LaPorte Ground Site Measurements

Statement of Work

Confirming the Presence and Extent of OXIDATON BY Cl in the Houston, TEXAS Urban Area Using Specific Isoprene Oxidation Products as Tracers

Dan Riemer, Univ. Miami

Eric Apel, NCAR

Atomic chlorine (Cl) has been identified as an important component of the atmospheric oxidation cycle in the marine boundary layer [Finlayson-Pitts, 1993] and certain urban areas such as Houston, Texas [David Allen, personal communication]. Approximately 95,000 kg of Cl2 were emitted in Harris county, Texas in 1993 [].

To determine the extent of Cl chemistry in the Houson area, a study is proposed, whereby specific tracers resulting from isoprene oxidation by Cl will be identified and measured. Isoprene is a highly reactive nonmethane hydrocarbon (NMHC) that is emitted by vegetation. Laboratory studies have shown that the reaction of isoprene with Cl forms of several unique tracers which could be used as evidence for Cl chemistry [Nordmeyer et al., 1997]. By investigating the isoprene oxidation process under ambient conditions, differentiation of the several oxidation processes should be achievable. Emphasis is on determining the presence of Cl and its importance as an oxidant relative to other oxidative species such as OH and O3.

Isoprene chemistry is driven primarily by OH and O3 reactions. In Houston Cl chemistry should occur and, under certain circumstances, NO3 nighttime chemistry should occur. Figure 1 depicts the primary pathways of isoprene oxidation. Isoprene oxidation by OH and O3 results in the formation of methacrolein (MACR), methyl vinyl ketone (MVK) and formaldehyde (CH2O). Isoprene oxidation by Cl forms the unique reaction product 1-chloro-3-methyl-3-butene-2-one (CMBO) and several isomers. 2-methylene-3-butenal is formed a result of H-abstraction by Cl. CMBO and its isomers should serve as robust tracers of Cl chemistry. Rate coefficients for the isoprene oxidation reactions are shown in Table 1. Isoprene lifetimes with respect to the different oxidation processes are presented for ranges of oxidation potential by the individual oxidants.

Although the mixing ratios of isoprene and its specific oxidation products are expected to be in the pptv - ppbv range, current technology will allow the measurements to be made accurately, precisely and rapidly. To sufficiently delineate the importance of Cl chemistry and its temporal patterns, samples need to be taken approximately every 10-15 minutes. In situ air enrichment followed by gas chromatography and mass spectrometric detection (GC-MS) of only select ions will be sufficient.

Preparation of standards for isoprene and its OH and O3 oxidation products is relatively uncomplicated [Apel et al., 1998]. CMBO has been synthesized, purified and characterized and a standard is presently being prepared.

Using the analytical techniques described above, the measurement of isoprene and its oxidation products, MACR, MVK, CMBO and its isomers will be possible. By characterizing the temporal patterns of the various oxidation products of isoprene, and specifically the tracers of Cl chemistry, the relative significance of the different oxidation processes in the Houston area will be determined.


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