Effects of Water Vapor and OH-scavenging on Nano-particle Formation from Ozone and Limonene Reaction

CHI PHUONG HOANG (1), Donghuyn Rim (1), Lance Wallace(1), Andrew Persily (1)

(1) National Institute of Standards and Technology

     Abstract Number: 623
     Last modified: May 14, 2010

Abstract
A number of recent studies on the ozonolysis of limonene have shown that these reactions are major sources of indoor secondary organic aerosols. Although the particle formation mechanism from limonene and ozone reaction has been widely discussed, newly-formed particles have not yet been well studied. In particular, the effects of some external factors (e.g. water vapor) on the formation of freshly nucleated aerosol particles are largely unknown.

In this study, a scanning mobility particle sizer with a nano-DMA (Model 3936, TSI, St. Paul, MN), was used to evaluate the influences of water vapor and OH-scavenging on new particle formation (sized from 3 nm to 100 nm). A 4-L glass flask served as a reactor for a wide range of limonene and ozone levels (50 ppb(v) to 1000 ppb(v) and 3 ppb(v) to 20 ppb(v) respectively). Relative humidity varied from <1 % (dry case) to 40 % (wet case), while cyclo-hexane was used to suppress the influence of OH radicals produced from limonene and ozone reaction.

The study results showed nucleation events were observable before particles started coagulating and low volatility compounds condensed on the particle surfaces leading to the growth of larger sizes. Water vapor showed little or no effect during the nucleation event. However, in terms of total particle counts, a small effect was found with the increase in relative humidity, specifically, a reduction by a factor of 1.1 to 1.3, which is comparable with those reported in a previous study. Also, the presence of cyclo-hexane displayed both negative and positive effects on aerosol particle concentration, depending if limonene is excessive (positive effect) or not. This may indicate the fact that besides the impact of direct limonene oxidation by the OH radicals, excessive limonene and cyclo-hexane may condense on extremely small particles to form larger, detectable partcicles.