Molecular understanding of new-particle formation from α-pinene between −50 and +25 °C

Abstract

Abstract. Highly oxygenated organic molecules (HOMs) contribute substantially to the formation and growth of atmospheric aerosol particles, which affect air quality, human health and Earth's climate. HOMs are formed by rapid, gas-phase autoxidation of volatile organic compounds (VOCs) such as α-pinene, the most abundant monoterpene in the atmosphere. Due to their abundance and low volatility, HOMs can play an important role in new-particle formation (NPF) and the early growth of atmospheric aerosols, even without any further assistance of other low-volatility compounds such as sulfuric acid. Both the autoxidation reaction forming HOMs and their NPF rates are expected to be strongly dependent on temperature. However, experimental data on both effects are limited. Dedicated experiments were performed at the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN to address this question. In this study, we show that a decrease in temperature (from +25 to −50 ∘C) results in a reduced HOM yield and reduced oxidation state of the products, whereas the NPF rates (J1.7 nm) increase substantially. Measurements with two different chemical ionization mass spectrometers (using nitrate and protonated water as reagent ion, respectively) provide the molecular composition of the gaseous oxidation products, and a two-dimensional volatility basis set (2D VBS) model provides their volatility distribution. The HOM yield decreases with temperature from 6.2 % at 25 ∘C to 0.7 % at −50 ∘C. However, there is a strong reduction of the saturation vapor pressure of each oxidation state as the temperature is reduced. Overall, the reduction in volatility with temperature leads to an increase in the nucleation rates by up to 3 orders of magnitude at −50 ∘C compared with 25 ∘C. In addition, the enhancement of the nucleation rates by ions decreases with decreasing temperature, since the neutral molecular clusters have increased stability against evaporation. The resulting data quantify how the interplay between the temperature-dependent oxidation pathways and the associated vapor pressures affect biogenic NPF at the molecular level. Our measurements, therefore, improve our understanding of pure biogenic NPF for a wide range of tropospheric temperatures and precursor concentrations.

Probing key organic substances driving new particle growth initiated by iodine nucleation in coastal atmosphere

“…C 18 Oleic acid, linoleic acid and linolenic acid are the most commonly found unsaturated FAs in macro algae. FAs with a carbon chain shorter than C 20 were used by atmospheric chemists as organic tracers of atmospheric aerosols from microbe or kitchen emission (Simoneit and Mazurek, 1982;Zheng et al, 2000;Guo et al, 2003;Rogge et al, 1991;DeMott et al, 2018;Willoughby et al, 2016). In our study, no saturated FAs were detected in 10-18 nm particles.…”

CRI-HOM: A novel chemical mechanism for simulating highly oxygenated organic molecules (HOMs) in global chemistry–aerosol–climate models

“…This provides a framework for incorporating a comprehensive description of pure biogenic nucleation into a global model and, ultimately, allowing for a more rigorous description of aerosol formation and the climatic consequences. α-Pinene is considered as it is the most widely studied and widely emitted monoterpene (∼ 32 Tg yr −1 ; Sindelarova et al, 2014) and, with measured HOM yields of around 3 %-10 % (Ehn et al, 2014;Jokinen et al, 2015), has the potential to produce 2-7 Tg HOM yr −1 with the range arising from uncertainties in emissions, HOM yield, and difference in mass between the precursor biogenic volatile organic compounds (BVOC) and the HOMs, which will have at least six additional oxygen atoms but, in some cases, considerably more. HOM yields from β-pinene (the second most widely emitted monoterpene; Sindelarova et al, 2014) and isoprene (the most widely emitted BVOC; Sindelarova et al, 2014) are negligible (Ehn et al, 2014).…”

“…α-Pinene is considered as it is the most widely studied and widely emitted monoterpene (∼ 32 Tg yr −1 ; Sindelarova et al, 2014) and, with measured HOM yields of around 3 %-10 % (Ehn et al, 2014;Jokinen et al, 2015), has the potential to produce 2-7 Tg HOM yr −1 with the range arising from uncertainties in emissions, HOM yield, and difference in mass between the precursor biogenic volatile organic compounds (BVOC) and the HOMs, which will have at least six additional oxygen atoms but, in some cases, considerably more. HOM yields from β-pinene (the second most widely emitted monoterpene; Sindelarova et al, 2014) and isoprene (the most widely emitted BVOC; Sindelarova et al, 2014) are negligible (Ehn et al, 2014). Limonene has emissions at around 25 % of α-pinene (Sindelarova et al, 2014) and is likely to have a higher HOM yield (Ehn et al, 2014;Jokinen et al, 2015) although a much wider range of values have been reported than for α-pinene.…”

“…HOM yields from β-pinene (the second most widely emitted monoterpene; Sindelarova et al, 2014) and isoprene (the most widely emitted BVOC; Sindelarova et al, 2014) are negligible (Ehn et al, 2014). Limonene has emissions at around 25 % of α-pinene (Sindelarova et al, 2014) and is likely to have a higher HOM yield (Ehn et al, 2014;Jokinen et al, 2015) although a much wider range of values have been reported than for α-pinene. Limonene thus may have the potential to produce a similar mass of HOMs to α-pinene, and its consideration may be an area of future work.…”

The potential role of organics in new particle formation and initial growth in the remote tropical upper troposphere

“…Recent observations of the composition of molecular clusters present during NPF have highlighted the role that organics may play (Kulmala et al, 2013;Smith et al, 2004). Murphy et al (2006) and Froyd et al (2009) found that larger particles (D p > 0.15 µm) in the UT contained significant organic matter that was likely secondary, which suggests that condensable gas-phase organic compounds are present in the UT.…”