Aerosol phase state is critical for quantifying aerosol effects on climate and air quality. However, significant challenges remain in our ability to predict and quantify phase state during its evolution in the atmosphere. Herein, we demonstrate that aerosol phase (liquid, semisolid, solid) exhibits a diel cycle in a mixed forest environment, oscillating between a viscous, semisolid phase state at night and liquid phase state with phase separation during the day. The viscous nighttime particles existed despite higher relative humidity and were independently confirmed by bounce factor measurements and atomic force microscopy. High-resolution mass spectrometry shows the more viscous phase state at night is impacted by the formation of terpene-derived and higher molecular weight secondary organic aerosol (SOA) and smaller inorganic sulfate mass fractions. Larger daytime particulate sulfate mass fractions, as well as a predominance of lower molecular weight isoprene-derived SOA, lead to the liquid state of the daytime particles and phase separation after greater uptake of liquid water, despite the lower daytime relative humidity. The observed diel cycle of aerosol phase should provoke rethinking of the SOA atmospheric lifecycle, as it suggests diurnal variability in gas–particle partitioning and mixing time scales, which influence aerosol multiphase chemistry, lifetime, and climate impacts.

The atmospheric evolution of organic compounds encompasses many thousands of compounds with varying volatility, polarity, and water solubility. The molecular-level chemical composition of this mixture plays a major, yet uncertain, role in its transformations and impacts. Here we perform a non-targeted molecular-level intercomparison of functionalized organic aerosol from three diverse field sites and a chamber. Despite similar bulk composition, we report large molecular-level variability between multi-hour organic aerosol samples at each site, with 66 ± 13% of functionalized compounds differing between consecutive samples. Single precursor environmental laboratory chamber experiments and fully chemically-explicit modeling confirm this variability is due to changes in emitted precursors, chemical age, and/or oxidation conditions. These molecular-level results demonstrate greater compositional variability than is typically observed in less-speciated measurements, such as bulk elemental composition, which tend to show less daily variability. These observations should inform future field and laboratory studies, including assessments of the effects of variability on aerosol properties and ultimately the development of strategic organic aerosol parameterizations for air quality and climate models.

Organic aerosol (OA) is a complex mixture of compounds with diverse elemental and structural features, and its composition affects its health and environmental impacts. A detailed speciation of the functional group distribution in OA is important for constraining atmospheric reaction pathways and products, evaluating chemical mechanisms and models, and understanding OA impacts. We used high-resolution tandem mass spectrometry to perform a nontargeted analysis of OA functional groups from three diverse ambient sites across times of day and seasons. We observed a range of oxygen-, nitrogen-, and/or sulfur-containing functional groups, including oxygenates such as hydroxyls (29−69%) and carboxylic acids (19−59%), that dominated the functional group distribution and that may participate in hydrogen bonding and thus impact the chemical and physical properties of OA (percentages indicate average ion abundance contributions across campaigns). We also observed esters (7−39%) and ethers (13−42%) that suggest the importance of oligomerization. On average, organonitrates represented only 12% of identified nitrogen-containing groups and organosulfates represented 21% of identified sulfur-containing groups, while we observed many other nitrogen-and/or sulfurcontaining structures that were important contributors to OA composition (e.g., amines, imines, nitrophenols, and sulfides). Most compounds (81%) were multifunctional and likely multigenerational oxidation products, which typically contained two to five functional groups in total.

The contamination of indoor nonsmoking environments with thirdhand smoke (THS) is an important, poorly understood public health concern. Real-time THS off-gassing from smokers into a nonsmoking movie theater was observed with online and offline high-resolution mass spectrometry. Prominent emission events of THS tracers (e.g., 2,5-dimethylfuran, 2-methylfuran, and acetonitrile) and other tobacco-related volatile organic compounds (VOCs) coincided with the arrival of certain moviegoers and left residual contamination. These VOC emission events exposed occupants to the equivalent of 1 to 10 cigarettes of secondhand smoke, including multiple hazardous air pollutants (e.g., benzene and formaldehyde) at parts-per-billion concentrations. Nicotine and related intermediate-volatility nitrogen-containing compounds, which vaporized from clothes/bodies and recondensed onto aerosol, comprised 34% of observed functionalized organic aerosol abundance. Exposure to THS VOC emission events will be considerably enhanced in poorly ventilated or smaller spaces in contrast with a large, well-ventilated theater—amplifying concentrations and potential impacts on health and indoor chemistry.

Abstract. Biomass burning is a large source of uncontrolled air pollutants, including particulate matter (i.e., PM2.5), black carbon (BC), volatile organic compounds (VOCs), and carbon monoxide (CO), which have significant effects on air quality, human health, and climate. Measurements of PM2.5, BC, and CO made at the Yale Coastal Field Station in Guilford, CT, and five other sites in the metropolitan New York City (NYC) area indicate long-distance transport of pollutants from wildfires and other biomass burning to surface-level sites in the region. Here, we examine two such events occurring on 16–17 and 27–29 August 2018. In addition to regionally consistent enhancements in the surface concentrations of gases and particulates associated with biomass burning, satellite imagery confirms the presence of smoke plumes in the NYC–Connecticut region during these events. Back-trajectory modeling indicates that air masses arriving at surface-level sites in coastal Connecticut on 16–17 August passed over the western coast of Canada, near multiple large wildfires. In contrast, air parcels arriving on 27–29 August passed over active fires in the southeastern United States. The results of this study demonstrate that biomass burning events throughout the US and Canada (at times more than 4000 km away), which are increasing in frequency, impact surface-level air quality beyond regional scales, including in NYC and the northeastern US.