Measurements of HNO₃, SO₂, High Resolution Aerosol SO_{4 2-} , and Selected Aerosol Species
Aboard the NASA DC-8 Aircraft During TRACE-P

Robert W. Talbot (robert.talbot@unh.edu)
Jack E. Dibb (jack.dibb@unh.edu)
Institute for the Study of Earth, Oceans, and Space
University of New Hampshire
Durham, NH 03824

Summary

Our investigation will provide measurements of selected acidic gases and aerosol species
aboard the NASA DC-8 research aircraft. Our investigation is focused on measuring HNO3 , SO2 ,
aerosol ionic species, and aerosol ²¹⁰ Pb and ⁷ Be. We are also implementing a new measurement for
TRACE-P, the measurement of fine (<2 Fm) aerosol SO_{4 2-} with approximately two minute time
resolution. This measurement should yield exciting new information on the distribution and inter-species
correlations of fine aerosol SO_{4 2-} with other trace gases in fresh and aged continental
outflow. It will also provide a unique data base on the relative distribution of the ratio SO₂ fine
aerosol SO_{4 2-} collected using simultaneous measurements with identical time resolution.

Instrument Description

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 our
sampling manifold is the capability for performing standard additions of HNO₃ and SO₂ down the
entire length of the inlet.

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 2-4 minutes for HNO₃ , SO₂ , and fine aerosol SO_{4 2-} .

The flow measurement and control assembly for the aerosol sampling fit on the top of a
double bay high rack spanning the entire footprint. This assembly is coupled to a venturi pumping
station situated aft of our sampling location. To sample the atmospheric aerosol we use a forward
facing tandem probe arrangement which is located in a 62E 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.

The aerosol soluble ions are determined after aqueous extraction with subsequent ion
chromatographic analysis. 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 ⁷ Be 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 ⁷ Be solution. Due to the short half-life of ⁷ Be
(53 days), samples are shipped from the field to UNH via express carriers. ²¹⁰ Pb activities are
determined by alpha spectrometric determination of ²¹⁰ Po (a ²¹⁰ Pb daughter). Filters are spiked with
known amounts of ²⁰⁸ Po (a yield tracer), and both Po isotopes are stripped from the aerosol filters
by strong acid and subsequently plated onto Ag planchets for counting.