Influence of Dynamics and Chemistry on the Diurnal Variation of VOCs in the Planetary Boundary Layer above a Mixed Forest Canopy in the Southeastern United States

Abstract:

The planetary boundary layer (PBL) is a region of inherent interest because reactive VOCs emitted from the forest canopy are mixed with the residual and free tropospheric air masses, oxidized, and/or otherwise removed in this region. The characterization of diurnal variation of VOCs in the PBL is limited due to the lack of appropriate sampling platforms that are able to probe all the regions of interest: from the surface to the entrainment zone. Here we present the application of the Whole Air Sample Profiler (WASP) system during the 2013 Southeast Atmosphere Study (SAS) campaign.

A total of 41 research flights (RFs) were carried out during the 2013 SAS campaign between June 1 and June 14 over the Alabama Aquatic Biodiversity Center (AABC) site and the SEARCH site. During each RF, ambient air sampling started from 50-100 m above the canopy top and stopped at ~1200 m above the mean sea level (a.m.s.l). The air samples were subsequently analyzed by using a proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOF-MS). Here we analyze the vertical profiles and averaged diurnal variation of the mixing ratios of several reactive VOC species, including isoprene, the sum of monoterpenes, and first generation oxidation products of isoprene: methyl vinyl ketone and methacrolein (MVK+MACR).

A MiXed Layer Chemistry (MXLCH) model, guided by the meteorological and chemical observations during the SAS campaign, is used to study the influence of boundary layer dynamics and new isoprene oxidation mechanism on the diurnal variation of major biogenic VOCs emitted from the forest canopy. The new scheme includes OH recycling through two pathways under low-NOx regime: (1) hydroxyl peroxy radicals (HOC5H8OO∙; ISOPO2) unimolecular isomerization, and (2) ISOPO2+HO2. The model is able to reproduce the evolution of the boundary layer dynamics (including potential temperature, and boundary layer height) during the selected simulation dates. Based on the model results, budget analyses are performed to study the roles that the boundary layer dynamics and chemistry play in controlling the evolution of VOCs in the PBL. Chemical tendencies of important species, including organic peroxy radicals (RO2), HO2, and OH are calculated to evaluate the capacity of the new isoprene scheme in explaining the photooxidation processes in the PBL.