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Airborne and ground-based measurements of aerosol concentrations, chemical composition, and gas-phase precursors were obtained in three valleys in northern Utah (USA). The measurements were part of the Utah Winter Fine Particulate Study (UWFPS) that took place in January–February 2017. Total aerosol mass concentrations of PM1 were measured from a Twin Otter aircraft, with an aerosol mass spectrometer (AMS). PM1 concentrations ranged from less than 2 µg m−3 during clean periods to over 100 µg m−3 during the most polluted episodes, consistent with PM2.5 total mass concentrations measured concurrently at ground sites. Across the entire region, increases in total aerosol mass above ∼2 µg m−3 were associated with increases in the ammonium nitrate mass fraction, clearly indicating that the highest aerosol mass loadings in the region were predominantly attributable to an increase in ammonium nitrate. The chemical composition was regionally homogenous for total aerosol mass concentrations above 17.5 µg m−3, with 74±5 % (average ± standard deviation) ammonium nitrate, 18±3 % organic material, 6±3 % ammonium sulfate, and 2±2 % ammonium chloride. Vertical profiles of aerosol mass and volume in the region showed variable concentrations with height in the polluted boundary layer. Higher average mass concentrations were observed within the first few hundred meters above ground level in all three valleys during pollution episodes. Gas-phase measurements of nitric acid (HNO3) and ammonia (NH3) during the pollution episodes revealed that in the Cache and Utah valleys, partitioning of inorganic semi-volatiles to the aerosol phase was usually limited by the amount of gas-phase nitric acid, with NH3 being in excess. The inorganic species were compared with the ISORROPIA thermodynamic model. Total inorganic aerosol mass concentrations were calculated for various decreases in total nitrate and total ammonium. For pollution episodes, our simulations of a 50 % decrease in total nitrate lead to a 46±3 % decrease in total PM1 mass. A simulated 50 % decrease in total ammonium leads to a 36±17 % µg m−3 decrease in total PM1 mass, over the entire area of the study. Despite some differences among locations, our results showed a higher sensitivity to decreasing nitric acid concentrations and the importance of ammonia at the lowest total nitrate conditions. In the Salt Lake Valley, both HNO3 and NH3 concentrations controlled aerosol formation.

Biomass burning aerosol is a major source of PM2.5, and significantly affects Earth's radiative budget. The magnitude of its radiative effect is poorly quantified due to uncertainty in the optical properties of aerosol formed from biomass burning. Using a broadband cavity-enhanced spectrometer with a recently increased spectral range (360–720 nm) coupled to a size-selecting aerosol inlet, we retrieve complex refractive indices of aerosol throughout the near-ultraviolet and visible spectral region. We demonstrate refractive index retrievals for two standard aerosol samples: polystyrene latex spheres and ammonium sulfate. We then retrieve refractive indices for biomass burning aerosol from 13 controlled fires during the 2016 Missoula Fire Science Laboratory Study. We demonstrate that the technique is highly sensitive to the accuracy of the aerosol size distribution method and find that while we can constrain the optical properties of brown carbon aerosol for many fires, fresh smoke dominated by fractal-like black carbon aerosol presents unique challenges and is not well-represented by Mie theory. For the 13 fires, we show that the accuracy of Mie theory retrievals decreases as the fraction of black carbon mass increases. At 475 nm, the average refractive index is 1.635 (±0.056) +0.06 (±0.12)i, and at 365 nm, the average refractive index is 1.605 (±0.041) +0.038 (±0.074)i.

We describe the design and performance of a lightweight broadband cavity-enhanced spectrometer for measurement of NO2 on uncrewed aerial vehicles and light aircraft. The instrument uses a light-emitting diode (LED) centered at 457 nm, high-finesse mirrors (reflectivity =0.999963 at 450 nm), and a grating spectrometer to determine optical extinction coefficients between 430 and 476 nm, which are fit with custom spectral fitting software and published absorption cross sections. The instrument weighs 3.05 kg and has a power consumption of less than 35 W at 25 ∘C. A ground calibration unit provides helium and zero air flows to periodically determine the reflectivity of the cavity mirrors using known Rayleigh scattering cross sections. The precision (1σ) for laboratory measurements is 43 ppt NO2 in 1 s and 7 ppt NO2 in 30 s. Measurement of air with known NO2 mixing ratios in the range of 0–70 ppb agreed with the known values within 0.3 % (slope =0.997±0.007; r2=0.99983). We demonstrate instrument performance using vertical profiles of the NO2 mixing ratio acquired on board an uncrewed aerial vehicle between 0 and 110 m above ground level in Boulder, Colorado.

