Semicontinuous measurements of gas–particle partitioning of organic acids in a ponderosa pine forest using a MOVI-HRToF-CIMS

摘要

Hundreds of gas- and particle-phase organic acids were measured in a ruralponderosa pine forest in Colorado, USA, during BEACHON-RoMBAS (Bio-hydro-atmosphereinteractions of Energy, Aerosols, Carbon, H₂O, Organics & Nitrogen –Rocky Mountain Biogenic Aerosol Study). A recently developedmicro-orifice volatilization impactor high-resolution time-of-flightchemical ionization mass spectrometer (MOVI-HRToF-CIMS) using acetate(CH₃C(O)O−) as the reagent ion was used to selectively ionize anddetect acids semicontinuously from 20 to 30 August 2011, with ameasurement time resolution of ~1.5 h. At this site 98%of the organic acid mass is estimated to be in the gas phase, with only~2% in the particle phase. We investigated gas–particlepartitioning, quantified as the fraction in the particle phase (F_p), ofC₁–C₁₈ alkanoic acids, six known terpenoic acids, and bulk organicacids vs. carbon number. Data were compared to the absorptive partitioningmodel and suggest that bulk organic acids at this site follow absorptivepartitioning to the organic aerosol mass. The rapid response(<1–2 h) of partitioning to temperature changes for bulk acids suggests thatkinetic limitations to equilibrium are minor, which is in contrast toconclusions of some recent laboratory and field studies, possibly due tolack of very low ambient relative humidities at this site. Time trends forpartitioning of individual and groups of acids were mostly captured by themodel, with varying degrees of absolute agreement. Species with predictedsubstantial fractions in both the gas and particle phases show betterabsolute agreement, while species with very low predicted fractions in onephase often show poor agreement, potentially due to thermal decomposition,inlet adsorption, or other issues. Partitioning to the aqueous phase ispredicted to be smaller than to the organic phase for alkanoic and bulkacids, and has different trends with time and carbon number than observedexperimentally. This is due to the limited additional functionalizationobserved for the bulk acids. Partitioning to water appears to only play arole for the most oxidized acids during periods of high aerosol liquidwater. Based on measurement–model comparison we conclude that species carbonnumber and oxygen content, together with ambient temperature, control thevolatility of organic acids and are good predictors for partitioning at thissite. Partitioning of bulk acids is more consistent with model predictionsfor hydroxy acids, hydroperoxyacids, or polyacids, and less so for keto acids.