UNH Acidic Trace Gases and Aerosol Chemistry

Measurements for PEM-Tropics B

Robert W. Talbot (robert.talbot@unh.edu) and

Jack E. Dibb (jack.dibb@unh.edu)

Institute for the Study of Earth, Oceans and Space

University of New Hampshire

Durham, NH 03824

The UNH group will provide a selected suite of measurements for important acidic gases and aerosol species on both the DC-8 and P-3B aircraft. Specifically, on the DC-8 we will provide measurements of gas phase HNO₃, SO₂, and the aerosol major ions NO₃^-, non-sea-salt (nss) SO₄^2-, methyl sulfonate (MS), NH₄⁺, ²¹⁰Pb, and ⁷Be. The radionuclides are excellent tracers of continental (²¹⁰Pb) and stratospheric (⁷Be) air parcels. On the P-3B aircraft we will focus on HNO₃ and aerosol nss SO₄^2-, and MS. Here bulk and size-fractionated (using a virtual impactor) aerosols will be sampled independently. A size cut of 1m will be used to examine the distribution of sulfur species in the fine and coarse aerosol fractions.

Instrument Description

Both the trace gas and aerosol sampling systems fit into one double-bay high rack. The flow measurement and control assembly for the aerosol sampling fit on the top of the rack spanning the entire rack footprint. This assembly is coupled to a venturi pumping station situated aft of our sampling location. The only power requirements for the aerosol system is 50 watts of 120V/60Hz for the flowmeters, temperature, and pressure sensors.

To sample the atmospheric aerosol we use a forward facing tandem probe arrangement which is located in a 62 port on both aircraft. Both probes are identical, using an 8 mm curved leading edge orfice that is housed inside a shroud to ensure isoaxial sampling. The probes are operated isokinetically, by adjusting the flow as flight parameters change. We use pitot tubes mounted inside the shrouds to determine the air velocity through them. The air velocity is typically 10% less than that of the aircraft. On the P-3B aircraft an additional probe will be used to sample size-fractionated aerosols with a virtual impactor. Inside the aircraft aerosols are collected on Zefluor teflon filters. Collected samples are stored in ultra-clean polyethylene heat sealable bags purged with ultra zero air and frozen immediately to -30C after sampling.

The aerosol soluble ions are determined after aqueous extraction with subsequent ion chromatographic analysis. We use custom built ion chromatographs equipped with temperature controlled concentrator and separation columns coupled to suppressed conductivity detection. NIST standard solutions are used for calibration purposes. We have the option of doing the analysis in-flight or on the ground after the flight. Typically, we do in-flight analysis on transit flights and after-flight analysis on science missions conducted from a main base of operations. Activities of ⁷Be are measured back at UNH using gamma spectroscopy. The system consists of a Ge-well detector coupled to a 4000 multi-channel analyzer. The photopeak at 477.6 keV is used, with calibration accomplished by frequent counts of a blank filter spiked with a NIST traceable ⁷Be solution. Due to the short half-life of ⁷Be (53 days), samples are shipped from the field to UNH via express carriers. ²¹⁰Pb activities are determined by alpha spectrometric determination of ²¹⁰Po (a ²¹⁰Pb daughter). Filters are spiked with known amounts of ²⁰⁸Po (a yield tracer), and both Po isotopes are stripped from the aerosol filters by strong acid and subsequently plated onto Ag planchets for counting.

For sampling acidic gases a heated high flow glass-coated manifold is used to bring ambient air into our instrument where it is then subsampled using mist chambers. We maintain a nearly constant volumetric air flow through the manifold (1500 LPM, liters per minute) to provide a residence time in the inlet of about 0.2 seconds. A venturi pump is used as the vacuum source. The manifold is 50 mm ID, with the orfice housed inside a diffuser. The diffuser adds 50-150 mbar of pressurization to our manifold above ambient conditions. This is enough to overcome reverse venturi effects on the inlet, which are most severe above 8 km altitude. An added feature of it is a "ramp" insert which slips around the manifold pipe and effectively diverts aerosol larger than 1 - 2 m diameter away from the inlet entrance. This gives the effect of a reverse facing inlet minimizing sampling of aerosols in the gas manifold.

We use a custom built dual ion chromatograph analytical system equipped with temperature controlled concentrator and separation columns coupled to suppressed conductivity detection. NIST standard solutions are used for calibration purposes. Computer controlled syringe pumps are used to move ultra-pure water between the mist chambers and analytical systems. This provides essentially a closed liquid handling system which minimizes extraneous contamination of the mist chamber samples. Sampling times are 2-6 minutes for HNO₃ and 2 minutes for its analytical detection. Chromatographic data is collected at 5 Hz in a computer controlled environment. Atmospheric mixing ratio data for HNO₃ is available immediately after each flight. Data for SO₂and aerosol ions is available within 24 hours after each flight.

