Secondary organic aerosol (SOA) contributes a significant fraction to aerosol mass and toxicity.Low-volatility organic vapors are critical intermediates connecting the oxidation of volatile organic compounds (VOCs) to SOA formation. However, the direct measurement of intermediate vapors poses a great challenge, further compounded by the difficulty of linking them to specific precursors from a cocktail of complex emission sources in the vast urbanized areas. Here, we present coordinated measurements of low-volatility oxidation products, termed oxygenated organic molecules (OOMs) in three most urbanized regions in China. With a newly-developed analysis methodology, we are able to assign these OOMs to their likely precursors and ultimately connect SOA formation to various VOCs. At all measurement locations, we find similar OOM

Highly oxidized multifunctional compounds (HOMs) have been demonstrated to be important for atmospheric secondary organic aerosols (SOA) and newparticle formation (NPF), yet it remains unclear which the main atmospheric HOM formation pathways are. In this study, a nitrate-ion-based chemical ionization atmosphericpressure-interface time-of-flight mass spectrometer (CI-APi-TOF) was deployed to measure HOMs in the boreal forest in Hyytiälä, southern Finland. Positive matrix factorization (PMF) was applied to separate the detected HOM species into several factors, relating these "factors" to plausible formation pathways. PMF was performed with a revised error estimation derived from laboratory data, which agrees well with an estimate based on ambient data. Three factors explained the majority ( > 95 %) of the data variation, but the optimal solution found six factors, including two nighttime factors, three daytime factors, and a transport factor. One nighttime factor is almost identical to laboratory spectra generated from monoterpene ozonolysis, while the second likely represents monoterpene oxidation initiated by NO 3 . The exact chemical processes forming the different daytime factors remain unclear, but they all have clearly distinct diurnal profiles, very likely related to monoterpene oxidation with a strong influence from NO, presumably through its effect on peroxy radical (RO 2 ) chemistry. Apart from these five "local" factors, the sixth factor is interpreted as a transport related factor. These findings improve our understanding of HOM production by confirming current knowledge and inspiring future research directions and provide new perspectives on using factorization methods to understand shortlived atmospheric species.

Pollutants during haze and Asian dust storms are transported out of the Asian continent, affecting the regional climate and the hydrological and biogeochemical cycles. Understanding the physicochemical properties of aerosol particles is essential to quantify their impacts. In order to determine physicochemical properties of aerosols before leaving the Asian continent, we carried out a field campaign from 14 April to 2 May 2011 at a background site in the path of Asian dust and haze outflows. We measured concentrations of gaseous pollutants (SO 2 , NO 2 , NO, O 3 , and CO), black carbon (BC), and particle number in situ and collected airborne particles for microscopic analysis. Pollutant concentrations (BC, 4 μg m À3; CO, 808 ppb; SO 2 , 24 ppb; NO 2 , 37 ppb) were highest during haze periods, except for PM 2.5 mass, which was highest (162 μg m À3) during a dust storm. Seventy-one percent of haze particles were coated with organic films and 32% were internal mixtures of sulfates and refractory particles (e.g., soot, metal/fly ash, and mineral). Seventy-nine percent of haze particles have deliquescence relative humidity at 68-70%. During a dust storm, soot particles were observed among dust particles. Most dust particles were hydrophobic, and no Ca(NO 3 ) 2 was observed in dust particles collected during the dust storms, but up to 32% of dust particles were found to be coated with Ca(NO 3 ) 2 after the main dust storm moved out of the sampling area. These results indicated that both natural and anthropogenic aerosol particles in Asian outflow can undergo significant physicochemical processes before leaving the Asian continent.

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Abstract. There are strong indications that particles containing secondary organic aerosol (SOA) exhibit amorphous solid or semi-solid phase states in the atmosphere. This may facilitate deposition ice nucleation and thus influence cirrus cloud properties. However, experimental ice nucleation studies of biogenic SOA are scarce. Here, we investigated the ice nucleation ability of viscous SOA particles. The SOA particles were produced from the ozone initiated oxidation of α-pinene in an aerosol chamber at temperatures in the range from −38 to −10 °C at 5–15 % relative humidity with respect to water to ensure their formation in a highly viscous phase state, i.e. semi-solid or glassy. The ice nucleation ability of SOA particles with different sizes was investigated with a new continuous flow diffusion chamber. For the first time, we observed heterogeneous ice nucleation of viscous α-pinene SOA in the deposition mode for ice saturation ratios between 1.3 and 1.4 significantly below the homogeneous freezing limit. The maximum frozen fractions found at temperatures between −36.5 and −38.3 °C ranged from 6 to 20 % and did not depend on the particle surface area. Global modelling of monoterpene SOA particles suggests that viscous biogenic SOA particles are indeed present in regions where cirrus cloud formation takes place. Hence, they could make up an important contribution to the global ice nuclei (IN) budget.

