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Abstract. We present the Fire Inventory from National Center for Atmospheric Research (NCAR) version 2.5 (FINNv2.5), a fire emissions inventory that provides publicly available emissions of trace gases and aerosols for various applications, including use in global and regional atmospheric chemistry modeling. FINNv2.5 includes numerous updates to the FINN version 1 framework to better represent burned area, vegetation burned, and chemicals emitted. Major changes include the use of active fire detections from the Visible Infrared Imaging Radiometer Suite (VIIRS) at 375 m spatial resolution, which allows smaller fires to be included in the emissions processing. The calculation of burned area has been updated such that a more rigorous approach is used to aggregate fire detections, which better accounts for larger fires and enables using multiple satellite products simultaneously for emissions estimates. Fuel characterization and emissions factors have also been updated in FINNv2.5. Daily fire emissions for many trace gases and aerosols are determined for 2002–2019 (Moderate Resolution Imaging Spectroradiometer (MODIS)-only fire detections) and 2012–2019 (MODIS + VIIRS fire detections). The non-methane organic gas emissions are allocated to the species of several commonly used chemical mechanisms. We compare FINNv2.5 emissions against other widely used fire emissions inventories. The performance of FINNv2.5 emissions as inputs to a chemical transport model is assessed with satellite observations. Uncertainties in the emissions estimates remain, particularly in Africa and South America during August–October and in southeast and equatorial Asia in March and April. Recommendations for future evaluation and use are given.

 

Abstract. This study characterizes the impact of the Chesapeake Bay and associated meteorological phenomena on aerosol chemistry during the second Ozone Water-Land Environmental Transition Study (OWLETS-2) field campaign, which took place from 4 June to 5 July 2018. Measurements of inorganic PM2.5 composition, gas-phase ammonia (NH3), and an array of meteorological parameters were undertaken at Hart-Miller Island (HMI), a land–water transition site just east of downtown Baltimore on the Chesapeake Bay. The observations at HMI were characterized by abnormally high NH3 concentrations (maximum of 19.3 µg m−3, average of 3.83 µg m−3), which were more than a factor of 3 higher than NH3 levels measured at the closest atmospheric Ammonia Monitoring Network (AMoN) site (approximately 45 km away). While sulfate concentrations at HMI agreed quite well with those measured at a regulatory monitoring station 45 km away, aerosol ammonium and nitrate concentrations were significantly higher, due to the ammonia-rich conditions that resulted from the elevated NH3. The high NH3 concentrations were largely due to regional agricultural emissions, including dairy farms in southeastern Pennsylvania and poultry operations in the Delmarva Peninsula (Delaware–Maryland–Virginia). Reduced NH3 deposition during transport over the Chesapeake Bay likely contributed to enhanced concentrations at HMI compared to the more inland AMoN site. Several peak NH3 events were recorded, including the maximum NH3 observed during OWLETS-2, that appear to originate from a cluster of industrial sources near downtown Baltimore. Such events were all associated with nighttime emissions and advection to HMI under low wind speeds (< 1 m s−1) and stable atmospheric conditions. Our results demonstrate the importance of industrial sources, including several that are not represented in the emissions inventory, on urban air quality. Together with our companion paper, which examines aerosol liquid water and pH during OWLETS-2, we highlight unique processes affecting urban air quality of coastal cities that are distinct from continental locations. 

