft. This ensures a well-mixed, uncontaminated sample. Comparisons with independent measurements during flight are de- scribed below and suggest negligible contamination of the sample. 2) MAINTAIN FLOW RATES IN THE FLOW INDEPENDENT REGIME In connecting the OS-B-2 to Teflon tubing, the effi- ciency of the miniaturized pump is severely compro- mised by the aerodynamic resistance of the tubing. However, during flight, air is forced through the tubing into the instrument. To ensure adequate, but not exces- sive, flow rates through the instrument, the intake tubing was arranged in a bypass mode that permitted the ozone- sonde to bleed off a portion of the flow into the aircraft. Wind tunnel tests with this configuration showed that the critical flow rate of 15 l min21, which is required to maintain the instrument in the flow-independent re- gime at lower tropospheric pressures was reached at aircraft speeds of 22 m s21, well below those reached during takeoff, vertical profiling maneuvers (35 m s21), and horizontal flight (50 m s21). 3) ENSURE SUFFICIENT RESOLUTION AT AIR SPEEDS OF APPROXIMATELY 50 M S21 With a specified time constant of 1 s for the OS-B- 2, the aircraft in horizontal flight can resolve fluctua- tions on wavelengths of approximately 100 m. This is deemed sufficient for the purposes discussed here (ba- sic aerial reconnaissance). For vertical profiling, where aircraft vertical velocity is of the order of 3 m s21 , fluctuations at wavelengths of approximately 6 m are resolvable. e. System components and costs In addition to the OS-B-2 ozonesonde, the system includes components for data acquisition, positioning, and monitoring of related meteorological variables. These components are listed in Table 1 with approx- imate costs. A high-resolution digital barometer is used for pressure measurements that may be used to correct ozone measurements (see above) as well as establish aircraft altitude. Air temperature is measured by therm- istor at both the air intake and within the cockpit. Air- craft location may be determined by reference to pre- established landmarks or by commercial global posi- tioning systems (GPSs). A GPS compatible with the Campbell Scientific CR10X data logger is listed in Ta- ble 1. All variables, including position data, are logged and stored at 3-s intervals on the data logger. A laptop computer may be used during flights to monitor all variables, although this is not necessary. With available memory in the CR10X (128K) and with a 3-s sampling rate, flights may extend for 2.5 h without downloading from data logger to computer. A 12-V gel cell battery is used to power both the data logger and OS-B-2. 3. Field validation During operations in the LFV, validation of aircraft measurements was based on two sources of independent data. a. Comparisons with surface monitors and electrochemical ozonesondes Figure 1 shows an ensemble of comparative mea- surements on occasions when the aircraft (at an altitude FEBRUARY 1998 139M c K E N D R Y E T A L . FIG. 1. Comparison of midboundary layer measurements with air- craft-mounted OS-B-2 and surface monitors directly below. Com- parisons with an Atmospheric Instrumentation Research electrochem- ical sonde flown on a tethered balloon at approximately the same time, height, and location as the aircraft are also shown. FIG. 2. Simultaneous, collocated afternoon boundary layer profiles from a tethered Atmospheric Instrumentation Research electrochem- ical sonde and the slowly spiraling aircraft. of ;500 m) flew directly over surface monitors or past a tethered ozonesonde (Atmospheric Instrumentation Research Inc.) at the same elevation. These observa- tions are derived from 5 days, reflecting the range of ozone concentrations typically encountered in the LFV. With the exception of the labelled early morning flights on 13 August 1994, all observations are associated with an afternoon convective boundary layer. For afternoon conditions, agreement between surface monitors and the airborne sensor is generally good (r 2 5 0.98). Air- craft concentrations are generally higher than surface concentrations due to the effects of surface deposition. This effect is most pronounced on the morning of 13 August when, at the surface, concentrations were very low (due to deposition overnight) compared to the rel- atively high concentrations persisting within the resid- ual mixed layer aloft overnight (McKendry et al. 1997). As expected, the agreement between observa- tions at the same elevation (electrochemical ozone- sonde on tethered balloon versus aircraft) is good, with no apparent bias. b. Simultaneous profiles A more rigorous test of the accuracy, and ability of the OS-B-2 to resolve ozone gradients during flight, is provided by simultaneous, collocated ozone profiles by tethersonde–ozonesonde and slowly spiraling aircraft. In the case shown in Fig. 2, the aircraft spiralled around the tethersonde during midafternoon in typical non- episode conditions. In this case, the mean difference between instruments was small and clearly demonstrates the ability of the airborne OS-B-2 to capture the fine- scale vertical ozone structure in the PBL (e.g., the in- crease in concentrations evident at 450 m). 