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Abstract The interaction between nitrogen monoxide (NO) and organic peroxy radicals (RO 2 ) greatly impacts the formation of highly oxygenated organic molecules (HOM), the key precursors of secondary organic aerosols. It has been thought that HOM production can be significantly suppressed by NO even at low concentrations. Here, we perform dedicated experiments focusing on HOM formation from monoterpenes at low NO concentrations (0 – 82 pptv). We demonstrate that such low NO can enhance HOM production by modulating the RO 2 loss and favoring the formation of alkoxy radicals that can continue to autoxidize through isomerization. These insights suggest that HOM yields from typical boreal forest emissions can vary between 2.5%-6.5%, and HOM formation will not be completely inhibited even at high NO concentrations. Our findings challenge the notion that NO monotonically reduces HOM yields by extending the knowledge of RO 2 -NO interactions to the low-NO regime. This represents a major advance towards an accurate assessment of HOM budgets, especially in low-NO environments, which prevails in the pre-industrial atmosphere, pristine areas, and the upper boundary layer. 

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.