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.