Abstract. Particle acidity (aerosol pH) is an important driver of atmospheric chemical processes and the resulting effects on human and environmentalhealth. Understanding the factors that control aerosol pH is critical when enacting control strategies targeting specific outcomes. This studycharacterizes aerosol pH at a land–water transition site near Baltimore, MD, during summer 2018 as part of the second Ozone Water-Land EnvironmentalTransition Study (OWLETS-2) field campaign. Inorganic fine-mode aerosol composition, gas-phase NH3 measurements, and all relevantmeteorological parameters were used to characterize the effects of temperature, aerosol liquid water (ALW), and composition on predictions ofaerosol pH. Temperature, the factor linked to the control of NH3 partitioning, was found to have the most significant effect on aerosol pHduring OWLETS-2. Overall, pH varied with temperature at a rate of −0.047 K−1 across all observations, though the sensitivity was−0.085 K−1 for temperatures > 293 K. ALW had a minor effect on pH, except at the lowest ALW levels(< 1 µg m−3), which caused a significant increase in aerosol acidity (decrease in pH). Aerosol pH was generally insensitive tocomposition (SO42-, SO42-:NH4+, total NH3 (Tot-NH3) = NH3 + NH4+), consistentwith recent studies in other locations. In a companion paper, the sources of episodic NH3 events (95th percentile concentrations,NH3 > 7.96 µg m−3) during the study are analyzed; aerosol pH was higher by only ∼ 0.1–0.2 pH unitsduring these events compared to the study mean. A case study was analyzed to characterize the response of aerosol pH to nonvolatile cations (NVCs)during a period strongly influenced by primary Chesapeake Bay emissions. Depending on the method used, aerosol pH was estimated to be either weakly(∼ 0.1 pH unit change based on NH3 partitioning calculation) or strongly (∼ 1.4 pH unit change based onISORROPIA thermodynamic model predictions) affected by NVCs. The case study suggests a strong pH gradient with size during the event and underscores the need to evaluate assumptions of aerosol mixing state applied to pH calculations. Unique features of this study, including the urban land–water transition site and the strong influence of NH3 emissions from both agricultural and industrial sources, add to the understanding of aerosol pH and its controlling factors in diverseenvironments. 

Cloud cycling plays a key role in the evolution of atmospheric particles and gases, producing secondary aerosol mass and transforming the optical properties and impacts of aerosols globally. In this study, bulk cloud water samples collected at Whiteface Mountain (Wilmington, NY) in the summer of 2017 were aerosolized, dried to 50% RH, and analyzed for the evaporative loss of water soluble organic carbon (WSOC) and for brown carbon (BrC) formation. Systematic WSOC evaporation occurred in all cloud water samples, while no evidence for drying induced BrC formation was observed. On average, 11% (±3%) of WSOC evaporated when the aerosolized cloud droplets were dried to 50% RH, though this represents a lower bound on the WSOC reversibly partitioned to clouds due to experimental constraints. To our knowledge, this represents the first direct measurements of organic evaporation from actual cloud water undergoing drying. Formate and acetate contributed 19%, on average, to the evaporated WSOC, while no oxalate evaporation occurred. GECKO-A model simulations were carried out to predict the production of WSOC compounds that reversibly partition to cloud water from photooxidation of an array of VOCs. The model results suggest that precursor VOC identity and oxidation regime (VOC:NO x ) have a dramatic effect on the reversible partitioning of WSOC to cloud water and the abundance of aqSOA precursors, though the higher abundance of reversibly partitioned WSOC predicted by the model may be due to aqueous production of low-volatility material in the actual cloud samples. This study underscores the importance of the large fraction of unidentified compounds that contribute to WSOC in cloud water and their aqueous processing. 

Inorganic salts present in the atmosphere may affect the composition and abundance of secondary organic aerosol. Here, we quantify the effects of salt identity, salt concentration (ionic strength), and solution pH on the partitioning of ambient water‐soluble organic gases (WSOC_g) at a site in the eastern United States. The experimental pH (pH = 1–6) and ionic strengths (10⁻³–10¹ mol kg⁻¹) span a wide range of conditions found in atmospheric particles, clouds, and fog droplets. Chloride salts (NaCl, NH₄Cl, and KCl) exhibit a strong salting‐out effect at all ionic strengths >0.005 mol kg⁻¹and pH = 1.8–6. In contrast, sulfate salts (Na₂SO₄, (NH₄)₂SO₄, and K₂SO₄) induce both salting‐in and salting‐out behaviors, depending on ionic strength and pH. These results suggest that monovalent cations have minimal effect, while ionic strength, pH, and anion identity exert strong effects on the partitioning of ambient organic gases in the atmosphere.