Particle Formation from Photooxidation of αpinene, Limonene, and Myrcene

Abstract

We present measurements of the effect of first-generation secondary organic aerosol (SOA) material on the growth of ∼10 nanometer diameter seed particles 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 SO 2 or a monoterpene. For typical conditions, organic compounds at ∼300 ppbv are exposed to photooxidation for a time of ∼80 s at a [OH] of about 6 × 10 6 cm −3 : thus, oxidation products have minimal OH exposure. The measured size changes of the sulfuric acid seed particles can then be attributed to the uptake of firstgeneration products. Along with descriptions of the apparatus and the procedure, the analysis to obtain SOA yields by comparing them to growth with H 2 SO 4 (g) is detailed. Results from photooxidation experiments of αpinene, limonene, and myrcene give SOA yields of 0.040, 0.084, and 0.16, respectively. These SOA yields roughly double with each addition of a double bond to the compound. The αpinene and limonene results are in accord with the results of many previous SOA experiments, while the myrcene SOA yield stands alone. Photooxidation of myrcene also led to significant nucleation, and the species responsible is comparable to H 2 SO 4 at a 35% relative humidity in its nucleation capability.

Effects of Precursor Structure on First-Generation Photo-Oxidation Organic Aerosol Formation

“…Our experimental conditions differ from the majority of SOA experiments where seed particles are ammonium sulfate (or bi-sulfate), oxidation is carried out for long time periods and the mass concentration of organic material is large (the present experiment has organic aerosol concentration at a maximum of about 0.005 μg/m 3 ) Comparisons with previous work are complicated for all of those differences but acid-catalyzed reactions are a special concern. We argue below (as we have previously, Hanson et al, 2022) that acid-catalyzed reactions are not the dominant factor driving SOA in our measurements. We also present measurements in the Supplement that support this viewpoint where α-pinene SOA was found to be nearly independent of relative humidity and thus acid activity: less than a 20% fluctuation in ΔD p measured over the range of 17-70% RH.…”

“…Hunter et al (2014) discussed a tethering effect for the multifunctional products form in the oxidation of ring structures. Studies have also related the number of double bonds in a compound to its SOA behavior (Ng et al, 2006) and to the rate of H-shift reactions once photooxidized (Møller et al, 2020); our previous experimental work (Hanson et al, 2022) with a series of monoterpenes is consistent with these ideas. Other studies have looked in great detail at a few molecules that are structurally similar and found differences in SOA yields (Sato et al, 2012;Chiappini et al, 2019).…”

“…Photo-oxidation experiments on alkanes, alkenes, alkynes, cyclic compounds, carbonyls and an alcohol were performed in a growth photolytic flow reactor, GroFR. The GroFR apparatus and methodology has been described in detail (Hanson et al, 2022) and here an overview of the technique is presented.…”

“…NO impurity levels were about 2-3% of HONO, determined with a TECO 14B/E; for most of the measurements initial NO was 0.8 to 1.3 nmol/mol (ppbv) but was up to 4.5 ppbv at the highest Q 4 . Typical residence time was 85 s but with laminar flow established over most of the reactor (supplement S2 of Hanson et al, 2022), the on-axis residence time is about half of that. Typical [OH] is 1 × 10 7 cm −3 (as modeled in Hanson et al, 2022) but for low vapor pressure compounds, OH levels can rise to ten times this and OH reaction with HONO becomes significant.…”

“…Typical residence time was 85 s but with laminar flow established over most of the reactor (supplement S2 of Hanson et al, 2022), the on-axis residence time is about half of that. Typical [OH] is 1 × 10 7 cm −3 (as modeled in Hanson et al, 2022) but for low vapor pressure compounds, OH levels can rise to ten times this and OH reaction with HONO becomes significant. For typical conditions, less than 1% of first-generation products react further with an OH molecule.…”