Air‐surface exchange of nonmethane organic compounds at a grassland site: Seasonal variations and stressed emissions

Abstract

Emissions of nonmethane organic compounds (NMOCs) were measured by a static enclosure technique at a grassland site in the midwestern United States during the growing seasons over a 2‐year period. A mixture of nonmethane hydrocarbons (NMHCs) and oxygenated hydrocarbons (OxHCs) was emitted from the surface at rates exhibiting large seasonal and year‐to‐year variations. The average emission rate (and standard error) of the total NMOCs around noontime on sunny days during the growing seasons for the 2‐year period was 1,300±170 μg m−2 h−1 (mass of the total NMOCs per area of enclosed soil surface per hour) or 5.5±0.9 μg g−1 h−1 (mass of the total NMOCs per mass of dry plant biomass in an enclosure per hour), with about 10% and 70% of the emissions being composed of tentatively identified NMHCs and OxHCs, respectively. Methanol was apparently derived from both the soil and vegetation and exhibited an average emission rate of 460±73 μg m−2 h−1 (1.4±0.2 μg g−1 h−1), which was the largest emission among the NMOCs. The year‐to‐year variation in the precipitation pattern greatly affected the NMOC emission rates. Emission rates normalized to biomass density exhibited a linear decrease as the growing season progressed. The emission rates of some NMOCs, particularly the OxHCs, from vegetation subjected to hypoxia, frost, and physical stresses were significantly greater than the average values observed at the site. Emissions of monoterpenes (α‐ and β−pinene, limonene, and myrcene) and cis‐3‐hexen‐l‐ol were accelerated during the flowering of the plants and were much greater than those predicted by algorithms that correlated emission rates with temperature. Herbaceous vegetation is estimated to contribute about 40% and 50% of the total NMOC and monoterpene emissions, respectively, in grasslands; the remaining contributions are from woody species within grasslands. Contributions of isoprene emissions from herbaceous vegetation in grasslands are negligible. Grasslands are estimated to contribute about 10% of the total biogenic NMOC emissions in the United States.