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Atmospheric aerosol particles play an important role for air quality and climate. Secondary organic aerosol (SOA) make up a significant mass fraction of these particles. SOA particles mostly forms from oxidation of gases, followed by gas-particle conversion of the oxidation products. Due to the variety of precursors and oxidation pathways involved in SOA formation, atmospheric SOA rank among the least understood aerosol types. To assess the impacts of SOA particles on air pollution and climate, knowledge of the number of phases in internal mixtures of different SOA types is critical. For example, gas-particle partitioning of organic species, and thus ultimately ambient SOA mass concentration, strongly depend on the number of phases in SOA particles. Atmospheric models traditionally assumed that different SOA types form a single condensed organic phase when internally mixed in individual particles. In case of mixed SOA particles with a single condensed phase uptake of semi-volatile vapors are enhanced, due to a lowering of the activities in the organic aerosol phase, and hence a lowering of the equilibrium partial pressure. By contrast, the equilibrium partial pressure is greater if the different SOA types form separate phases due to repulsive intermolecular forces between immiscible organic molecules. Consequently, enhancement of vapor uptake and ambient SOA mass concentrations will be smaller or absent in the case of phase-separated SOA particles.Here, using fluorescence microscopy, we directly observed the number of phase in individual particles containing mixtures of different SOA types. A total of 6 different SOA types were generated in environmental chambers from oxidation of single precursors. This included both biogenic and anthropogenic SOA types, having elemental oxygen-to-carbon (O/C) ratios between 0.34 and 1.05, covering values characteristic for aged and fresh atmospheric SOA. The number of phases of all possible internal mixtures of two different SOA types, termed SOA+SOA particles, was investigated as a function of humidity between 90% and 0% relative humidity (RH). We found that the number of phases was independent of RH within the range investigated and that 6 out of 15 SOA+SOA mixtures resulted in particles with two condensed organic phases. The observation of phase separated SOA+SOA particles challenges the approach of assuming a single condensed organic phase when representing SOA formation in atmospheric models. Specifically, we demonstrate that the difference in the average O/C ratio between the two SOA types of a mixture (ΔO/C) is a good predictor of the number of phases in particles that are internal mixtures of different SOA types: two-phase SOA+SOA particles formed for ΔO/C ≥ 0.47, while one-phase SOA+SOA particles formed for ΔO/C < 0.47. This threshold ΔO/C provides a simple, yet powerful parameter to predict whether mixtures of fresh and aged SOA particles form one- or two-phase particles in models.

Episodes with high concentrations of particulate matter (PM) across the seasons were investigated during four one-month campaigns at a suburban site in Hong Kong. High-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) measurements revealed that both regional transport and secondary formation contributed to high PM levels during the episodes at this site. Based on distinct meteorological conditions, episodes were categorized into three types: liquid water content (LWC), solar irradiance (IR), and long-range transport (LRT). Despite the difference in meteorological conditions, all episodes were characterized by a high fraction of sulfate (45 %–56 %) and organics (23 %–34 %). However, aerosols in LWC episodes were less aged, consisting of the lowest fraction of secondary organics aerosols (SOA) and the highest fraction of small particles. Large particles mixed internally while freshly formed small particles mixed externally in LWC episodes. Aerosols in LRT episodes, by contrast, were the most aged and consisted of the highest proportion of low-volatility oxygenated organic aerosols (LVOOA) and the lowest proportion of small particles. Both small and large particles mixed externally in LRT episodes. The highest proportion of semi-volatile oxygenated organic aerosols (SVOOA) and a medium proportion of small particles were observed in IR episodes. Both small and large particles were likely externally mixed during IR episodes. Unlike in the other two types of episodes, in IR episodes aerosols experienced the most dramatic size increase and diurnal variation, with a time lag between SVOOA and LVOOA and a gradual increase in carbon oxidation state (OSc ≈ 2 × O : C – H : C). Five out of ten episodes were of the IR type, further reflecting the importance of this type of episode. The evolution of aerosol components in one particular episode of the IR type, which exhibited a clear land-sea breeze pattern, was examined in detail. Sulfate and SOA due to photochemical aging were very efficiently produced during the course of six hours. The “fresh” SOA (SVOOA) was initially formed at a higher rate than the “aged” SOA (LVOOA). The SVOOA transformed to LVOOA at the later stage of photochemical aging. This tranformation was further supported by mass spectral analysis, which showed an increase in the most oxidized ion (CO2+) and decreases in moderately oxidized ones (C2H3O+, C3H3O+ and C3H5O+). By measuring the physical and chemical properties of PM in a highly time-resolved manner, the current study was able to demonstrate the dynamic and complex nature of PM transformation during high-PM episodes.