DC-8 IN SITU OZONE AND FINE AEROSOL MEASUREMENTS

Dr. Gerald L. Gregory¹, Dr. Bruce E. Anderson¹, Dr. Melody A. Owens^{2, .} Mr. Wesley R. Cofer¹

The DC-8 in situ instrument package will include: an ozone detector employing the nitric oxide+ozone chemiluminescent detection principle; multiple (3) condensation nuclei counters (CN) with inlet heaters for detecting ultra-fine and fine aerosol and differentiating between refractory (e.g., soot and sea-salt) and volatile (sulfate) aerosols; and a pair of optical scattering spectrometer probes for determining the size distribution of particles over the 0.1 to 20 um diameter range. All have previously flown during GTE missions.

DC-8 In Situ Ozone Measurement.

The ozone detector employs the nitric oxide+ozone (NO+O₃) chemiluminescent principle. Samples are delivered to the sensor via a J-probe – forward-facing, window-mounted, and Teflon-lined. The NO+O₃ chemiluminescent detection principle is a reverse application of the standard technique [Clough and Thrust, 1967; Fontijn et al., 1970] used for detection of oxides of nitrogen and has been used by numerous investigators [Eastman and Stedman, 1977; Pearson and Stedman, 1980; Gregory et al., 1987]. Reaction of O₃ in the sample and nitric oxide (NO) supplied via gas cylinder produces an electronically excited state of NO₂ which emits (via chemiluminescence) light. The quantity of light detected by PMT circuitry is proportional to O₃ in the sample. The NO+O₃ chemiluminescent reaction is sufficiently fast (>10 Hz) and has been used in flux studies [Ritter et al., 1992 and 1994); however, the DC-8 instrument is configured for 2 Hz measurements. The instrument is calibrated by gas-phase titration, traceable to NIST nitric oxide gas standards. Corrections for water vapor quenching [Mathews et al., 1977] are applied, post-flight, using inflight data – pressure, temperature, and dew point temperature. Such corrections are at maximum ~10% for near-surface, warm, and moist (100% humidity) environments. Above about 3-km altitude corrections are negligible.

DC-8 In Situ Aerosol Measurements.

The CN counting system is comprised of three butanol-based condensation nuclei counters operated within a constant pressure and flow sampling system [Anderson et al., 1996 and 1997]. Samples are delivered to the sensors via a window-mounted, shrouded inlet and are diluted with filtered cabin air to prevent sensor saturation during encounters with air masses of high aerosol concentration. Each CN instrument is composed of a porous saturator tube partially immersed in an n-butyl alcohol reservoir, a thermoelectrically cooled condense column, a flow accelerating/focusing jet, a laser diode/photodiode detector pair and associated focusing and collecting optics, and signal processing and temperature controlling circuitry. In operation, sample air is drawn through the alcohol-saturated tube, then into the chilled column where alcohol condenses onto particles larger than a threshold size diameter dictated by the alcohol saturation ratio achieved within the condenser. The sample stream exits the condenser through a small orifice jet and passes through the focus of a 3-mW diode laser beam where activated particles scatter light into the collecting optics of the photodiode detector. Two different counters, the TSI models 3025 and 3760, are used in the system to provide size discrimination between fine and ultra-fine aerosols; the 3025 has a 50% lower cut-size of 4 nm whereas that of the 3760 is 20 nm. The sample for one of the 3760 counters will be preheated to 250 C prior to entering the counter for purposes of providing information on the volatile and non-volatile components of the fine aerosol.

The aerosol instrument package will also include a pair of Particle Measuring Systems’ aerosol spectrometer probes to determine aerosol number density as a function of size diameter over the 0.1 to 20 um size range. These probes have been modified by Droplet Measurement Technologies to include state of the art electronics which provide additional size channels (40 channels per probe) and a greater immunity to aircraft generated noise. The PCASP probe, which covers the 0.1 to 3 um size diameter range, has been used during previous NASA flight programs [Anderson et al., 1993 and 1996; Gregory et al., 1992]. The FSSP-300, which provides size discrimination for 0.5 to 20 um particles, is a relatively new sensor, but has been used successfully in several airborne campaigns. The PCASP has a resistive heater on the inlet which prevents ice formation during penetration of clouds and acts to dehydrate aerosol samples before measurement. Both probes size particles by measuring the amount of light scattered into collection optics during aerosol transport through a focused He-Ne laser beam. The FSSP-300 has an open sample cavity and thus avoids many of the problems associated with anisokinetic sampling which are particularly serious for super micron-sized aerosols. The PCASP has an enclosed sample cavity into which samples are drawn through a short (about 4 cm) length of capillary tubing. Calculations suggest that, because the probe is used to size small aerosols which are much less sensitive to inertial and gravitational effects, this sample extraction technique minimally disturbs the size and number density of the ambient aerosol. Both probes are designed for aircraft use and are installed in wing-tip instrumentation pods, accessing flow minimally perturbed by the aircraft presence. The probes are inherently fast response and can provide data at > 5 Hz rates. Laboratory calibrations of the probes using particles of similar size and composition as those anticipated to be sampled will be used to evaluate particle losses and instrument sensitivity to changes in pressure and temperature. A similar study has already been accomplished for the fine and ultra-fine aerosol sensors [Cofer et al., 1997].

