<i>α</i>-Pinene secondary organic aerosol at low temperature: chemical composition and implications for particle viscosity

Abstract

Abstract. Chemical composition, size distributions, and degree of oligomerization of secondary organic aerosol (SOA) from α-pinene (C10H16) ozonolysis were investigated for low-temperature conditions (223 K). Two types of experiments were performed using two simulation chambers at the Karlsruhe Institute of Technology: the Aerosol Preparation and Characterization (APC) chamber, and the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber. Experiment type 1 simulated SOA formation at upper tropospheric conditions: SOA was generated in the AIDA chamber directly at 223 K at 61 % relative humidity (RH; experiment termed “cold humid”, CH) and for comparison at 6 % RH (experiment termed “cold dry”, CD) conditions. Experiment type 2 simulated SOA uplifting: SOA was formed in the APC chamber at room temperature (296 K) and < 1 % RH (experiment termed “warm dry”, WD) or 21 % RH (experiment termed “warm humid”, WH) conditions, and then partially transferred to the AIDA chamber kept at 223 K, and 61 % RH (WDtoCH) or 30 % RH (WHtoCH), respectively. Precursor concentrations varied between 0.7 and 2.2 ppm α-pinene, and between 2.3 and 1.8 ppm ozone for type 1 and type 2 experiments, respectively. Among other instrumentation, a chemical ionization mass spectrometer (CIMS) coupled to a filter inlet for gases and aerosols (FIGAERO), deploying I− as reagent ion, was used for SOA chemical composition analysis. For type 1 experiments with lower α-pinene concentrations and cold SOA formation temperature (223 K), smaller particles of 100–300 nm vacuum aerodynamic diameter (dva) and higher mass fractions (> 40 %) of adducts (molecules with more than 10 carbon atoms) of α-pinene oxidation products were observed. For type 2 experiments with higher α-pinene concentrations and warm SOA formation temperature (296 K), larger particles (∼ 500 nm dva) with smaller mass fractions of adducts (< 35 %) were produced. We also observed differences (up to 20 ∘C) in maximum desorption temperature (Tmax) of individual compounds desorbing from the particles deposited on the FIGAERO Teflon filter for different experiments, indicating that Tmax is not purely a function of a compound's vapor pressure or volatility, but is also influenced by diffusion limitations within the particles (particle viscosity), interactions between particles deposited on the filter (particle matrix), and/or particle mass on the filter. Highest Tmax were observed for SOA under dry conditions and with higher adduct mass fraction; lowest Tmax were observed for SOA under humid conditions and with lower adduct mass fraction. The observations indicate that particle viscosity may be influenced by intra- and inter-molecular hydrogen bonding between oligomers, and particle water uptake, even under such low-temperature conditions. Our results suggest that particle physicochemical properties such as viscosity and oligomer content mutually influence each other, and that variation in Tmax of particle desorptions may have implications for particle viscosity and particle matrix effects. The differences in particle physicochemical properties observed between our different experiments demonstrate the importance of taking experimental conditions into consideration when interpreting data from laboratory studies or using them as input in climate models.

Response to referee comments

“…We also observe good correlations (Pearson's R: 0.85; Fig. S7a) between our summer mass spectra and the summer daytime mass spectra acquired in 2016 near Karlsruhe (a city in southwest Germany, about 70 km northwest of Stuttgart; Huang et al, 2019), indicative of the regional nature of sources and/or chemistry in summer. We therefore conclude that the majority of the precursor VOCs for OOA presented here in summer are most likely of biogenic origin, despite the urban location of the measurement site.…”

Local Arctic Air Pollution: A Neglected but Serious Problem

“…For SOA formed from the ozonolysis of ɑ-pinene, Kristensen et al (2017) observed suppressed formation of dimer esters, relative to carboxylic acids, at 258 K, compared to 293 K, suggesting reduced oligomerization at lower temperatures. However, at an even lower temperature of 223 K, Huang et al (2018) observed increased mass fractions of oligomers in the SOA formed from ɑ-pinene ozonolysis, which was attributed to increased hydrogen bonding in the higher viscosity SOA (compared to SOA formed at 296 K). The studies of Fairbanks pollution, for example, have only used standard OC/elemental carbon analysis (Chow et al, 2004), which has little information about the actual chemical structures of the organic carbon species.…”

High-molecular-weight esters in <i>α</i>-pinene ozonolysis secondary organic aerosol: structural characterization and mechanistic proposal for their formation from highly oxygenated molecules

“…Hilfiker et al (2015) and Huang et al (2018), who examined the thermal behavior of α-pinene/O 3 SOA and found that cis-pinic acid and hydroxypinonic acid can also arise from thermal decomposition of unstable hetero-oligomers. In addition, it is consistent with the findings by Mutzel et al (2015) that intact HOMs detected in the gas phase are carbonyl-containing compounds.…”

“…It was also demonstrated in the latter study that the oligomer content is strongly correlated with cloud condensation nuclei activities of SOA particles. Furthermore, it could be demonstrated in laboratory chamber experiments that the ratio of monomers to oligomers and the oligomer content in α-pinene ozonolysis SOA are enhanced at low temperature and low precursor concentrations, conditions that are relevant for the upper troposphere (Huang et al, 2018).…”