Modeling secondary organic aerosol formation from isoprene oxidation under dry and humid conditions

Abstract

Abstract.A new model for the formation of secondary organic aerosol (SOA) from isoprene was developed. This model uses surrogate molecular species (hydroxyhydroperoxides, tetrols, methylglyceric acid, organic nitrates) to represent SOA formation. The development of this model used available experimental data on yields and molecular composition of SOA from isoprene and methacrolein oxidation. This model reproduces the amount of particles measured in smog chambers under both low-NO x and high-NO x conditions. Under low-NO x conditions, the model reproduces the transitional formation of hydroxy-hydroperoxides particles, which are photolyzed and lead to SOA mass decrease after reaching a maximum. Under high-NO x conditions, particles are assumed to be formed mostly from the photo-oxidation of a PAN-type molecule derived from methacrolein (MPAN). This model successfully reproduces the complex NO x -dependence of isoprene oxidation and suggests a possible yield increase under some high-NO x conditions. Experimental data correspond to dry conditions (RH < 10%). However, particles formed from isoprene are expected to be highly hydrophilic, and isoprene oxidation products would likely partition between an aqueous phase and the gas phase at high humidity in the atmosphere. The model was extended to take into account the hydrophilic properties of SOA, which are relevant under atmospheric conditions, and investigate the effect of particulate liquid water on SOA formation. An important increase in SOA mass was estimated for humid conditions due to the hydrophilic properties. Experiments under high relative humidity conditions should be conducted to confirm the results of this study, which have implications for SOA modeling.

Secondary organic aerosol formation in cloud droplets and aqueous particles (aqSOA): a review of laboratory, field and model studies

“…This overlap in low volatility and water-solubility makes it difficult to derive accurate yields. The trends in our yield predictions follow those from a previous model study where significant aqSOA formation from isoprene was predicted (Couvidat and Seigneur, 2011). However, both this latter study and our study assume identical partitioning parameters (α i , K p,i ) for isoprene under high and low RH conditions although recent chamber experiments suggest that at high RH aqSOA formation might play a role and this cannot be fully parameterized by Eq.…”

Modelling organic aerosol concentrations and properties during ChArMEx summer campaigns of 2012 and 2013 in the western Mediterranean region

“…As detailed in Couvidat and Seigneur (2011), isoprene may form tetrols and methyl dihydroxy dihydroperoxide under low NO x , and methyl glyceric acids and organic nitrates under high NO x . Oxidation of isoprene by the nitrate radical NO 3 is also modelled.…”

“…The surrogate BiPER is supposed to be a methyl dihydroxy dihydroperoxide. Although BiDER is not identified, it is assumed to have the properties of a methyl tetrol (Couvidat and Seigneur, 2011). If we assumed that this compound has the same properties as a sulfate ester of formula C 6 H 11 O 3 SO 4 , the ratios OM / OC and O / C increase to get closer to measurements (Fig.…”

Development of an inorganic and organic aerosol model (CHIMERE 2017&lt;i&gt;β&lt;/i&gt; v1.0): seasonal and spatial evaluation over Europe

“…Henry's law constants of hydrophilic species are taken from Couvidat et al (2012). For hydrophobic species, they are calculated using the activity coefficients at infinite dilution as in Couvidat and Seigneur (2011), by using the saturation vapor pressure P 0 and the activity coefficient of compound i at infinite dilution computed with UNIFAC γ ∞ i such that…”