Roles of Relative Humidity and Particle Size on Chemical Aging of Tropical Peatland Burning Particles: Potential Influence of Phase State and Implications for Hygroscopic Property

Abstract

Peatland fires in Southeast Asia are an important source of primary organic aerosol (POA). Chemical aging of POA in the atmosphere produces oxygenated POA (OPOA). The OPOA production influences optical and hygroscopic properties, modulating the regional climate. However, the roles of environmental parameters such as relative humidity (RH) on chemical aging of peatland burning particles have rarely been investigated. Utilizing the potential aerosol mass (PAM) reactor, we conducted laboratory experiments for POA aging of particles generated from smoldering combustions of surface peat, fern, and Acacia leaves. The corresponding experiments for secondary organic aerosol formation were also separately conducted. Properties and chemical compositions of the resulting particle were quantified by both the hygroscopic tandem differential mobility analyzer and time‐of‐flight aerosol chemical speciation monitor (ToF‐ACSM). Conversion of peat combustion from POA to OPOA was pronounced when RH in the PAM reactor was higher. Considering that previous electromicroscopic observations demonstrated that peat combustion POA is likely (semi)solid, we postulate that oxidation of fresh peatland burning particles is faster at an elevated RH due to reduced viscosity following hygroscopic growth. Hygroscopicity parameter (κ) of aged POA particles linearly correlated with the mass fraction of OPOA that was quantified by the ToF‐ACSM. The above results highlight the importance of simultaneously measuring the chemical aging and particle phase state of peatland burning POA for quantifying their climatic impacts.

Existence of Crystalline Ammonium Sulfate Nuclei Affects Chemical Reactivity of Oleic Acid Particles Through Heterogeneous Nucleation

“…The experimental data were combined with annual average temperature and relative humidity (RH) data on the global scale, suggesting that the atmospheric lifetime of light-absorbing species in biomass burning OA depended on altitude due to variations in temperature and RH. Chen et al (2022) consistently demonstrated that oxidation of biomass burning POA was faster under high RH conditions. Influence of phase state of OA on atmospheric lifetime of POA has long been investigated (Knopf et al, 2005;J.…”

“…Chen et al. (2022) consistently demonstrated that oxidation of biomass burning POA was faster under high RH conditions. Influence of phase state of OA on atmospheric lifetime of POA has long been investigated (Knopf et al., 2005; J. Li et al., 2020; J. Li & Knopf, 2021; Shiraiwa et al., 2011), yet it is still not sufficiently understood for accurately simulating in global models.…”

“…Most of the current global models assume that the oxidation lifetime of POA is 1–1.2 days (Cooke et al., 1999; Heald et al., 2011; Mahowald et al., 2008). However, recent studies suggested that the time scale for POA aging may not be represented as a single parameter due to phase transition (Chen et al., 2022; J. Li et al., 2020; J. Li & Knopf, 2021; Pratap et al., 2019; Ravishankara, 1997; Schnitzler et al., 2022).…”