Abstract. New particle formation (NPF) is one of the major sources of atmospheric ultrafine particles. Due to the high aerosol and trace gas concentrations, the mechanism and governing factors for NPF in the polluted atmospheric boundary layer may be quite different from those in clean environments, which is however less understood. Herein, based on long-term atmospheric measurements from January 2018 to March 2019 in Beijing, the nucleation mechanism and the influences of H2SO4 concentration, amine concentrations, and aerosol concentration on NPF are quantified. The collision of H2SO4-amine clusters is found to be the dominating mechanism to initialize NPF in urban Beijing. The coagulation scavenging due to the high aerosol concentration is a governing factor as it limits the concentration of H2SO4 amine clusters and new particle formation rates. Besides, the effective amine concentration is another limiting factor in Beijing because amine is sometimes insufficient for nucleation at the kinetic limit. Based on the synergistic effects of these factors on H2SO4-amine nucleation, governing factors for H2SO4-amine nucleation for different conditions are summarized.

Abstract. Under certain conditions, secondary organic aerosol (SOA) particles can exist in the atmosphere in an amorphous solid or semi-solid state. To determine their relevance to processes such as ice nucleation or chemistry occurring within particles requires knowledge of the temperature and relative humidity (RH) range for SOA to exist in these states. In the CLOUD experiment at CERN, we deployed a new in-situ optical method to detect the viscosity of α-pinene SOA particles and measured their transition from the amorphous viscous to liquid state. The method is based on the depolarising properties of laboratory-produced non-spherical SOA particles and their transformation to non-depolarising spherical liquid particles during deliquescence. We found that particles formed and grown in the chamber developed an asymmetric shape through coagulation. A transition to spherical shape was observed as the RH was increased to between 35 % at −10 °C and 80 % at −38 °C, confirming previous calculations of the viscosity transition conditions. Consequently, α-pinene SOA particles exist in a viscous state over a wide range of ambient conditions, including the cirrus region of the free troposphere. This has implications for the physical, chemical and ice-nucleation properties of SOA and SOA-coated particles in the atmosphere.

Abstract. The impact of aerosols on climate and air quality remains poorly understood due to multiple factors. One of the current limitations is the incomplete understanding of the contribution of oxygenated products, generated from the gas-phase oxidation of volatile organic compounds (VOC), to aerosol formation. Indeed, atmospheric gaseous chemical processes yield thousands of (highly) oxygenated species, spanning a wide range of chemical formulas, functional groups and, consequently, volatilities. While recent mass spectrometric developments have allowed extensive on-line detection of a myriad of oxygenated organic species, playing a central role in atmospheric chemistry, the detailed quantification and characterization of this diverse group of compounds remains extremely challenging. To address this challenge, we evaluated the capability of current state-of-the-art mass spectrometers equipped with different chemical ionization sources to detect the oxidation products formed from α-pinene ozonolysis under various conditions. Five different mass spectrometers were deployed simultaneously for a chamber study. Two chemical ionization atmospheric pressure interface time-of-flight mass spectrometers (CI-APi-TOF) with nitrate and amine reagent ion chemistries and an iodide chemical ionization time-of-flight mass spectrometer (TOF-CIMS). Additionally, a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF 8000) and a new “Vocus” PTR-TOF were also deployed. In the current study, we compared around 1000 different compounds between each of the five instruments, with the aim to determine which oxygenated VOC (OVOC) the different methods were sensitive to, and identifying regions where two or more instruments were able to detect species with similar molecular formulae. We utilized a large variability in conditions (including different VOC, ozone, NOx and OH scavenger concentrations) in our newly constructed atmospheric simulation chamber for a comprehensive correlation analysis between all instruments. This analysis, combined with estimated concentrations for identified molecules in each instrument, yielded both expected and surprising results. As anticipated based on earlier studies: the PTR instruments were the only ones able to measure the precursor VOC; the iodide TOF-CIMS efficiently detected many semi-volatile organic compounds (SVOC) with 3 to 5 oxygen atoms; and the nitrate CI-APi-TOF was mainly sensitive to highly-oxygenated organic (O > 5) molecules (HOM). In addition, the Vocus showed good agreement with the iodide TOF-CIMS for the SVOC, including also a range of organonitrates. The amine CI-APi-TOF agreed well with the nitrate CI-APi-TOF for HOM dimers. However, the loadings in our experiments caused the amine reagent ion to be considerably depleted, causing non-linear responses for monomers. This study explores and highlights both benefits and limitations of currently available chemical ionization mass spectrometry instrumentation for characterizing the wide variety of OVOC in the atmosphere. While specifically shown for the case of α-pinene ozonolysis, we expect our general findings to be valid also for a wide range of other VOC-oxidant systems. As discussed in this study, no single instrument configuration can be deemed better or worse than the others, as the optimal instrument for a particular study ultimately depends on the specific target of the study.