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Asphalt-related emissions are an understudied source of reactive organic compounds with the potential to form organic aerosol (OA). Ambient aerosol mass spectrometry (AMS) measurements of asphalt-related aerosols near a month-long road paving project showed enhanced ambient OA concentrations with a mix of primary and secondary OA signatures. For comparison, gas-phase emissions from real-world road asphalt samples at application (e.g., 140 °C) and in-use (e.g., 60 °C) temperatures were injected into an environmental chamber and an oxidation flow reactor to simulate varying degrees of oxidative aging while measuring their gas- and aerosol-phase oxidation products. Secondary OA formation was observed via both self-nucleation and condensation, with chemical properties dependent on asphalt temperature and reaction conditions. The chemical composition of less-aged asphalt-related OA observed in outdoor and laboratory measurements was similar to OA from other petrochemical-based sources and hydrocarbon-like OA source factors observed via AMS in previous urban studies. The composition of aged OA varied with the degree of oxidation, similar to oxidized OA factors observed in ambient air. Taken together, these field and laboratory observations suggest that contributions to urban OA during and after application may be challenging to deconvolve from other traditional sources in ambient measurements. 

Abstract Aqueous‐phase uptake and processing of water‐soluble organic compounds can promote secondary organic aerosol (SOA) production. We evaluated the contributions of aqueous‐phase chemistry to summertime urban SOA at two sites in New York City. The relative role of aqueous‐phase processing varied with chemical and environmental conditions, with evident daytime SOA enhancements (e.g., >1 μg/m³) during periods with relative humidities (RH) exceeding 65% and often higher temperatures. Oxygenated organic aerosol (OOA) production was also sensitive to secondary inorganic aerosols, in part through their influence on aerosol liquid water. On average, high‐RH periods exhibited a 69% increase in less‐oxidized OOA production in Queens, NY. These enhancements coincided with southerly backward trajectories and greater inorganic aerosol concentrations, yet showed substantial intra‐city variability between Queens and Manhattan. The observed aqueous‐phase SOA production, even with historically low sulfate and nitrate aerosol loadings, highlights both opportunities and challenges for continued reductions in summertime PM_2.5in urban communities. 

Abstract Isoprene (C₅H₈) is the non-methane hydrocarbon with the highest emissions to the atmosphere. It is mainly produced by vegetation, especially broad-leaved trees, and efficiently transported to the upper troposphere in deep convective clouds, where it is mixed with lightning NO_x. Isoprene oxidation products drive rapid formation and growth of new particles in the tropical upper troposphere. However, isoprene oxidation pathways at low temperatures are not well understood. Here, in experiments at the CERN CLOUD chamber at 223 K and 243 K, we find that isoprene oxygenated organic molecules (IP-OOM) all involve two successive$${{{\rm{OH}}}}^{\bullet}$$ ${\mathrm{OH}}^{\bullet }$oxidations. However, depending on the ambient concentrations of the termination radicals ($${{{{\rm{HO}}}}_{2}}^{\bullet},\,{{{\rm{NO}}}}^{\bullet}$$ ${{\mathrm{HO}}_2}^{\bullet },{\mathrm{NO}}^{\bullet }$, and$${{{\rm{NO}}}}_{2}^{\bullet}$$ ${\mathrm{NO}}_2^{\bullet }$), vastly-different IP-OOM emerge, comprising compounds with zero, one or two nitrogen atoms. Our findings indicate high IP-OOM production rates for the tropical upper troposphere, mainly resulting in nitrate IP-OOM but with an increasing non-nitrate fraction around midday, in close agreement with aircraft observations. 

Abstract Ambient ozone (O₃) concentrations in Southeast Michigan (SEMI) can exceed the U.S. National Ambient Air Quality Standard. Despite past efforts to measure O₃precursors and elucidate reaction mechanisms, changing emission patterns and atmospheric composition in SEMI warrant new measurements and updated mechanisms to understand the causes of observed O₃exceedances. In this study, we examine the chemical drivers of O₃exceedances in SEMI, based on the Phase I MOOSE (Michigan‐Ontario Ozone Source Experiment) field study performed during May to June 2021. A zero‐dimensional (0‐D) box model is constrained with measurement data of meteorology and trace gas concentrations. Box model sensitivity simulations suggest that the formaldehyde to nitrogen dioxide ratio (HCHO/NO₂) for the transition between the volatile organic compounds (VOCs)‐ and nitrogen oxides (NO_x)‐limited O₃production regimes is 3.0 ± 0.3 in SEMI. The midday (12:00–16:00) averaged HCHO/NO₂ratio during the MOOSE Phase I study is 1.62 ± 1.03, suggesting that O₃production in SEMI is limited by VOC emissions. This finding implies that imposing stricter regulations on VOC emissions should be prioritized for the SEMI O₃nonattainment area. This study, through its use of ground‐based HCHO/NO₂ratios and box modeling to assess O₃‐VOC‐NO_xsensitivities, has significant implications for air quality policy and the design of effective O₃pollution control strategies, especially in O₃nonattainment areas. 