4. Application in the Lower Fraser Valley a. Background The LFV of southwestern British Columbia (Fig. 3a) is one of three areas in Canada where the National Am- bient Air Quality Objective for ozone of 82 ppb (hourly) is frequently exceeded during the summer months. Such events are usually associated with stagnating anticy- clonic situations when local thermotopographic circu- lations such as sea/land and mountain/valley winds are well developed, and pollutants emanating from greater Vancouver (Fig. 3a) are generally transported eastward (McKendry 1994; Steyn and McKendry 1988). Recent studies show that during such episodes, pollutants are often transported beyond the existing surface monitor- ing network within the LFV (e.g., into tributary valleys) and may also become trapped in layers aloft where they may persist or become mixed to ground (McKendry et al. 1997). Understanding the vertical and horizontal dis- tribution of ozone is therefore essential to impact as- sessment, modeling, and mass budget analyses. The fol- lowing examples demonstrate how an airborne sensor may be used to complement standard surface-based measurements. 140 VOLUME 15J O U R N A L O F A T M O S P H E R I C A N D O C E A N I C T E C H N O L O G Y FIG. 3. Observations on 19 July 1995 in the LFV and tributary valleys. (a) Map showing flightpath and 1500 PST ozone isopleths (ppb) derived from surface observation network, (b) aircraft-derived ozone concentrations along the flight path, and (c) aircraft altitude during flight. b. Horizontal surveys and vertical profiling On 19 July 1995, ozone concentrations in the central LFV reached 72 ppb under warm, anticyclonic condi- tions. Moderate (4–5 m s21) west to southwesterly winds prevailed during the afternoon in the lower troposphere over the LFV, while southerly up-valley winds were evident in the tributary valleys to the north of the LFV. Such thermally driven flows are typical of ozone epi- sodes in the LFV and are described in detail in Steyn and McKendry (1988). In Fig. 3a, ozone isopleths at 1500 PST, based on the surface observing network, show the typically observed pattern of low concentra- tions over the western LFV and relatively high concen- trations downwind of the greater Vancouver source re- gion. In the period 1200–1500 PST, the aircraft flight path included three legs within the LFV and three legs in the complex, forested terrain to the north of the LFV. Leg BC was within the Pitt Valley, a valley known to suffer from degraded air quality due to the northward advection of photochemically active air from the LFV (McKendry et al. 1997). At the northern end of Pitt Lake, the aircraft turned back toward C and began an FEBRUARY 1998 141M c K E N D R Y E T A L . FIG. 4. Vertical profile at end of Harrison Lake in the vicinity of point D on 19 July 1995 during the flight shown in Fig. 4. ascent to approximately 2500 m (Fig. 3a). It then trav- eled northeast (leg CD) to the northern end of Harrison Lake, where it then descended back into the middle boundary layer and flew southward over Harrison Lake, making the first known observations of ozone in this locality (leg DE). Finally, the aircraft completed the sortie by traveling westward along the length of the LFV (legs EF and FA). The northward flight leg AB showed mean ozone con- centrations of 53 ppb across the LFV (Fig. 3b). Con- centrations measured by surface monitor at the entrance to Pitt Valley at this time (;1200 PST) were about 51 ppb, in good agreement with those observed by the air- craft. Within the Pitt Valley (leg BC), concentrations rose steadily to a peak of 79 ppb at the major bend in the lake and then declined again toward the northern end of the lake. This pattern has been observed in many flights into the Pitt Valley and suggests that the tributary valleys of the LFV may have the poorest air quality in the region. Possible mechanisms for the high concen- trations observed include the effects of reduced surface deposition over water and the role of local thermoto- pographic circulations (McKendry et al. 1997). As the aircraft climbed at the northern end of Pitt Lake, concentrations dropped from values of approxi- mately 50 ppb in the PBL to 35–40 ppb. At 2000 m, as the aircraft headed toward Harrison Lake on leg CD, concentrations abruptly increased to 67 ppb and re- mained in the range 65–70 ppb until immediately prior to the descent into Harrison Lake at D, when they again dropped to 40 ppb. The aircraft descent at D demonstrates the profiling capability of the system in complex terrain and also sheds light on the unusually high concentrations ob- served at an altitude of approximately 2500 on leg CD. The profile of ozone near D (Fig. 4) shows a distinct polluted layer from 2000–2250 m with concentrations twice those above and below. As observed in other stud- ies of elevated layers in the LFV (McKendry et al. 1997) and the Los Angeles Basin (McElroy and Smith 1993), this layer of ozone aloft was associated with an inver- sion. If it is assumed that the layer was horizontally widespread, a likely explanation for the variations in ozone concentrations observed on leg CD is that the aircraft initially flew within the elevated layer. As it approached D it then briefly climbed out of the layer. On descent at D, the aircraft then encountered the layer again, giving the profile shown in Fig. 4. Layers of ozone at this altitude have been previously observed over the LFV (McKendry et al. 1997), how- ever, the mechanisms responsible for their formation are not clear. Possible causes include the venting of pol- lutants from the LFV into the lower troposphere along heated slopes (the ‘‘chimney effect’’) and plumes from biomass burning. The latter have been observed to have high ozone concentrations and travel considerable dis- tances (Kirchhoff and Marinho 1994). A careful wind analysis and more comprehensive