Instrument Performance.

Ozone:

Dynamic Range: 0.8 to 1000 ppbv

Response: 2-Hz (based upon sample exchange rate at sea level; faster at

higher altitudes)

Accuracy: 3% or 2 ppbv

Precision: 1% or 0.8 ppbv

Sensitivity: Independent of pressure to 15-km altitude

Calibration: Multiple point (5 to 500 ppbv) by NO gas phase titration.

Data Rate: Recorded at 6 Hz

Data Reporting: 5-sec average (in the field); 2 Hz (final archive)

CN Aerosol Number Density

Dynamic Range: 1 count to 9999 counts/count period

Size Range: Fine aerosol (TSI 3760), 50% cut @ 17 nm diameter

Ultra-fine aerosol (TSI 3035), 50% cut @ 4 nm diameter

Heat: Fine aerosol CNC (TSI 3760)

CN 1 - sample unheated

CN 2 - sample heated to 250 C

Response: 0.5 sec integration

Accuracy: 20%

Precision: 5%

Data Rate: Recorded at 2 Hz

Data Reporting: 10-sec average (in the field); 1, 5, or 10 second averages (final archive).

Aerosol Size Distribution

Dynamic Range: 0 to 10⁴ particles per cm³ per size bin

Size Range: 0.1 to 3 um, 40 channels (PCASP); 0.5 to 20 um, 40 channels

(FSSP-300)

Response: 0.5 sec integration

Accuracy: 30% (estimated)

Precision: 10%

Data Rate: Recorded at 2 Hz

Data Reporting: 10-sec average size distributions along with total surface and

volume values; 1-sec average total number densities.

References.

Anderson, B. E., G.L. Gregory, J.D. Barrick, J.E. Collins, Jr., G.W. Sachse, M.C. Shipham, J.D. Bradshaw, and S.T. Sandholm, The impact of U.S. continental outflow of ozone and aerosol distributions over the western Atlantic, J. Geophys. Res., 98, 23,477-23,490,1993.

Anderson, B. E., W.B. Grant, G.L. Gregory, E.V. Browell, J.E. Collins, Jr., G.W. Sachse,D.R. Bagwell, C.H. Hudgins, D.R. Blake, and N.J Blake, Aerosols from biomass burningover the south tropical Atlantic region: Distributions and impacts, J. Geophys. Res., 101,24,117-24,138, 1996.

Anderson, B. E., W.R. Cofer, D.R. Bagwell, K.E. Brunke, J.W. Barrick, C.H. Hudgins, and G.D. Nowicki, Airborne observations of aircraft aerosol emissions I: Emission indicesfor volatile and nonvolatile particles, accepted by Geophys. Res. Lett., 1997.

Clough, P.N. and B.A. Thrush, Mechanism of chemiluminescent reaction between nitric oxideand ozone, Trans. Faraday Soc., 63, 915-925, 1967.

Cofer, W. R. III, B.E. Anderson, E.L. Winstead, and D.R. Bagwell, Calibration anddemonstration of a condensation nuclei counting system for airborne measurements ofaircraft exhausted particles, Atmos. Environ., 32, 169-177, 1997.

Eastman, J.A. and D.H. Stedman, A fast response sensor for ozone eddy-correlation fluxmeasurements, Atmos. Environ., 11, 1209-1211, 1977.

Fontijn, A., A.J. Sabadell, and R.J. Ronco, Homogenous chemiluminescent measurement ofnitric oxide with ozone, Anal. Chem., 42, 575-579, 1970.

Gregory, G.L., C.H. Hudgins, J. Ritter, and M. Lawrence, In situ ozone instrumentation for10-Hz measurements: Development and evaluation, Proceedings of Sixth Symposium on Meteorological Observations and Instrumentation, New Orleans, LA, Jan. 12-16, 1987,pp 136-139.

Gregory, G. L., B.E. Anderson, L.S . Warren, E.V. Browell, D.R. Bagwell, and C.H. Hudgins,Tropospheric ozone and aerosol observations: The Alaskan Arctic, J. Geophys. Res., 97,16,451-16,472, 1992.

Mathews, R.D., R.F. Sawyer, and R.W. Schefer, Interferences in chemiluminescent measurementof NO and NO₂ emissions from combustion systems, Environ. Sci. Technol., 11,1092-1096, 1977.

Ritter, J. A., J.D. Barrick, G.W. Sachse, G.L. Gregory, M.A. Woerner, C.E. Watson, G.F. Hill,J.E. Collins, Jr., Airborne flux measurements of trace species in an arctic boundary layer, J. Geophys. Res., 97, 16,601-16,626, 1992.

Ritter, J. A., J.D. Barrick, C.E. Watson, G.W. Sachse, G.L. Gregory, B.E. Anderson, M.A. Woerner, and J.E. Collins, Jr., Airborne boundary layer flux measurements of tracespecies over Canadian boreal forest and northen wetland regions, J. Geophys. Res., 99,1671-1686, 1994.

Pearson, R.W. and D.H. Stedman, Instrumentation for fast response ozone measurements fromaircraft, Atmos. Tech., 12, 1980.

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1. NASA Langley Research Center; Atmospheric Sciences Division; Mail Stop 483; Hampton, Virginia 23681-0001

2. Hampton University; Department of Physics; Hampton, Virginia 23688