Abstract
Abstract. Silver birch (Betula pendula) and three Southeast Asian tropical plant species (Ficus cyathistipula, Ficus benjamina and Caryota millis) from the pantropical fig and palm genera were grown in a purpose-built and environment-controlled whole-tree chamber. The volatile organic compounds emitted from these trees were characterised and fed into a linked photochemical reaction chamber where they underwent photo-oxidation under a range of controlled conditions (relative humidity or RH ~65–89%, volatile organic compound-to-NOx or VOC / NOx ~3–9 and NOx ~2 ppbV). Both the gas phase and the aerosol phase of the reaction chamber were monitored in detail using a comprehensive suite of on-line and off-line chemical and physical measurement techniques. Silver birch was found to be a high monoterpene and sesquiterpene but low isoprene emitter, and its emissions were observed to produce measurable amounts of secondary organic aerosol (SOA) via both nucleation and condensation onto pre-existing seed aerosol (YSOA 26–39%). In contrast, all three tropical species were found to be high isoprene emitters with trace emissions of monoterpenes and sesquiterpenes. In tropical plant experiments without seed aerosol there was no measurable SOA nucleation, but aerosol mass was shown to increase when seed aerosol was present. Although principally isoprene emitting, the aerosol mass produced from tropical fig was mostly consistent (i.e. in 78 out of 120 aerosol mass calculations using plausible parameter sets of various precursor specific yields) with condensation of photo-oxidation products of the minor volatile organic compounds (VOCs) co-emitted; no significant aerosol yield from condensation of isoprene oxidation products was required in the interpretations of the experimental results. This finding is in line with previous reports of organic aerosol loadings consistent with production from minor biogenic VOCs co-emitted with isoprene in principally isoprene-emitting landscapes in Southeast Asia. Moreover, in general the amount of aerosol mass produced from the emissions of the principally isoprene-emitting plants was less than would be expected from published single-VOC experiments, if co-emitted species were solely responsible for the final SOA mass. Interpretation of the results obtained from the fig data sets leaves room for a potential role for isoprene in inhibiting SOA formation under certain ambient atmospheric conditions, although instrumental and experimental constraints impose a level of caution in the interpretation of the results. Concomitant gas- and aerosol-phase composition measurements also provide a detailed overview of numerous key oxidation mechanisms at work within the systems studied, and their combined analysis provides insight into the nature of the SOA formed.
Highlights
Atmospheric aerosols change the radiative balance of the Earth through scattering and absorbing incident solar radiation (Kim and Ramanathan, 2008); they directly and indirectly affect the properties and formation of clouds, altering the hydrological cycle (Gunthe et al, 2009; Junkermann et al, 2009; Stevens and Feingold, 2009); and they may have an impact on the efficiency of plant photosynthesis (Mercado et al, 2009), thereby modifying the uptake of atmospheric carbon
We studied silver birch (Betula pendula), which emits predominantly monoterpenes, with some t o the starting body of hydrocarbon material during such an e xperiment, the total measured mass (i.e. volatile organic compounds (VOCs) + secondary organic aerosol (SOA)) within the system would be expected to increase with time
Our Betula pendula experiments showed significant SOA measurement uncertainties involved in producing these data formation (Fig. 4) in both the presence and the absence of duction and growth of SOA observed in earlier published and influences imposed by the chamber walls
Summary
Atmospheric aerosols change the radiative balance of the Earth through scattering and absorbing incident solar radiation (Kim and Ramanathan, 2008); they directly and indirectly affect the properties and formation of clouds, altering the hydrological cycle (Gunthe et al, 2009; Junkermann et al, 2009; Stevens and Feingold, 2009); and they may have an impact on the efficiency of plant photosynthesis (Mercado et al, 2009), thereby modifying the uptake of atmospheric carbon. There is further evidence from chamber studies using temperate tree species such as birch, spruce and pine that isoprene may suppress SOA formation from other VOC precursors, when present (KiendlerScharr et al, 2009a; Kanawade et al, 2011). It should be noted at this point that it is unclear in most cases how wall effects have been considered in the production of such SOA yield values and whether the treatments employed are adequate to ensure that the yields are comparable between chambers, or between experiments. Seeded (ammonium sulfate) and unseeded experiments were carried out to allow studies of both fresh nucleation and condensation onto pre-existing aerosol
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