Atmospheric HONO mixing ratios in indoor and outdoor environments span a range of less than a few parts per trillion by volume (pptv) up to tens of parts per billion by volume (ppbv) in combustion plumes. Previous HONO calibration sources have utilized proton transfer acid displacement from nitrite salts or solutions, with output that ranges from tens to thousands of ppbv. Instrument calibrations have thus required large dilution flows to obtain atmospherically relevant mixing ratios. Here we present a simple universal source to reach very low HONO calibration mixing ratios using a nitrite-coated reaction device with the addition of humid air and/or HCl from a permeation device. The calibration source developed in this work can generate HONO across the atmospherically relevant range and has high purity (> 90 %), reproducibility, and tunability. Mixing ratios at the tens of pptv level are easily reached with reasonable dilution flows. The calibration source can be assembled to start producing stable HONO mixing ratios (relative standard error, RSE ≤ 2 %) within 2 h, with output concentrations varying ≤ 25 % following simulated transport or complete disassembly of the instrument and with ≤ 10 % under ideal conditions. The simplicity of this source makes it highly versatile for field and lab experiments. The platform facilitates a new level of accuracy in established instrumentation, as well as intercomparison studies to identify systematic HONO measurement bias and interferences.

Particle morphology is an important parameter affecting aerosol optical properties that are relevant to climate and air quality, yet it is poorly constrained due to sparse in situ measurements. Biomass burning is a large source of aerosol that generates particles with different morphologies. Quantifying the optical contributions of non-spherical aerosol populations is critical for accurate radiative transfer models, and for correctly interpreting remote sensing data. We deployed a laser imaging nephelometer at the Missoula Fire Sciences Laboratory to sample biomass burning aerosol from controlled fires during the FIREX intensive laboratory study. The laser imaging nephelometer measures the unpolarized scattering phase function of an aerosol ensemble using diode lasers at 375 and 405 nm. Scattered light from the bulk aerosol in the instrument is imaged onto a charge-coupled device (CCD) using a wide-angle field-of-view lens, which allows for measurements at 4–175∘ scattering angle with ∼ 0.5∘ angular resolution. Along with a suite of other instruments, the laser imaging nephelometer sampled fresh smoke emissions both directly and after removal of volatile components with a thermodenuder at 250 ∘C. The total integrated aerosol scattering signal agreed with both a cavity ring-down photoacoustic spectrometer system and a traditional integrating nephelometer within instrumental uncertainties. We compare the measured scattering phase functions at 405 nm to theoretical models for spherical (Mie) and fractal (Rayleigh–Debye–Gans) particle morphologies based on the size distribution reported by an optical particle counter. Results from representative fires demonstrate that particle morphology can vary dramatically for different fuel types. In some cases, the measured phase function cannot be described using Mie theory. This study demonstrates the capabilities of the laser imaging nephelometer instrument to provide realtime, in situ information about dominant particle morphology, which is vital for understanding remote sensing data and accurately describing the aerosol population in radiative transfer calculations.

The sum of all reactive nitrogen species (NOy) includes NOx (NO2 + NO) and all of its oxidized forms, and the accurate detection of NOy is critical to understanding atmospheric nitrogen chemistry. Thermal dissociation (TD) inlets, which convert NOy to NO2 followed by NO2 detection, are frequently used in conjunction with techniques such as laser-induced fluorescence (LIF) and cavity ring-down spectroscopy (CRDS) to measure total NOy when set at > 600 °C or speciated NOy when set at intermediate temperatures. We report the conversion efficiency of known amounts of several representative NOy species to NO2 in our TD-CRDS instrument, under a variety of experimental conditions. We find that the conversion efficiency of HNO3 is highly sensitive to the flow rate and the residence time through the TD inlet as well as the presence of other species that may be present during ambient sampling, such as ozone (O3). Conversion of HNO3 at 400 °C, nominally the set point used to selectively convert organic nitrates, can range from 2 to 6 % and may represent an interference in measurement of organic nitrates under some conditions. The conversion efficiency is strongly dependent on the operating characteristics of individual quartz ovens and should be well calibrated prior to use in field sampling. We demonstrate quantitative conversion of both gas-phase N2O5 and particulate ammonium nitrate in the TD inlet at 650 °C, which is the temperature normally used for conversion of HNO3. N2O5 has two thermal dissociation steps, one at low temperature representing dissociation to NO2 and NO3 and one at high temperature representing dissociation of NO3, which produces exclusively NO2 and not NO. We also find a significant interference from partial conversion (5–10 %) of NH3 to NO at 650 °C in the presence of representative (50 ppbv) levels of O3 in dry zero air. Although this interference appears to be suppressed when sampling ambient air, we nevertheless recommend regular characterization of this interference using standard additions of NH3 to TD instruments that convert reactive nitrogen to NO or NO2.

