Enhanced Oxidation of Isoprene and Monoterpenes in High and Low NOx Conditions

Abstract:

In the troposphere, the photochemical oxidation of volatile organic compounds (VOCs) is primarily initiated by their reactions with the hydroxyl radical (OH) which yields peroxy radicals (HO₂ and RO₂). Concentrations of OH and the rates of VOC oxidation depend on the efficiency of peroxy radical recycling to OH. Radical recycling mainly occurs through reaction of HO₂ with NO to produce NO₂ and, ultimately, ozone (O₃). Hence, the rate of VOC oxidation is dependent on NO_x (=NO+NO₂) concentration.

The Shale Oil and Natural Gas Nexus (SONGNEX) campaign was conducted from March 17 to April 29, 2015 with the main goal of identifying and quantifying industrial sources of pollutants throughout the United States, in particular those associated with the production of oil and natural gas. In this work, a case study of biogenic VOC oxidation within and outside a power plant plume in the Haynesville basin near the border of Texas and Louisiana is presented. Isoprene, monoterpenes and their oxides were measured by H₃O⁺ chemical ionization mass spectrometry (H₃O⁺ CIMS) in high time resolution (1 s). Further, an improved Whole Air Sampler (iWAS) was used to collect samples for post-flight analysis by gas chromatography mass spectrometric detection (GC-MS) and yielded speciated quantification of biogenic VOCs. The monoterpene oxide to monoterpene ratio follows the spatial extent of the plume as judged by another tracer (NO_x), tracking the enhancement of oxidation rates by NO_x. The observations are rationalized with the aid of box modeling using the Master Chemical Mechanism (MCM).