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Abstract. The molecular composition and volatility of gaseous organiccompounds were investigated during April–July 2019 at the Station forMeasuring Ecosystem – Atmosphere Relations (SMEAR) II situated in a borealforest in Hyytiälä, southern Finland. In order to obtain a morecomplete picture and full understanding of the molecular composition andvolatility of ambient gaseous organic compounds (from volatile organiccompounds, VOCs, to highly oxygenated organic molecules, HOMs), twodifferent instruments were used. A Vocus proton-transfer-reactiontime-of-flight mass spectrometer (Vocus PTR-ToF; hereafter Vocus) wasdeployed to measure VOCs and less oxygenated VOCs (i.e., OVOCs). Inaddition, a multi-scheme chemical ionization inlet coupled to an atmosphericpressure interface time-of-flight mass spectrometer (MION API-ToF) was usedto detect less oxygenated VOCs (using Br− as the reagent ion; hereafterMION-Br) and more oxygenated VOCs (including HOMs; using NO3- asthe reagent ion; hereafter MION-NO3). The comparison among differentmeasurement techniques revealed that the highest elemental oxygen-to-carbonratios (O : C) of organic compounds were observed by the MION-NO3 (0.9 ± 0.1, average ± 1 standard deviation), followed by the MION-Br(0.8 ± 0.1); lowest O : C ratios were observed by Vocus (0.2 ± 0.1). Diurnal patternsof the measured organic compounds were found to vary among differentmeasurement techniques, even for compounds with the same molecular formula,suggesting contributions of different isomers detected by the differenttechniques and/or fragmentation from different parent compounds inside theinstruments. Based on the complementary molecular information obtained fromVocus, MION-Br, and MION-NO3, a more complete picture of the bulkvolatility of all measured organic compounds in this boreal forest wasobtained. As expected, the VOC class was the most abundant (about 53.2 %), followed by intermediate-volatility organic compounds (IVOCs, about45.9 %). Although condensable organic compounds (low-volatility organiccompounds, LVOCs; extremely low volatility organic compounds, ELVOCs; andultralow-volatility organic compounds, ULVOCs) only comprised about 0.2 %of the total gaseous organic compounds, they play an important role in newparticle formation as shown in previous studies in this boreal forest. Ourstudy shows the full characterization of the gaseous organic compounds inthe boreal forest and the advantages of combining Vocus and MION API-ToF formeasuring ambient organic compounds with different oxidation extents (fromVOCs to HOMs). The results therefore provide a more comprehensiveunderstanding of the molecular composition and volatility of atmosphericorganic compounds as well as new insights into interpreting ambientmeasurements or testing/improving parameterizations in transport and climatemodels. 

Intense and frequent new particle formation (NPF) events have been observed in polluted urban environments, yet the dominant mechanisms are still under debate. To understand the key species and governing processes of NPF in polluted urban environments, we conducted comprehensive measurements in downtown Beijing during January–March, 2018. We performed detailed analyses on sulfuric acid cluster composition and budget, as well as the chemical and physical properties of oxidized organic molecules (OOMs). Our results demonstrate that the fast clustering of sulfuric acid (H2SO4) and base molecules triggered the NPF events, and OOMs further helped grow the newly formed particles toward climate- and health-relevant sizes. This synergistic role of H2SO4, base species, and OOMs in NPF is likely representative of polluted urban environments where abundant H2SO4 and base species usually co-exist, and OOMs are with moderately low volatility when produced under high NOx concentrations. 

Abstract. While the role of highly oxygenated molecules (HOMs) in new particleformation (NPF) and secondary organic aerosol (SOA) formation is not indispute, the interplay between HOM chemistry and atmospheric conditionscontinues to draw significant research attention. During the Influence ofBiosphere-Atmosphere Interactions on the Reactive Nitrogen budget (IBAIRN)campaign in September 2016, profile measurements of neutral HOMs below andabove the forest canopy were performed for the first time at the borealforest SMEAR II station. The HOM concentrations and composition distributionsbelow and above the canopy were similar during daytime, supporting awell-mixed boundary layer approximation. However, much lower nighttime HOMconcentrations were frequently observed at ground level, which was likely dueto the formation of a shallow decoupled layer below the canopy. Near theground HOMs were influenced by the changes in the precursors and oxidants andenhancement of the loss on surfaces in this layer, while the HOMs above thecanopy top were not significantly affected. Our findings clearly illustratethat near-ground HOM measurements conducted under stably stratifiedconditions at this site might only be representative of a small fraction ofthe entire nocturnal boundary layer. This could, in turn, influence thegrowth of newly formed particles and SOA formation below the canopy where thelarge majority of measurements are typically conducted. 

A major fraction of atmospheric aerosol particles, which affect both air quality and climate, form from gaseous precursors in the atmosphere. Highly oxygenated organic molecules (HOMs), formed by oxidation of biogenic volatile organic compounds, are known to participate in particle formation and growth. However, it is not well understood how they interact with atmospheric pollutants, such as nitrogen oxides (NO x ) and sulfur oxides (SO x ) from fossil fuel combustion, as well as ammonia (NH 3 ) from livestock and fertilizers. Here, we show how NO x suppresses particle formation, while HOMs, sulfuric acid, and NH 3 have a synergistic enhancing effect on particle formation. We postulate a novel mechanism, involving HOMs, sulfuric acid, and ammonia, which is able to closely reproduce observations of particle formation and growth in daytime boreal forest and similar environments. The findings elucidate the complex interactions between biogenic and anthropogenic vapors in the atmospheric aerosol system.