Research Program Area: Atmospheric Processes
Topic Areas: Chemistry & Reactivity
An experimental and modeling study was carried out to assess the ground-level atmospheric ozone impacts of representative pesticide-related VOCs. Environmental chamber experiments were carried out to develop and evaluate mechanisms for methyl isothiocyanate (MITC), S-ethyl N,N-di-npropyl thiocarbamate (EPTC), 1-3-dichloropropenes, kerosene, and carbon disulfide. The first four are important compounds in the California pesticide emissions inventory where ozone impact data are not available, and carbon disulfide is a known pesticide degradation product. In addition, results of previous experiments on the pesticide chloropicrin were used to evaluate an updated mechanism for this compound. Chamber data were also used to derive rate constants for the reactions of OH radials with the MITC and EPTC, with the results indicating rate constants of 1.72 x 10-12 and 2.21 x 10-11 cm3 molec-1 s-1, respectively, which are both somewhat different from previously measured values. The UCR EPA environmental chamber was employed, and most experiments were “incremental reactivity” experiments to determine effects of adding the test compounds to experiments simulating representative ambient chemical conditions. These employed NOx levels of 25-30 ppb and at reactive organic gas (ROG)/NOx ratios representing maximum incremental reactivity (MIR) and NOx-limited conditions. Mechanisms for the compounds studied were developed based on available information in the literature and the results of the chamber experiments, and the mechanism for chloropicrin was updated. Mechanisms for other thiocarbamates were estimated based on the mechanism derived for EPTC. The SAPRC-99 mechanism was used as the starting point, to which an updated chlorine mechanism was added so the reactions of the chlorine-containing compounds could be modeled. It was necessary to adjust uncertain portions of the mechanisms for MITC, CS2, and EPTC to give predictions that were consistent with the chamber data, and it was also necessary for the 1,3-dichloropropene mechanism to include an explicit representation of chloroacetaldehyde undergoing photolysis to form chlorine atoms at near-unit quantum yields to simulate the reactivity data for these compounds. The experiments with kerosene were found to be consistent with the predictions of the model derived from the available compositional data without the need for adjustments. The mechanisms were then used to derive quantitative ozone impacts for these and other pesticide compounds in the MIR and other incremental reactivity scales. The MIR values derived (in units of grams O3 per gram VOC) were 1,3-dichloropropenes: 5.4; chloropicrin: 2.2; EPTC and pebulate: 1.8; kerosene and molinate: 1.7; thiobencarb: 0.7; MITC: 0.35; and CS2: 0.25-0.28. (For comparison, the mixture used to represent reactive VOCs from all sources has an MIR of 3.7, and ethane has an MIR of 0.3). In addition, relative impacts of these compounds on particulate matter (PM) formation were determined, with kerosene having the greatest PM impact on a mass basis, followed by the sulfurcontaining compounds, and with the 1,3-dichloropropenes having no PM impact. Areas of uncertainties and needs for future research are discussed.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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