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.
Highlights
Atmospheric aerosol particles play a key role in the regulation of climate by influencing Earth’s radiative energy balance (IPCC, 2013)
In order to affect the solar radiation budget by acting as cloud condensation nuclei (CCN), newly formed particles have to reach a size of 50 to 100 nm (Dusek et al, 2006); i.e., they need to grow fast enough to avoid coagulation scavenging by preexisting particles
Gaseous sulfuric acid (Ball et al, 1999; Kuang et al, 2008), ammonia (Kirkby et al, 2011; Kürten et al, 2016), amines (Kurtén et al, 2008; Almeida et al, 2013; Kürten et al, 2014), iodine (O’Dowd et al, 2002; Sipilä et al, 2016) and biogenic volatile organic compounds (BVOCs; Donahue et al, 2013; Riccobono et al, 2014; Kirkby et al, 2016; Bianchi et al, 2016) have been identified as key vapors involved in atmospheric New-particle formation (NPF)
Summary
Atmospheric aerosol particles play a key role in the regulation of climate by influencing Earth’s radiative energy balance (IPCC, 2013). New-particle formation (NPF) is observed in many environments and under various conditions around the globe, from remote locations such as forested areas or marine and coastal regions to polluted urban areas; from warm environments, such as the tropics, to cold polar and alpine regions; and from Earth’s surface to the free troposphere (Kulmala et al, 2004; Kerminen et al, 2018). Gaseous sulfuric acid (Ball et al, 1999; Kuang et al, 2008), ammonia (Kirkby et al, 2011; Kürten et al, 2016), amines (Kurtén et al, 2008; Almeida et al, 2013; Kürten et al, 2014), iodine (O’Dowd et al, 2002; Sipilä et al, 2016) and biogenic volatile organic compounds (BVOCs; Donahue et al, 2013; Riccobono et al, 2014; Kirkby et al, 2016; Bianchi et al, 2016) have been identified as key vapors involved in atmospheric NPF. The chemical composition of the newly formed particles is widely influenced by volatile organic compounds (VOCs), which undergo atmospheric reactions to form secondary organic aerosols (SOAs; Jimenez et al, 2009; Hallquist et al, 2009; Riipinen et al, 2012)
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