Secondary Organic Aerosol from First-Generation Photo-oxidation Products of alpha-Pinene, Limonene and Myrcene

We present measurements of the effect of first-generation SOA material on the growth of nanoparticles composed of sulfuric acid and water. Experiments were performed in an atmospheric pressure, vertically-aligned flow reactor where OH was produced from HONO photolysis in the presence of either SO2 or a monoterpene. This new reactor, GroFR, is based on the flow reactor (PhoFR) from our previous work on photolytic H2SO4 formation (from photo-oxidation of SO2) and subsequent nucleation. For typical conditions, organic compounds at ~300 ppbv are exposed to photo-oxidation for a time of ~80 s at an [OH] of about 3x106 cm-3: thus first-generation products have minimal OH exposure. The measured size changes are primarily due to the uptake of first-generation oxidation products. This talk will focus on the apparatus and the analytical methods we used to get SOA yields from the measured change in size of ~ 10 nm diameter seed particles. In short, SOA yields were evaluated by comparing to growth with H2SO4(g), accomplished by substituting SO2 for the monoterpene. Results from photo-oxidation experiments of -pinene, limonene and myrcene will be presented where SOA yields of 0.05, 0.1 and 0.2, respectively were derived. The alpha-pinene SOA yields are in accord with early results of Burkholder et al. [2006] and Ng et al. [2006]. These SOA yields do not compare well to measured HOM (highly oxidized molecule) yields [Jokinen et al. 2015], a finding similar to that of Krasnomowitz et al. [2019] who reported SOA yields that are much higher than measured HOM yields in the ozone plus a-pinene system. The present SOA yields increase geometrically with the number of double bonds in the compound, a finding that enhances the early hypothesis on the SOA capabilities of these monoterpenes [Ng et al., 2006]. Nucleation was observed to occur in the photo-oxidation of myrcene where particles up to 12 nm in diameter and number densities of 5x105 cm-3 were observed. These were comparable to particles nucleated with H2SO4 from SO2 photo-oxidation under similar conditions. Nucleation was not observed in the photo-oxidation of alpha-pinene and a small amount was observed in limonene photo-oxidation.