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.