Journal Article

New particle formation in forests inhibited by isoprene emissions

Mentel, Thomas F., Wildt, Jürgen, Kleist, Einhard, Kiendler-Scharr, Astrid, Hohaus, Thorsten, Tillmann, Ralf, Uerlings, Ricarda, Maso, Miikka Dal, Wahner, Andreas, Schurr, Uli

2009

Overview

Atmospheric chemistry: forest isoprene clears the air Terrestrial vegetation releases vast amounts of volatile organic compounds (VOCs) into the atmosphere, mainly isoprene and derivatives such as monoterpenes and sesquiterpenes, some familiar as the aroma of pine trees. It has been suggested that these compounds are involved in the formation of organic aerosols, which act as 'seeds' for cloud formation and hence as cooling agents via an effect on radiative forcing. Experiments in a plant chamber simulating forest conditions show that isoprene can significantly inhibit new particle formation owing to its high hydroxyl radical reactivity. This surprising result may explain the observed seasonality in the frequency of aerosol nucleation events, as terpene emissions peak in summer, when there are fewer nucleation events than in autumn and spring. This work suggests that an increase in the isoprene content of VOCs in response to climate or land use change might reduce the potential for the formation of new aerosol particles, introducing a previously unrecognized element of climate warming. Volatile organic compounds, such as isoprene and monoterpenes, are emitted by terrestrial vegetation and have been suggested to be involved in organic aerosol formation, which in turn affects radiative forcing and climate. Simulation experiments conducted in a plant chamber now reveal that isoprene can significantly inhibit new particle formation; this may explain the observed seasonality in the frequency of aerosol nucleation events. It has been suggested that volatile organic compounds (VOCs) are involved in organic aerosol formation, which in turn affects radiative forcing and climate 1 . The most abundant VOCs emitted by terrestrial vegetation are isoprene and its derivatives, such as monoterpenes and sesquiterpenes 2 . New particle formation in boreal regions is related to monoterpene emissions 3 and causes an estimated negative radiative forcing 4 of about -0.2 to -0.9 W m -2 . The annual variation in aerosol growth rates during particle nucleation events correlates with the seasonality of monoterpene emissions of the local vegetation, with a maximum during summer 5 . The frequency of nucleation events peaks, however, in spring and autumn 5 . Here we present evidence from simulation experiments conducted in a plant chamber that isoprene can significantly inhibit new particle formation. The process leading to the observed decrease in particle number concentration is linked to the high reactivity of isoprene with the hydroxyl radical (OH). The suppression is stronger with higher concentrations of isoprene, but with little dependence on the specific VOC mixture emitted by trees. A parameterization of the observed suppression factor as a function of isoprene concentration suggests that the number of new particles produced depends on the OH concentration and VOCs involved in the production of new particles undergo three to four steps of oxidation by OH. Our measurements simulate conditions that are typical for forested regions and may explain the observed seasonality in the frequency of aerosol nucleation events, with a lower number of nucleation events during summer compared to autumn and spring 5 . Biogenic emissions of isoprene are controlled by temperature and light 2 , and if the relative isoprene abundance of biogenic VOC emissions increases in response to climate change or land use change, the new particle formation potential may decrease, thus damping the aerosol negative radiative forcing effect.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: Aerosols / Aerosols - analysis / Aerosols - metabolism / Air - analysis / Atmosphere / Atmospheric nucleation / Atoms & subatomic particles / Betula - drug effects / Betula - metabolism / Butadienes - analysis / Butadienes - pharmacology / Carbon - analysis / Climate change / Earth, ocean, space / Environment, Controlled / Exact sciences and technology / External geophysics / Fagus - drug effects / Fagus - metabolism / Forest ecology / Hemiterpenes - analysis / Hemiterpenes - pharmacology / Hemiterpenes - secretion / Humanities and Social Sciences / Hydroxyl Radical - analysis / Hydroxyl Radical - metabolism / letter / Light / Meteorology / Monoterpenes - metabolism / Monoterpenes - pharmacology / multidisciplinary / Nucleation / Observations / Oxidation / Oxidation-Reduction / Particles and aerosols / Pentanes - analysis / Pentanes - pharmacology / Picea - drug effects / Picea - metabolism / Properties / Science / Science (multidisciplinary) / Seasons / Service oriented architecture / Temperature / Time Factors / Trees - drug effects / Trees - metabolism / Volatile organic compounds / Volatile Organic Compounds - analysis / Volatile Organic Compounds - metabolism