Forests emit large quantities of volatile organic compounds (VOCs) to the atmosphere. Their condensable oxidation products can form secondary organic aerosol, a significant and ubiquitous component of atmospheric aerosol, which is known to affect the Earth's radiation balance by scattering solar radiation and by acting as cloud condensation nuclei. The quantitative assessment of such climate effects remains hampered by a number of factors, including an incomplete understanding of how biogenic VOCs contribute to the formation of atmospheric secondary organic aerosol. The growth of newly formed particles from sizes of less than three nanometres up to the sizes of cloud condensation nuclei (about one hundred nanometres) in many continental ecosystems requires abundant, essentially non-volatile organic vapours, but the sources and compositions of such vapours remain unknown. Here we investigate the oxidation of VOCs, in particular the terpene α-pinene, under atmospherically relevant conditions in chamber experiments. We find that a direct pathway leads from several biogenic VOCs, such as monoterpenes, to the formation of large amounts of extremely low-volatility vapours. These vapours form at significant mass yield in the gas phase and condense irreversibly onto aerosol surfaces to produce secondary organic aerosol, helping to explain the discrepancy between the observed atmospheric burden of secondary organic aerosol and that reported by many model studies. We further demonstrate how these low-volatility vapours can enhance, or even dominate, the formation and growth of aerosol particles over forested regions, providing a missing link between biogenic VOCs and their conversion to aerosol particles. Our findings could help to improve assessments of biosphere-aerosol-climate feedback mechanisms, and the air quality and climate effects of biogenic emissions generally.

Abstract. We describe a novel inlet that allows measurement of both gas and particle molecular composition when coupled to mass spectrometric, chromatographic, or optical sensors: the Filter Inlet for Gases and AEROsols (FIGAERO). The design goals for the FIGAERO are to allow unperturbed observation of ambient air while simultaneously analyzing gases and collecting particulate matter on a Teflon ® (hereafter Teflon) filter via an entirely separate sampling port. The filter is analyzed periodically by the same sensor on hourly or faster timescales using temperature-programmed thermal desorption. We assess the performance of the FI-GAERO by coupling it to a high-resolution time-of-flight chemical-ionization mass spectrometer (HRToF-CIMS) in laboratory chamber studies of α-pinene oxidation and field measurements at a boreal forest location. Low instrument backgrounds give detection limits of ppt or lower for compounds in the gas-phase and in the picogram m −3 range for particle phase compounds. The FIGAERO-HRToF-CIMS provides molecular information about both gases and particle composition on the 1 Hz and hourly timescales, respectively for hundreds of compounds. The FIGAERO thermal desorptions are highly reproducible (better than 10 %), allowing a calibrated assessment of the effective volatility of desorbing compounds and the role of thermal decomposition during the desorption process. We show that the often multi-modal desorption thermograms arising from secondary organic aerosol (SOA) provide additional insights into molecular composition and/or particle morphology, and exhibit changes with changes in SOA formation or aging pathways.

NO (nitric oxide) production from sunflower plants (Helianthus annuus L.), detached spinach leaves (Spinacia oleracea L.), desalted spinach leaf extracts or commercial maize (Zea mays L.) leaf nitrate reductase (NR, EC 1.6.6.1) was continuously followed as NO emission into the gas phase by chemiluminescence detection, and its response to post-translational NR modulation was examined in vitro and in vivo. NR (purified or in crude extracts) in vitro produced NO at saturating NADH and nitrite concentrations at about 1% of its nitrate reduction capacity. The K(m) for nitrite was relatively high (100 microM) compared to nitrite concentrations in illuminated leaves (10 microM). NO production was competitively inhibited by physiological nitrate concentrations (K(i)=50 microM). Importantly, inactivation of NR in crude extracts by protein phosphorylation with MgATP in the presence of a protein phosphatase inhibitor also inhibited NO production. Nitrate-fertilized plants or leaves emitted NO into purified air. The NO emission was lower in the dark than in the light, but was generally only a small fraction of the total NR activity in the tissue (about 0.01-0.1%). In order to check for a modulation of NO production in vivo, NR was artificially activated by treatments such as anoxia, feeding uncouplers or AICAR (a cell permeant 5'-AMP analogue). Under all these conditions, leaves were accumulating nitrite to concentrations exceeding those in normal illuminated leaves up to 100-fold, and NO production was drastically increased especially in the dark. NO production by leaf extracts or intact leaves was unaffected by nitric oxide synthase inhibitors. It is concluded that in non-elicited leaves NO is produced in variable quantities by NR depending on the total NR activity, the NR activation state and the cytosolic nitrite and nitrate concentration.

Secondary organic aerosol contributes to the atmospheric particle burden with implications for air quality and climate. Biogenic volatile organic compounds emitted from plants are important secondary organic aerosol precursors with isoprene emissions dominating globally. However, its yield of particle mass from oxidation is generally modest compared to that of other terpenoids. Here we show that isoprene, carbon monoxide and methane can suppress the instantaneous mass and the overall mass yield derived from monoterpenes in mixtures. We find that isoprene scavenges hydroxyl radicals preventing reaction with monoterpenes and the resulting isoprene peroxy radicals scavenge highly oxygenated monoterpene products. These effects reduced the yield of low-volatility products that would otherwise form secondary organic aerosol. Global model calculations indicate that oxidant and product scavenging can operate effectively in the real atmosphere. Highly reactive, modest aerosol yield compounds are not necessarily net producers and their oxidation can suppress both particle number and mass.

It has been suggested that volatile organic compounds (VOCs) are involved in organic aerosol formation, which in turn affects radiative forcing and climate. The most abundant VOCs emitted by terrestrial vegetation are isoprene and its derivatives, such as monoterpenes and sesquiterpenes. New particle formation in boreal regions is related to monoterpene emissions and causes an estimated negative radiative forcing 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. The frequency of nucleation events peaks, however, in spring and autumn. 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. Biogenic emissions of isoprene are controlled by temperature and light, 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.