Isoprene in poplar emissions: effects on new particle formation and OH concentrations

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 (C 5 D 8 ) 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.

Secondary aerosol formation from stress-induced biogenic emissions and possible climate feedbacks

“…We derive incremental mass yields of 20 ± 3 % for MeSa and of 17 ± 1 % for SQT from Experiments #1c and #2, respectively (Table 4). The SQT and MeSa incremental yields are about the same within the errors and we conclude that SQT and MeSa must have contributed according to their turnover to the BSOA yields of ≈ 20 % in Experiment #1 (Table 4 and Kiendler-Scharr et al, 2012). Compared to the incremental yields of MT-SOAs, the yields of SQT-SOAs and MeSa-SOAs are about a factor of 4 larger.…”

Global Modeling of Secondary Organic Aerosol With Organic Nucleation

“…We examined the sensitivity of the formation of newSOA to the inclusion of the suppression by isoprene in EX2 as described in section based on field measurements that indicated that NPF rarely takes place when the concentration of isoprene is high (Kiendler‐Scharr et al, ; Lee et al, ). The assumption of isoprene suppression in the model causes organic nucleation to shut off in many places, so that the annual global average organic nucleation rate is decreased by 67% (Table ).…”

“…The second sensitivity case (EX2) was designed to examine the uncertainty of organic nucleation under an isoprene‐rich environment. Some field measurements indicated that isoprene suppresses organic nucleation (Kiendler‐Scharr et al, ; Kiendler‐Scharr et al, ; Lee et al, ). It is clear that NPF rarely takes place when emissions of isoprene are high, although the chemical and physical mechanism of suppression is not clear.…”

Isoprene suppression of new particle formation: Potential mechanisms and implications

“…Despite the important impacts of BVOCs on climate, little is known about how individual BVOC species emitted from trees is involved in NPF processes. In particular, isoprene, which accounts for nearly half of the total global BVOC budget [ Guenther et al ., ], is controversial regarding whether this dominant biogenic compound either promotes or suppresses NPF [ Kiendler‐Scharr et al ., ; W. Xu et al ., ]. Additionally, the previous chemical mechanism proposed to explain isoprene's suppression of biogenic NPF contradicts atmospheric observations, as outlined below.…”