2.5. FLASK MEASUREMENTS OF SF/NO

This project is a collaborative effort between the CCG and NOAH groups within CMDL funded by the Atmospheric Chemistry Project of NOAA's Climate and Global Change Program. A custom built gas chromatograph-electron capture detection (GC-ECD) system was installed to measure NO and SF in the air samples collected from the CCG air sampling network. This GC uses technology developed in the NOAH Group, which is the same as that used for the tower GCs and ACATS­IV (see Elkins, et al., 1996, for further instrumental details). Near the end of 1995 this system was used to analyze flasks from a subset of sites in the CCG network; eventually all of the sites in the network will be phased in.

A primary goal of the NO measurement program is to gain a better understanding of the budget of this compound. Both the natural and the anthropogenic sources of NO are poorly quantified, and the effectiveness of in situ field measurements is limited due to the extremely heterogeneous nature of NO emissions. These CCG flask measurements will complement the already existing background measurements made by NOAH in several ways: The CCG network has more continental sites that will give a closer look at the land-based NO sources. The CCG network also includes regular ocean cruise sampling that will help us better understand the oceanic source of NO and the effect that El Niño/QBO phenomenon has on this natural NO source. More generally, the increased spatial coverage of the CCG network will improve our ability to use inverse modeling techniques to derive NO sources and sinks on a more regional level, as has been done for CO [Tans, et al., 1989; 1990].

There are several motivations for the SF flask measurement program. (For more details on SF sources, analysis, calibration, and CMDL references, see the NOAH section 5.1.2 of this report). The global mean growth rate of this strong greenhouse gas will be elucidated from the NOAH baseline station flask sampling. With the CCG flask measurements, however, the more detailed variations in this compound's atmospheric distribution can be looked at. Because of its extreme inertness in the atmosphere and its well-understood sources, SF6 is a nearly ideal tracer of atmospheric dynamics. To this end, the SF flask data can be used to help keep track of interannual variations in interhemispheric mixing and to better characterize the "geographical history" of the air masses being sampled at our CCG network sites. Since it is a purely anthropogenic compound, the spatial and temporal variations that are observed in SF will aid in the ability to interpret the variations that are observed in the carbon gases and NO, which all have a combination of biogenic and anthropogenic sources. Initial findings show that, as expected, the continental sites (such as HUN, UTA, LEF; see Table 2.7 for acronyms) have SF levels that are on average »0.2 picomol/mol (abbreviated as ppt) higher than the marine sites (which are at »3.5 ppt), and some of our coastal and near shore sites (MHT, RPB, BME) show regular incursions of polluted continental air.

Long-term flask storage tests were conducted for NO using our standard glass flasks with Teflon o­rings filled with humidified air. After 1 year, a loss of »1.0 ppb of NO was measured most likely due to slow diffusion into the flasks' Teflon o­ring. Because the goal is to try to discern gradients about 1­3 ppb, this may rule out the use of NO data from flasks that have a long delay time between sample collection and analysis, primarily high latitude southern hemisphere sites. Similar long-term storage tests for SF are currently being conducted. br>

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