New insights into the nonlinear effects of NO_x on SOA formation from isoprene photo-oxidation

Abstract. Atmospheric isoprene can be oxidizene 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 High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (HR-TOF-CIMS) and High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-TOF-AMS), along with an Observation-Based Model (OBM) incorporated with the Master Chemical Mechanism (OBM-MCM model) simulation. We found that NO_x could influence the formation of the ultralow volatility organic compounds (ULVOCs, log₁₀ C^* < −8.5), low volatility organic compounds (LVOCs, −4.5 < log₁₀ C^* < −0.5) and extremely low volatility organic compounds (ELVOCs, −8.5 < log₁₀ C^* < −4.5) by changing the RO₂ fate, which are the critical compounds in nucleation and condensation in particle phase respectively. The SOA of isoprene photooxidation was mainly from RO₂+HO₂ and RO₂+NO pathways. When RO₂+HO₂ was the dominant RO₂ fate, the SOA yield increased with the fraction of RO₂+HO₂ and RO₂+NO increasing. While when NO is the major sink for RO₂, RO₂+NO would inhibit the formation low volatile VOCs and affect the SOA yield. The branching ratio term (β) is used to denote the competitive relationship between the two RO₂ fates (RO₂+HO₂ and RO₂+NO). The loss rate of RO₂+HO₂ pathway was maximized at a branching ratio β of 0.5 ([NO_x]/[Isoprene]=0.77), when more low volatiles were produced and the SOA yield reached maximum. The branching rate term (β) can be used as a reference for field campaign and modeling.