DOI: 10.7280/d1b674
title: Supporting Data for Relative Humidity Effect on the Formation of Highly Oxidized Molecules and New Particles during Monoterpene Oxidation

description: 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.
description: 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.
description: 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).