Abstract
Abstract. Measurements of particle formation following the gas phase oxidation of volatile organic compounds (VOCs) emitted by Scots pine (Pinus sylvestris L.) seedlings are reported. Particle formation and condensational growth both from ozone (O3) and hydroxyl radical (OH) initiated oxidation of pine emissions (about 20-120 ppb) were investigated in a smog chamber. During experiments, tetramethylethylene (TME) and 2-butanol were added to control the concentrations of O3 and OH. Particle formation and condensational growth rates were interpreted with a chemical kinetic model. Scots pine emissions mainly included α-pinene, β-pinene, Δ3-carene, limonene, myrcene and β-phellandrene, composing more than 95% of total emissions. Modeled OH concentrations in the O3- and OH-induced experiments were on the order of ~106 molecules cm−3. Our results demonstrate that OH-initiated oxidation of VOCs plays an important role in the nucleation process during the initial new particle formation stage. The highest average particle formation rate of 360 cm−3 s−1 was observed for the OH-dominated nucleation events and the lowest formation rate of less than 0.5 cm−3 s−1 was observed for the case with only O3 present as an oxidant. In contrast to the particle formation process, ozonolysis of monoterpenes appears to be much more efficient to the aerosol growth process following nucleation. Higher contributions of more oxygenated products to the SOA mass loadings from OH-dominated oxidation systems were found as compared to the ozonolysis systems. Comparison of mass and volume distributions from the aerosol mass spectrometer and differential mobility analyzer yields estimated SOA effective densities of 1.34±0.06 g cm−3 for the OH+O3 oxidation systems and 1.38±0.03 g cm−3 for the O3 dominated chemistry.
Highlights
On a worldwide basis, the annual biogenic volatile organic compounds (VOCs) budget is estimated to be in the range of 491∼1150 Tg C (Guenther et al, 1995; Griffin et al, 1999)
The observed VOCs emitted from the Scots pine seedlings included monoterpenes, sum of isoprene and 2-methyl-3buten-2-ol (MBO), and minor amounts of other small molecular compounds, e.g. methanol, with monoterpenes dominating the total emissions in this work
OH oxidation rate is low (0.72×106 molecules cm−3 s−1), though the O3 oxidation rate was quite high (49.4×106 molecules cm−3 s−1). These results indicate that OH-initiated oxidation reactions play a very important role in nucleation and the early stages of particle growth, whereas the ozonolysis of monoterpenes is less effectively involved in these processes under the current ex
Summary
The annual biogenic volatile organic compounds (VOCs) budget is estimated to be in the range of 491∼1150 Tg C (Guenther et al, 1995; Griffin et al, 1999). Biogenic VOCs react with O3, OH and nitrate radical (NO3), leading to the formation of secondary organic aerosols (SOA). The annual SOA production from these biogenic precursors is estimated to range from 2.5 to 44.5 Tg C (Tsigaridis and Kanakidou, 2003), comprising about 60% of the organic aerosol mass on the global scale and an even higher fraction regionally (Kanakidou et al, 2005). SOA may have adverse effects on human health through their inhalation. Understanding these effects requires a greater knowledge of the mechanisms inducing aerosol formation in the atmosphere
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