Chemical Sources for the On-line Calibration of Gas-phase Atmospheric Measurement Instrumentation

Abstract:

Measurement of chemical species in the atmosphere is challenging endeavor that can be complicated by the fact that many of these species are inherently reactive or unstable with respect to surface adsorption or decomposition. The calibration of atmospheric instrumentation used for these measurements is a vital part of this enterprise and often presents unique challenges. Static systems, i.e. systems that depend on a particular analyte sample being stable in a cylinder or permeation tube for long periods of time, are often not appropriate for highly absorptive or reactive compounds. On-line systems that involve immediate production of a low concentration stream of analyte species in a controlled manner are often the only practical way to provide a calibration source. This presentation describes several types of on-line sources: diffusion tubes, and chemical reactors, and how to predict and optimize their performance. We also summarize our experience with several systems that we have devised for calibrating species of interest in atmospheric chemistry such as nitrous acid (HONO), isocyanic acid (HNCO), cyanogen halides (XCN, X = Cl, Br, I), nitryl chloride (ClNO₂), hypobromous acid (HOBr), acyl peroxynitrates (PAN compounds), and dinitrogen pentoxide (N₂O₅).