Abstract
Abstract. Stress-induced volatile organic compound (VOC) emissions from transgenic Grey poplar modified in isoprene emission potential were used for the investigation of photochemical secondary organic aerosol (SOA) formation. In poplar, acute ozone stress induces the emission of a wide array of VOCs dominated by sesquiterpenes and aromatic VOCs. Constitutive light-dependent emission of isoprene ranged between 66 nmol m−2 s−1 in non-transgenic controls (wild type WT) and nearly zero (<0.5 nmol m−2 s−1) in isoprene emission-repressed plants (line RA22), respectively. Nucleation rates of up to 3600 cm−3 s−1 were observed in our experiments. In the presence of isoprene new particle formation was suppressed compared to non-isoprene containing VOC mixtures. Compared to isoprene/monoterpene systems emitted from other plants the suppression of nucleation by isoprene was less effective for the VOC mixture emitted from stressed poplar. This is explained by the observed high efficiency of new particle formation for emissions from stressed poplar. Direct measurements of OH in the reaction chamber revealed that the steady state concentration of OH is lower in the presence of isoprene than in the absence of isoprene, supporting the hypothesis that isoprenes' suppressing effect on nucleation is related to radical chemistry. In order to test whether isoprene contributes to SOA mass formation, fully deuterated isoprene (C5D8) was added to the stress-induced emission profile of an isoprene free poplar mutant. Mass spectral analysis showed that, despite the isoprene-induced suppression of particle formation, fractions of deuterated isoprene were incorporated into the SOA. A fractional mass yield of 2.3% of isoprene was observed. Future emission changes due to land use and climate change may therefore affect both gas phase oxidation capacity and new particle number formation.
Highlights
Land vegetation is the major source of volatile organic compounds (VOC) in the troposphere (Guenther et al, 1995)
The use of Grey poplar mutants differing in isoprene emission potential served as a unique option to investigate the influence of isoprene on new particle formation in a natural emitted VOC mixture
The photochemical formation of secondary organic aerosols (SOA) was found to be efficient in this VOC mixture with number efficiency of 6.4 (±1.2) s−1 cm−3 ppb C−1
Summary
Land vegetation is the major source of volatile organic compounds (VOC) in the troposphere (Guenther et al, 1995). In regions with low pre-existing particulate matter, VOC oxidation can produce low volatile products which are involved in new particle formation (Tunved et al, 2006). Recent literature points out that isoprene - the most important single VOC emitted from land vegetation - is poorly understood both in terms of gas phase oxidation mechanism (Lelieveld et al, 2008; Hofzumahaus et al, 2009; Paulot et al, 2009; Peeters and Muller, 2010) and its impact on new particle formation (Kiendler-Scharr et al, 2009b). Observations of atmospheric OH concentrations indicate that under low NOx conditions isoprene may be involved in a so far unidentified recycling mechanism that converts HO2 into OH
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