Influence of temperature on the molecular composition of ions and charged clusters during pure biogenic nucleation

Abstract

Abstract. It was recently shown by the CERN CLOUD experiment that biogenic highly oxygenated molecules (HOMs) form particles under atmospheric conditions in the absence of sulfuric acid, where ions enhance the nucleation rate by 1-2 orders of magnitude. The biogenic HOMs were produced from ozonolysis of α-pinene at 5 • C. Here we extend this study to compare the molecular composition of positive and negative HOM clusters measured with atmospheric pressure interface time-of-flight mass spectrometers (APi-TOFs), at three different temperatures (25, 5 and −25 • C). Most negative HOM clusters include a nitrate (NO − 3 ) ion, and the spectra are similar to those seen in the nighttime boreal forest. On the other hand, most positive HOM clusters include an ammonium (NH + 4 ) ion, and the spectra are characterized by mass bands that differ in their molecular weight by ∼ 20 C atoms, corresponding to HOM dimers. At lower temperatures the average oxygen to carbon (O : C) ratio of the HOM clusters decreases for both polarities, reflecting an overall reduction of HOM formation with decreasing temperature. This indicates a decrease in the rate of autoxidation with temperature due to a rather high activation energy as has previously been determined by quantum chemical calculations. Furthermore, at the lowest temperature (−25 • C), the presence of C 30 clusters shows that HOM monomers start to contribute to the nucleation of positive clusters. These experimental findings are supported by quantum chemical calculations of the binding energies of representative neutral and charged clusters.

New Particle Formation in the Atmosphere: From Molecular Clusters to Global Climate

“…The same model over a larger domain found that in China, NPF contributes 26% of CCN number at 1% supersaturation despite the very high emissions of primary particles (Matsui et al, 2013). In Europe, similar studies with PMCAMx (Fountoukis et al, 2012) and GLOMAP (Reddington et al, 2011) suggest NPF is more important, increasing concentrations of particles with at least 50 nm diameter by as much as a factor of 2. Julin et al (2018) used PMCAMx with nucleation mechanisms involving both ammonia and amines to predict the consequences of future emission controls in Europe.…”

“…Recent studies have shown that oxygenated organics formed from BVOCs can grow newly nucleated particles in a wide range of troposphere temperatures (D. Stolzenburg et al, ). This is because gas phase autoxidation reactions involved in the formation of HOMs usually have strong temperature dependencies, with higher reaction rates at higher temperatures (Frege et al, ), whereas nucleation is favored at lower temperatures. Despite the low reaction rates, Bianchi et al () observed high concentrations of HOMs during NPF at a high‐elevation site with low temperatures, with an average temperature of −5 °C at noontime.…”

“…The localized NPF in power plant plumes was also parameterized for large-scale models by Stevens and Pierce (2013) using large eddy simulations. The nucleation mechanisms in the PMCAMx study (Fountoukis et al, 2012) highlight the importance of ammonia in boundary layer nucleation, while those of the GLOMAP study highlight the importance of organics. A nucleation parameterization including the synergy between these compounds is a priority for future research.…”

Relative humidity effect on the formation of highly oxidized molecules and new particles during monoterpene oxidation

“…The widely observed anti-correlation between NPF and RH in the field experiments can be attributed to the indirect influence of water. For example, high RH often corresponds to greater cloud cover, which can lead to lower ground-level concentrations of photo-oxidized precursors such as H 2 SO 4 and highly oxidized molecules (HOMs) as well as an increased condensation sink that leads to scavenging of precursors and clusters (Hamed et al, 2011). On the other hand, water vapor may also directly influence NPF by regulating the formation of gas-phase precursors or by participating in cluster formation.…”

Impact of anthropogenic and biogenic sources on the seasonal variation in the molecular composition of urban organic aerosols: a field and laboratory study using ultra-high-resolution mass spectrometry

“…Field deployments of an aerosol mass spectrometer (AMS, Canagaratna et al, 2007) at several European stations have revealed a large impact of primary wood-burning emissions on OA (e.g. between 11 % and 59 % in Switzerland), while primary traffic emissions have a smaller contribution (4 %-14 %) (Lanz et al, 2010;Gilardoni et al, 2011;Daellenbach et al, 2017). The results are consistent with radiocarbon measurements (Zotter et al, 2014a), showing that during extreme winter pollution episodes non-fossil organic carbon may account for up to 97 % of organic carbon (OC) at Alpine valley sites (Magadino -83 %, San Vittore -97 %) and 74 % at an urban background site in Zurich and are also associated with levoglucosan, a pyrolysis product of cellulose.…”