Smog chamber study on the NO_x dependence of SOA from isoprene photo-oxidation: Implication on RO₂ chemistry

Atmospheric isoprene can be oxidized and form secondary organic aerosol (SOA) by OH, NO₃ radicals and O₃ simultaneously. Previous studies have reported a nonlinear relationship between isoprene-SOA mass yield and NO_x levels, but the inherent mechanism is still not fully elucidated. In this work, the nonlinear dependence of isoprene SOA yield on NO_x concentrations was investigated by performing a series of batch chamber experiments; both the gas and aerosol phase chemical species were characterized using HR-TOF-CIMS and HR-TOF-AMS, along with an Observation-Based Model incorporated with the Master Chemical Mechanism (OBM-MCM model) simulation. We found that NO_x could influence the formations of low/extremely low/ultralow volatility organic compounds (L/EL/ULVOCs) by changing the RO₂ fates, which are the critical compounds in nucleation and condensation on particle phase. The OBM-MCM model results revealed that when RO₂ + HO₂ is the dominant RO₂ reaction pathway, the SOA yield increases with an increase in total loss rate of RO₂ + HO₂ and RO₂ + NO. However, with a continuous increase in NO_x concentration NO becomes the major sink for RO₂, in which RO₂ + NO would inhibit the formation of low volatile VOCs and thus reduce the SOA yield. By combing the smog chamber results and the OBM-MCM model estimations, we found that the isoprene SOA yield reaches the maximum when the ratio (β) of loss rate of [RO₂ + HO₂] to the total loss rate of [RO₂ + HO₂ and RO₂ + NO] is 0.5, at which the reaction rates of RO₂ + HO₂ and RO₂ + NO are equal and the [NO]/[HO₂] ratio is about 2.