Our sampling manifold has ports built into it for performing standard additions of HNO₃down the entire length of the inlet. Two permeation tubes are housed in separate ovens heated to 60C. We use one high emission tube for polluted and stratospherically influenced air masses and a low emission source for normal remote tropospheric measurements. The oven output (25 cm³ min^-1) is diluted into a 1.5 LPM flow of ultra-zero air which is then diverted to pass through either: (1) a nylon filter, (2) 98% of the standard addition delivery line leading to our sampling manifold or, (3) 100% of the standard addition line and into our sampling manifold. The HNO₃ delivery lines are heated to 40C, and have separate temperature controllers for lines inside and outside the aircraft fuselage. Flow configuration ^#1 is used to verify the permeation output. We use flow configuration ^#2 above to equilibrate the delivery line without actually adding HNO₃ to the sampled air stream. The standard additions on ambient air (^#3) are performed at the 50-100% level above the ambient HNO₃ mixing ratio. It is crucial that the standard additions are conducted at this level rather than several ppbv to rigorously test the passing efficiency of the inlet.

The salient characteristics of the various UNH measurements can be summarized as follows:

HNO₃/SO₂

Time resolution: 2 - 6 minutes

Limit of Detection: 5 pptv HNO₃, 10 pptv SO₂

Overall Uncertainty: <20 pptv - 30-35%; 25-100 pptv - 20%; >100 pptv - 15%

Absolute accuracy is believed to be 10-20%.

Aerosol Species

Soluble Ions - time resolution (5-15 minutes); limit of detection (10-30 pptv); overall uncertainty ( 20-25%).

Radionuclides - time resolution (20 minutes), limit of detection (²¹⁰Pb - 0.1 fCi m^-3, ⁷Be - 75 fCi m^-3); overall uncertainty (10-20%).

References

Talbot, R. W., J. D. Bradshaw, S. T. Sandholm, S. Smyth, D. R. Blake, N. Blake, G. W. Sachse, J. Collins, B. G. Heikes, B. E. Anderson, G. L. Gregory, H. B. Singh, B. L. Lefer, and A. S. Bachmeier, Chemical characteristics of continental outflow over the tropical south Atlantic Ocean from Brazil and Africa, J. Geophys., 101, 24,187-24,202, 1996.

Talbot, R. W., E. M. Scheuer, B. L. Lefer, and W. T. Luke, Measurements of sulfur dioxide during GASIE with the mist chamber technique, J. Geophys. Res.,102, 16,273-16,278, 1997.

Talbot, R. W., J. E. Dibb, B. L. Lefer, J. D. Bradshaw, S. T. Sandholm, D. R. Blake, N. J. Blake, G. W. Sachse, J. E. Collins, Jr., B. G. Heikes, J. T. Merrill, G. L. Gregory, B. E. Anderson, H. B. Singh, D. C. Thornton, A. R. Bandy, and R. F. Pueschel, Chemical characteristics of continental outflow from Asia to the troposphere over the western Pacific Ocean during February-March 1994: Results from PEM-West B, J. Geophys. Res., 102, 28,255-28,274, 1997.

Talbot, R. W., J. E. Dibb, B. L. Lefer, E. Scheuer, J. D. Bradshaw, S. T. Sandholm, S. Smyth, D. R. Blake, N. J. Blake, G. W. Sachse, J. E. Collins, Jr., and G. L. Gregory, Large scale distributions of tropospheric nitric, formic, and acetic acids over the western Pacific basin during wintertime, J. Geophys. Res., 102, 28,303-28,313, 1997.

Dibb, J. E., R. W. Talbot, B. L. Lefer, and E. Scheuer, Distributions of beryllium-7 and lead-210 over the western Pacific: PEM-West B, Februray-March1994, J. Geophys. Res., 102, 28,287-28,302, 1997.

Talbot, R. W., J. E. Dibb, E. M. Scheuer, D. R. Blake, N. J. Blake, G. L. Gregory, G. W. Sachse, J. D. Bradshaw, S. T. Sandholm, and H. B. Singh, Influence of biomass combustion emissions on the distribution of acidic trace gases over the southern Pacific basin during austral springtime, J. Geophys. Res., in press, 1998.

Dibb, J. E., Talbot, R. W., E. M. Scheuer, D. R. Blake, N. J. Blake, G. L. Gregory, G. W. Sachse, and D. C. Thornton, Aerosol chemical composition and distribution during Pacific Exploratory Mission - Tropics, J. Geophys. Res., in press, 1998.