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Ambient pine forest air was oxidized by OH, O3, or NO3radicals using an oxidation flow reactor (OFR) during the BEACHON-RoMBAS(Bio–hydro–atmosphere interactions of Energy, Aerosols, Carbon, H2O,Organics and Nitrogen – Rocky Mountain Biogenic Aerosol Study) campaign tostudy biogenic secondary organic aerosol (SOA) formation and organic aerosol(OA) aging. A wide range of equivalent atmospheric photochemical ages wassampled, from hours up to days (for O3 and NO3) or weeks (for OH).Ambient air processed by the OFR was typically sampled every 20–30 min, inorder to determine how the availability of SOA precursor gases in ambient airchanged with diurnal and synoptic conditions, for each of the three oxidants.More SOA was formed during nighttime than daytime for all three oxidants,indicating that SOA precursor concentrations were higher at night. At alltimes of day, OH oxidation led to approximately 4 times more SOA formationthan either O3 or NO3 oxidation. This is likely because O3 andNO3 will only react with gases containing C  =  C bonds (e.g., terpenes)to form SOA but will not react appreciably with many of their oxidationproducts or any species in the gas phase that lacks a C  =  C bond (e.g.,pinonic acid, alkanes). In contrast, OH can continue to react with compoundsthat lack C  =  C bonds to produce SOA. Closure was achieved between the amountof SOA formed from O3 and NO3 oxidation in the OFR and the SOApredicted to form from measured concentrations of ambient monoterpenes andsesquiterpenes using published chamber yields. This is in contrast toprevious work at this site(Palm et al., 2016),which has shown that a source of SOA from semi- and intermediate-volatilityorganic compounds (S/IVOCs) 3.4 times larger than the source from measuredVOCs is needed to explain the measured SOA formation from OH oxidation. Thiswork suggests that those S/IVOCs typically do not contain C  =  C bonds.O3 and NO3 oxidation produced SOA with elemental O : C and H : Csimilar to the least-oxidized OA observed in local ambient air, and neitheroxidant led to net mass loss at the highest exposures, in contrast to OHoxidation. An OH exposure in the OFR equivalent to several hours ofatmospheric aging also produced SOA with O : C and H : C values similar toambient OA, while higher aging (days–weeks) led to formation of SOA withprogressively higher O : C and lower H : C (and net mass loss at the highestexposures). NO3 oxidation led to the production of particulate organicnitrates (pRONO2), while OH and O3 oxidation (under low NO) didnot, as expected. These measurements of SOA formation provide the firstdirect comparison of SOA formation potential and chemical evolution from OH,O3, and NO3 oxidation in the real atmosphere and help to clarifythe oxidation processes that lead to SOA formation from biogenichydrocarbons

atmospheric chemistry and physics

Secondary organic aerosol formation from in situ OH, O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;, and NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; oxidation of ambient forest air in an oxidation flow reactor