Determination of the stable carbon isotopic compositions of 2-methyltetrols for four forest areas in Southwest China: The implications for the δ13C values of atmospheric isoprene and C3/C4 vegetation distribution

Abstract

Isoprene is the most abundant non-methane hydrocarbons (NMHCs) in the troposphere and is released predominantly by vegetation. The δ13C values of atmospheric isoprene vary with different plant types (e.g. C3 and C4 plants). In this work, aerosol samples were collected in four forest areas in Sichuan Province, China, i.e., the Baima Spring Scenic Area (BM), the Panzhihua Cycas Nature Reserve (PZ), the Gongga Mountain National Nature Reserve (GG) and the Wolong National Nature Reserve (WL) during the summers of 2010–2012. The stable carbon isotopic compositions of 2-methyltetrols, the stable products of isoprene oxidation by OH, were measured using a GC/C/IRMS (gas chromatography/combustion/isotopic ratio mass spectrometry) with methylboronic acid derivatization. The stable carbon isotopic fractionation coefficient of isoprene oxidized by OH (OHεi) was derived in laboratory. With the δ13C values of 2-methyltetrols, OHεi and meteorological parameters, the δ13C values of atmospheric isoprene were calculated. The results show that forests can remarkably change the δ13C values of isoprene in the regional scales, making significant contributions to isoprene emissions. Moreover, C3/C4 proportions of shrubs and grasses depend on altitudes. The average δ13C values of atmospheric isoprene are −24.18 ± 1.72‰, −25.81 ± 1.36‰, −24.96 ± 0.94‰, −25.89 ± 1.35‰ for BM, PZ, GG and WL, respectively. The average δ13C value of atmospheric isoprene in SW China and the surrounding areas was −25.23 ± 1.44‰. C4 plants emitted 26.9 ± 10.3% of isoprene in the research atmosphere.