Abstract Aircraft observations have revealed ubiquitous new particle formation in the tropical upper troposphere over the Amazon^1,2and the Atlantic and Pacific oceans^3,4. Although the vapours involved remain unknown, recent satellite observations have revealed surprisingly high night-time isoprene mixing ratios of up to 1 part per billion by volume (ppbv) in the tropical upper troposphere⁵. Here, in experiments performed with the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber, we report new particle formation initiated by the reaction of hydroxyl radicals with isoprene at upper-tropospheric temperatures of −30 °C and −50 °C. We find that isoprene-oxygenated organic molecules (IP-OOM) nucleate at concentrations found in the upper troposphere, without requiring any more vapours. Moreover, the nucleation rates are enhanced 100-fold by extremely low concentrations of sulfuric acid or iodine oxoacids above 10⁵ cm⁻³, reaching rates around 30 cm⁻³ s⁻¹at acid concentrations of 10⁶ cm⁻³. Our measurements show that nucleation involves sequential addition of IP-OOM, together with zero or one acid molecule in the embryonic molecular clusters. IP-OOM also drive rapid particle growth at 3–60 nm h⁻¹. We find that rapid nucleation and growth rates persist in the presence of NO_xat upper-tropospheric concentrations from lightning. Our laboratory measurements show that isoprene emitted by rainforests may drive rapid new particle formation in extensive regions of the tropical upper troposphere^1,2, resulting in tens of thousands of particles per cubic centimetre. 

This study introduces an atmospheric pressure chemical ionization method that relies on low-energy thermal collisions (i.e., <0.05 eV) of aerosolized analytes with bipolar ions pre-seeded in a sample dilution flow and allows for the detection of weakly bound molecular clusters. Herein, the potential of the method is explored in the context of soot inception by performing mass spectrometric analysis of a laminar premixed flame of ethylene and air whose products are sampled through a tiny orifice and quickly diluted in nitrogen pre-flowed through a Kr85 based neutralizer to generate the bipolar ions. Analyses were performed with an Atmospheric Pressure Interface Time-of-Flight (APi-TOF, Tofwerk AG) Mass Spectrometer whose high sensitivity, mass accuracy, and resolution (over 4000) allowed for the discrimination of the flame products from the pre-seeded ions. Since ionization of neutrals occurs by either ion attachment or charge exchange following ion collision, the identification of the origin of each peak in the measured mass spectra is not-trivial. Nevertheless, the results provide valuable information on the overall elemental composition of the neutral flame products ionized in either polarity. Results show that the clustering of hydrocarbons lighter than 400 Da and having a C/H ratio between 2 and 3 leads to soot inception in the flame. The dehydrogenation of the flame products, expected to occur as they are convected in the flame, is observed only for measurements in positive polarity because of a higher probability of soot nuclei and precursors to get a positive rather than a negative charge. 

Abstract Emission factors (EFs) are crucial in understanding the effects of wildfire emissions on air quality. We examined the variability of EFs of three wildfires (Nethker, Castle, and 204 Cow) during the 2019 Western US wildfire season using the Aerodyne Mobile Laboratory (AML) and compared them to previous studies. The AML sampling captured the high degree of variability present in wildfires, and we report results for a range of combustion conditions that is more extensive than previous field and laboratory studies. For instance, we captured emissions from freshly started flaming fuels and we report rare EF measurements at very high modified combustion efficiencies (MCEs); MCEs >0.9. Differences in emissions between AML‐observed wildfires were attributed to burning state/MCE rather than fuel type. A comparison of EFs versus MCE was made and linear fits were compared to previous observations to reveal important differences that incorporate these high MCEs. For some species, there remains an EF dependence on MCE at these high values, while others reach a minimum value and exhibit either no or a weak dependence above it. EF differences were found for many of the studied compounds when comparing ground‐based and airborne observations, with generally greater airborne EFs possibly due to photochemical oxidation. The largest differences were from monoterpenes and acetaldehyde. Comparisons were made between AML‐observed wildfires, aircraft observations, and the values in literature for EFs and emission ratios, with mixed agreement due to the high degree of variability caused by differences in MCE. Differences in MCE drove the diurnal EF differences.