chemical measure- ments in such layers are required to determine their source. As the flight continued southward (DE) within the PBL over Harrison Lake (Fig. 3a), concentrations re- mained near background levels (35–40 ppb) until the aircraft encountered a frontlike discontinuity in which concentrations increased abruptly to 70 ppb in polluted air traveling northward past Long Island (Figs. 3a,b). Concentrations over the lake farther south exceeded 80 ppb and then dropped to 60 ppb near Agassiz. This is the first known observation of the apparent transport of polluted air emanating from the LFV up Harrison Lake, a distance of approximately 110 km from Greater Van- couver. As in the case of the Pitt Valley, concentrations over the water of Harrison Lake were generally higher than those observed in the LFV. On the return flight within the LFV (leg EF), observed concentrations in the PBL between Agassiz and Chil- liwack were mostly in the range 70–80 ppb. From Chil- liwack to the vicinity of Boundary Bay Airport (leg FG), concentrations decreased to the 60–70 ppb range. These values appear quite consistent with the observed ground-level concentrations at approximately the same time (Fig. 3a). This example demonstrates the extent to which air- craft can usefully extend observations into remote ter- rain beyond the surface monitoring network. In so do- ing, previously undocumented aspects of the regional distribution of pollutants and potentially important structures (such as elevated layers) may be identified. 142 VOLUME 15J O U R N A L O F A T M O S P H E R I C A N D O C E A N I C T E C H N O L O G Y c. Mass budget studies In addition to basic surveys of the horizontal and vertical distribution of ozone as described above, well- designed aircraft observations using the OS-B-2 system can provide the basis for more fundamental research such as mass budget studies. In the latter technique, a simple mass budget model in which the boundary layer is treated as a slab (Lenschow et al. 1981) can be used to investigate the relative importance of photochemical processes, advection, and vertical fluxes in contributing to observed concentrations. For any atmospheric con- stituent, S (kg m23) assuming horizontal homogeneity for the mean and turbulent flux terms, and negligible mean vertical velocity the budget is ]S ]S ]S9w9 5 Q 2 u 2 ,s]t ]x ]z terms 1 2 3 4 where u is the mean wind aligned along the x axis, z is the vertical coordinate, and Qs is the source or sink of S (Lenschow et al. 1981). Therefore, for ozone, the rate change in a given volume at a fixed location (term 1) is comprised of ozone production or destruction within the volume (term 2), the mean horizontal advection (term 3), and the vertical flux divergence (term 4). With the exception of term 2, and if necessary with estimates of entrainment and deposition velocities, all the terms can be measured using the aircraft system described above. Term 3 is easily calculated from ozone gradients over horizontal legs (e.g., FA in Fig. 3a) and using available wind data. With successive flights over the same flight leg, term 1 may also be estimated on the basis of the mean difference between successive time series. Finally, if the entire boundary layer is the volume considered, then flux divergence (term 4) is calculated on the basis of flux across the top of the mixed layer and deposition of ozone at the ground. Entrainment at the top of the boundary can be estimated on the basis of profile data derived from the aircraft (or tethered balloon), while deposition may be estimated on the basis of published or measured values. 5. Conclusions A lightweight, fast-response chemiluminescent sys- tem for continuous monitoring of ozone and other me- teorological variables from light aircraft has been de- scribed and tested in applications in the LFV, British Columbia. This system represents an efficient, afford- able, and versatile alternative to more commonly used, but expensive, research configurations. Although mounted in a light aircraft in this application, it could be used on a range of light platforms (microlight, hang glider, drone), thereby permitting observations in en- virons inaccessible to larger, fast research aircraft (e.g., close to ground or slopes, and in complex terrain). As shown in the LFV, the system is a viable alternative to other methods of ozone profiling and is also particularly useful for routine aerial surveys of ozone distribution and process studies (e.g., mass budget analyses). In the single case discussed here, the system provided valuable insights into the horizontal and vertical distribution of ozone in the complex terrain surrounding the LFV. Fi- nally, the system as described has the potential to be expanded to include other lightweight instruments. For example, Pisano et al. (1996) describe a lightweight NO2 sensor that could be integrated with the system. The addition of other chemical sensors would permit more rigorous analysis of mechanisms contributing to, and sources of, observed structures. Acknowledgments. We are grateful to Laurence Armi, whose experience with a portable aircraft data acqui- sition system and generous advice provided a focus for the present work. We are also grateful to Dr. Wilfried Hans (GFAS) for advice on adapting the sonde to air- craft applications. The Greater Vancouver Regional Dis- trict generously provided surface data and a monitor for calibration. Several students ably assisted in tethersonde operations. 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