Measurements
of HNO3, SO2, High Resolution Aerosol SO4 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 SO4
2- with approximately two minute time
resolution. This measurement should yield exciting new information on the distribution
and inter-species
correlations of fine aerosol SO4
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 SO4 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 SO4 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.
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