Laboratory and Modeling Studies of the Sources and Sinks of Oxygenated Volatile Organic Compounds in Forests

Abstract

The biosphere acts as a significant source and sink of reactive organic carbon that can regulate atmospheric composition via formation of pollutants such as tropospheric ozone (O3) and secondary organic aerosol (SOA). Both O3 and SOA can influence the radiative properties of the atmosphere and contribute negatively to human health. To better understand how the biosphere impacts secondary pollutant formation, this thesis covers projects that constrain the emission and dry deposition of oxygenated volatile organic compounds (OVOCs) such as formaldehyde (HCHO) and 1,2-isoprene hydroxyhydroperoxide (1,2-ISOPOOH) to vegetation via a combination of laboratory and modeling studies. First, we investigate the bidirectional exchange of HCHO, which is one of the most abundant carbonyls in the troposphere and is often used as a tracer for overall VOC oxidation processes that form O3 and SOA. Our laboratory studies at the leaf level for deciduous and evergreen tree leaves show that HCHO has a compensation point (CP) between 0.4 - 0.8 ppbv that rises exponentially with temperature. Ambient mixing ratios of HCHO are generally above this CP range, suggesting that trees act as a net sink rather than a net source of HCHO. Additionally, our measurements of the HCHO exchange velocity provide evidence that HCHO deposition is stomatally controlled. Simulations using a 1-D canopy box model show that dry deposition of HCHO is the major sink in the forest canopy and that the Wesely deposition scheme better represents the empirical data compared to the Karl deposition scheme. Second, we provide detailed measurements of the dry deposition of 1,2-ISOPOOH. Our studies show that 1,2-ISOPOOH converts efficiently (70% to 90% yield) to HCHO and methyl vinyl ketone (MVK) on both deciduous and evergreen tree leaves. The deposition and conversion of 1,2-ISOPOOH on both tree species was shown to be largely controlled by the cuticle. Simulations using a 1-D canopy box model show that the heterogeneous conversion of 1,2-ISOPOOH greatly influences both the HCHO and MVK source budgets. Our deposition results also show that the Nguyen deposition scheme agrees with the evergreen tree deposition results while the Karl deposition scheme agrees with the oak tree deposition results, showing the complexities of constraining organic peroxide dry deposition. Third, we suggest future studies that can help elucidate the factors contributing to the conversion of 1,2-ISOPOOH on leaf cuticles observed in the 1,2-ISOPOOH deposition study. Experiments showing the 1,2-ISOPOOH dry deposition velocity to leaf surfaces perturbed by soapy water are discussed in addition to studies looking at the impact of microbes on 1,2-ISOPOOH conversion to HCHO. Future studies should focus on measuring and perturbing leaf surface composition and exploring the deposition of other organic peroxides such as hydroxymethyl hydroperoxide.