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An oxidation flow reactor (OFR) is a vessel inside which the concentrationof a chosen oxidant can be increased for the purpose of studying SOAformation and aging by that oxidant. During the BEACHON-RoMBAS(Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O,Organics & Nitrogen–Rocky Mountain Biogenic Aerosol Study) fieldcampaign, ambient pine forest air was oxidized by OH radicals in an OFR tomeasure the amount of SOA that could be formed from the real mix of ambientSOA precursor gases, and how that amount changed with time as precursorschanged. High OH concentrations and short residence times allowed forsemicontinuous cycling through a large range of OH exposures ranging fromhours to weeks of equivalent (eq.) atmospheric aging. A simple model isderived and used to account for the relative timescales of condensation oflow-volatility organic compounds (LVOCs) onto particles; condensational lossto the walls; and further reaction to produce volatile, non-condensingfragmentation products. More SOA production was observed in the OFR atnighttime (average 3 µg m−3 when LVOC fate corrected) compared todaytime (average 0.9 µg m−3 when LVOC fate corrected), with maximumformation observed at 0.4–1.5 eq. days of photochemical aging. SOAformation followed a similar diurnal pattern to monoterpenes,sesquiterpenes, and toluene+p-cymene concentrations, including asubstantial increase just after sunrise at 07:00 local time. Higherphotochemical aging (> 10 eq. days) led to a decrease in new SOAformation and a loss of preexisting OA due to heterogeneous oxidationfollowed by fragmentation and volatilization. When comparing two differentcommonly used methods of OH production in OFRs (OFR185 and OFR254-70),similar amounts of SOA formation were observed. We recommend the OFR185 modefor future forest studies. Concurrent gas-phase measurements of air after OHoxidation illustrate the decay of primary VOCs, production of small oxidizedorganic compounds, and net production at lower ages followed by netconsumption of terpenoid oxidation products as photochemical age increased.New particle formation was observed in the reactor after oxidation,especially during times when precursor gas concentrations and SOA formationwere largest. Approximately 4.4 times more SOA was formed in the reactorfrom OH oxidation than could be explained by the VOCs measured in ambientair. To our knowledge this is the first time that this has been shown whencomparing VOC concentrations with SOA formation measured at the same time, rather thancomparing measurements made at different times. Several recently developedinstruments have quantified ambient semivolatile and intermediate-volatility organiccompounds (S/IVOCs) that were not detected by a proton transfer reactiontime-of-flight mass spectrometer (PTR-TOF-MS). An SOA yield of18–58 % from those compounds can explain the observed SOA formation.S/IVOCs were the only pool of gas-phase carbon that was large enough to explain the observed SOA formation. Thiswork suggests that these typically unmeasured gases play a substantial rolein ambient SOA formation. Our results allow ruling out condensation stickingcoefficients much lower than 1. These measurements help clarify themagnitude of potential SOA formation from OH oxidation in forestedenvironments and demonstrate methods for interpretation of ambient OFRmeasurements

In situ secondary organic aerosol formation from ambient pine forest air using an oxidation flow reactor