Relative humidity effect on the formation of highly oxidized molecules and new particles during monoterpene oxidation

It has been widely observed around the world that the frequency and intensity of new particle formation (NPF) events are reduced during periods of high relative humidity (RH). The current study focuses on how RH affects the formation of highly oxidized molecules (HOMs), which are key components of NPF and initial growth caused by oxidized organics. The ozonolysis of α-pinene, limonene, and Δ3-carene, with and without OH scavengers, were carried out under low NOx conditions under a range of RH (from ∼3 % to ∼92 %) in a temperature-controlled flow tube to generate secondary organic aerosol (SOA). A Scanning Mobility Particle Sizer (SMPS) was used to measure the size distribution of generated particles, and a novel transverse ionization chemical ionization inlet with a high-resolution time-of-fight mass spectrometer detected HOMs. A major finding from this work is that neither the detected HOMs nor their abundance changed significantly with RH, which indicates that the detected HOMs must be formed from water-independent pathways. In fact, the distinguished OH- and O3-derived peroxy radicals (RO2), HOM monomers, and HOM dimers could mostly be explained by the autoxidation of RO2 followed by bimolecular reactions with other RO2 or hydroperoxy radicals (HO2), rather than from a water-influenced pathway like through the formation of a stabilized Criegee intermediate (sCI). However, as RH increased from ∼3 % to ∼92 %, the total SOA number concentrations decreased by a factor of 2–3 while SOA mass concentrations remained relatively constant. These observations show that, while high RH appears to inhibit NPF as evident by the decreasing number concentration, this reduction is not caused by a decrease in RO2-derived HOM formation. Possible explanations for these phenomena were discussed.