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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