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 △³-carene, with and without OH-scavenger, were carried out under low NOx conditions under a range of RH (from ~3% to ~90%) in a temperature-controlled flow tube. 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 changed from 3 to 90% the particle number concentrations decreased by a factor of 2~3 while particle mass concentrations increased or decreased slightly within a factor of 2. 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 HOMs formation. One possible explanation is the existence of other extremely low volatility compounds (ELVOCs), like gas phase formed sCI-included accretion products, which are responsible for the very first steps of NPF but are not detected by nitrate-based chemical ionization mass spectrometry. These ELVOCs may be preferentially reduced at high RH compared to more volatile compounds, the latter of which mainly determine the final mass concentration of particles.  Another possibility is that a fraction of HOMs cluster with water (but detected as the declustered molecules) at high RH in such a way that they may no longer be able to participate in cluster formation, thereby suppressing NPF.

The interested reader is referred to the following article for details about the experiments performed here:

Li, X., Chee, S., Hao, J., Abbatt, J. P. D., Jiang, J., and Smith, J. N.: Relative humidity effect on the formation of highly oxidized molecules and new particles during monoterpene oxidation, Atmos. Chem. Phys., 19, 1555-1570, https://doi.org/10.5194/acp-19-1555-2019, 2019.

These tables contained processed data that are plotted and tabulated in the manuscript referred to above. All experiments numbers in these tables refer to those described in the article. Data format is comma-delimited ASCII. 

Additional questions and requests for other processed data associated with the article can be directed to James Smith (jimsmith@uci.edu).