Oligomer formation in the troposphere: from experimental knowledge to 3-D modeling

Abstract

Abstract. The organic fraction of atmospheric aerosols has proven to be a critical element of air quality and climate issues. However, its composition and the aging processes it undergoes remain insufficiently understood. This work builds on laboratory knowledge to simulate the formation of oligomers from biogenic secondary organic aerosol (BSOA) in the troposphere at the continental scale. We compare the results of two different modeling approaches, a first-order kinetic process and a pH-dependent parameterization, both implemented in the CHIMERE air quality model (AQM) (www.lmd.polytechnique.fr/chimere), to simulate the spatial and temporal distribution of oligomerized secondary organic aerosol (SOA) over western Europe. We also included a comparison of organic carbon (OC) concentrations at two EMEP (European Monitoring and Evaluation Programme) stations. Our results show that there is a strong dependence of the results on the selected modeling approach: while the irreversible kinetic process leads to the oligomerization of about 50 % of the total BSOA mass, the pH-dependent approach shows a broader range of impacts, with a strong dependency on environmental parameters (pH and nature of aerosol) and the possibility for the process to be reversible. In parallel, we investigated the sensitivity of each modeling approach to the representation of SOA precursor solubility (Henry's law constant values). Finally, the pros and cons of each approach for the representation of SOA aging are discussed and recommendations are provided to improve current representations of oligomer formation in AQMs.

Nanoparticle growth by particle-phase chemistry

“…More recent models have included reversibility (Roldin et al, 2014;Trump and Donahue, 2014), which is needed to reproduce perturbations of freshly formed SOA such as changes induced by isothermal dilution, thermal degradation, and/or aging. Regional air quality models show that oligomerization has the potential to significantly increase the SOA mass concentration (Aksoyoglu et al, 2011;Lemaire et al, 2016), and accurately representing this chemistry in these models is perhaps the greatest uncertainty for predicting SOA formation (Shrivastava et al, 2016).…”

Modeling of Gas-Wall Partitioning of Organic Compounds Using a Quantitative Structure–Activity Relationship

“…However, τ decreases for highly reactive organics (i.e., glyoxal) in aerosol or volatile SVOCs (i.e., pinonic acid and pinonaldehyde, °< 10 -3 mmHg). Many studies have found a high fraction of oligomeric matter in SOA 345 mass, (Jang et al, 2002;Tolocka et al, 2004;Gao et al, 2004;Beardsley and Jang, 2016;Lemaire et al, 2016), evidence that heterogeneous chemistry of organic species is an important mechanism by which SOA forms. For example, Lemaire et al (2016) reported 50% of the oligomeric mass of isoprene SOA.…”

Response to comments by Referee #2

“…for constraining aerosol formation models. Future modeling of size-dependent molecular composition should include physico-chemical parameters such as diffusion, phase separation, and reaction reversibility (Liu et al, 2014;Mai et al, 2015;Riipinen et al, 2012;Shiraiwa et al, 2012;Song et al, 2015;Trump and Donahue, 2014;Zaveri et al, 2014) that may influence the contribution of particlephase chemistry to nanoparticle growth.…”