Relative Humidity Impact on Organic New Particle Formation from Ozonolysis of α- and β-Pinene at Atmospherically Relevant Mixing Ratios

Abstract

The impact of relative humidity (RH) on organic new particle formation (NPF) from ozonolysis of monoterpenes remains an area of active debate. Previous reports provide contradictory results indicating both depression and enhancement of NPF under conditions of moderate RH, while others do not indicate a potential impact. Only several reports have suggested that the effect may depend on absolute mixing ratio of the precursor volatile organic compound (VOC, ppbv). Herein we report on the impact of RH on NPF from dark ozonolysis of α- and β-pinene at mixing ratios ranging from 0.2 to 80 ppbv. We show that RH enhances NPF (by a factor of eight) at the lowest α-pinene mixing ratio, with a very strong dependence on α-pinene mixing ratio from 4 to 22 ppbv. At higher mixing ratios, the effect of RH plateaus, with resulting modest decreases in NPF. In the case of α- and β-pinene, NPF is enhanced at low mixing ratios due to a combination of chemistry, accelerated kinetics, and reduced partitioning of semi-volatile oxidation products to the particulate phase. Reduced partitioning would limit particle growth, permitting increased gas-phase concentrations of semi- and low-volatility products, which could favor NPF.

Nontrivial Impact of Relative Humidity on Organic New Particle Formation from Ozonolysis of cis-3-Hexenyl Acetate

“…At lower RHs, NPF appears to be more sensitive to RH than does C SOA . NPF from α-pinene ozonolysis at similar ξ VOCs , on the other hand, remained insensitive to the presence of RH, in accord with previous reports [26,27,[84][85][86]. We also present evidence to demonstrate that the effective shutdown of particle formation for CHA-derived SOA in the presence of water is not a physical process but rather is due to gaseous water playing an important role in the chemical mechanism and leading to the shutdown of NPF.…”

“…In contrast to CHA, α-pinene shows only a slight decrease in NPF, while SOA mass remains constant [26][27][28][29][30][31]. Due to the cyclic alkene in α-pinene, the ozonolysis of the endocyclic double bond results in a single C10-CI that is much larger and heavier and with a lower VP than the CHA-derived CIs.…”

“…In a manner analogous to Heinritzi et al [83], we conducted mixed system experiments using CHA and α-pinene, anticipating that the presence of the small C5-CIs from the ozonolysis of CHA would sequester C10-CIs from α-pinene, preventing the formation of C20-dimers necessary for NPF. We have shown previously that gas-phase water does not suppress NPF from α-pinene ozonolysis at low ξ VOC [26]. Therefore, if CHA reduces α-pinene-derived SOA formation, and CHA CIs are efficiently sequestered by water, then the addition of water vapor to this quaternary system should aid in recovering SOA formation from α-pinene ozonolysis (Table 3).…”

“…It is unlikely that results obtained at these high ξ VOC /ξ O3 are representative of the natural environment, thereby providing inaccurate data and guidance to atmospheric modelling efforts. For example, recently, we reported on significant enhancements in NPF for the dark ozonolysis of αand β-pinene at ξ VOC < 20 ppb v in the presence of high RH [26].…”

“…Similarly, the vast majority of studies to date on NPF and C SOA from the dark ozonolysis of monoterpenes have been conducted at fixed and uncharacteristically low levels of RH that are not representative of the lower troposphere [19], which is where most of the NPF and C SOA are hypothesized to occur [9,[20][21][22][23][24][25]. Recently, studies have been reported to better understand the role of RH in NPF and SOA generation from the ozonolysis of several monoterpenes, including αand β-pinene [26][27][28][29][30][31], limonene [27,28], and 3 ∆-carene [27,28]. Reported results have been contradictory, with some reporting an enhancement in NPF and C SOA as a result of higher RH [27,[31][32][33], while others report Air 2023, 1 223 sharp decreases [28,31,[34][35][36][37][38].…”