Mountain basins in Northern Utah, including the Salt Lake Valley (SLV), suffer from wintertime air pollution events associated with stagnant atmospheric conditions. During these events, fine particulate matter concentrations (PM2.5) can exceed national ambient air quality standards. Previous studies in the SLV have found that PM2.5 is primarily composed of ammonium nitrate (NH4NO3), formed from the condensation of gas-phase ammonia (NH3) and nitric acid (HNO3). Additional studies in several western basins, including the SLV, have suggested that production of HNO3 from nocturnal heterogeneous N2O5 uptake is the dominant source of NH4NO3 during winter. The rate of this process, however, remains poorly quantified, in part due to limited vertical measurements above the surface, where this chemistry is most active. The 2017 Utah Winter Fine Particulate Study (UWFPS) provided the first aircraft measurements of detailed chemical composition during wintertime pollution events in the SLV. Coupled with ground-based observations, analyses of day- and nighttime research flights confirm that PM2.5 during wintertime pollution events is principally composed of NH4NO3, limited by HNO3. Here, observations and box model analyses assess the contribution of N2O5 uptake to nitrate aerosol during pollution events using the NO3- production rate, N2O5 heterogeneous uptake coefficient (γ(N2O5)), and production yield of ClNO2 (φ(ClNO2)), which had medians of 1.6 µg m−3 h−1, 0.076, and 0.220, respectively. While fit values of γ(N2O5) may be biased high by a potential under-measurement in aerosol surface area, other fit quantities are unaffected. Lastly, additional model simulations suggest nocturnal N2O5 uptake produces between 2.4 and 3.9 µg m−3 of nitrate per day when considering the possible effects of dilution. This nocturnal production is sufficient to account for 52 %–85 % of the daily observed surface-level buildup of aerosol nitrate, though accurate quantification is dependent on modeled dilution, mixing processes, and photochemistry.

Wildfires are an important source of carbonaceous aerosol in the atmosphere. Organic aerosol that absorbs light in the ultraviolet to visible spectral range is referred to as brown carbon (BrC), and its impact on Earth's radiative budget has not been well characterized. We collected water-soluble brown carbon using a particle-into-liquid sampler (PILS) on board a Twin Otter aircraft during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign. Samples were collected downwind of wildfires in the western United States from August to September 2019. We applied size-exclusion chromatography (SEC) with ultraviolet-visible spectroscopy to characterize the molecular size distribution of BrC chromophores. The wildfire plumes had transport ages of 0 to 5 h, and the absorption was dominated by chromophores with molecular weights <500 Da. With BrC normalized to a conserved biomass burning tracer, carbon monoxide, a consistent decrease in BrC absorption with plume age was not observed during FIREX-AQ. These findings are consistent with the variable trends in BrC absorption with plume age reported in recent studies. While BrC absorption trends were broadly consistent between the offline SEC analysis and the online PILS measurements, the absolute values of absorption and their spectral dependence differed. We investigate plausible explanations for the discrepancies observed between the online and offline analyses. This included solvent effects, pH, and sample storage. We suspect that sample storage impacted the absorption intensity of the offline measurements without impacting the molecular weight distribution of BrC chromophores.

Ozone profile measurements at high temporal and vertical resolution are needed to better understand physical and chemical processes driving tropospheric ozone variability and to validate the tropospheric ozone measurements from spaceborne missions such as TEMPO (Tropospheric Emissions: Monitoring Pollution). As part of the Tropospheric Ozone Lidar Network (TOLNet) efforts allocated to provide such measurements and leveraging on the experience of more than 20 years of ozone lidar measurements at Table Mountain Facility, the JPL lidar group developed the SMOL (Small Mobile Ozone Lidar), an affordable differential absorption lidar (DIAL) system covering all altitudes from 200 to 10 km above ground level (a.g.l.). The transmitter is based on a quadrupled Nd:YAG laser, which is further converted into a 289/299 nm wavelength pair using Raman shifting cells, and the receiver consists of three ozone DIAL pairs, including one that is 266/289 nm and two that are 289/299 nm. Two units were deployed in the Los Angeles basin area during the Synergistic TEMPO Air Quality Science (STAQS) and Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) campaigns in summer 2023. The comparison with airborne in situ and lidar measurements shows very good agreement, with systematic differences below 10 % throughout most of the measurement range. An additional comparison with nearby surface ozone measuring instruments indicates unbiased measurements by the SMOL lidars down to 200 m a.g.l. Further comparison with the Goddard Earth Observing System Composition Forecast (GEOS-CF) model suggests that such lidars are a critical tool to perform model validation and can potentially be used for assimilation to air quality forecasts.

Ammonium-containing aerosols are a major component of wintertime air pollution in many densely populated regions around the world. Especially in mountain basins, the formation of persistent cold-air pools (PCAPs) can enhance particulate matter with diameters less than 2.5 µm (PM2.5) to levels above air quality standards. Under these conditions, PM2.5 in the Great Salt Lake region of northern Utah has been shown to be primarily composed of ammonium nitrate; however, its formation processes and sources of its precursors are not fully understood. Hence, it is key to understanding the emission sources of its gas phase precursor, ammonia (NH3). To investigate the formation of ammonium nitrate, a suite of trace gases and aerosol composition were sampled from the NOAA Twin Otter aircraft during the Utah Winter Fine Particulate Study (UWFPS) in January and February 2017. NH3 was measured using a quantum cascade tunable infrared laser differential absorption spectrometer (QC-TILDAS), while aerosol composition, including particulate ammonium (pNH4), was measured with an aerosol mass spectrometer (AMS). The origin of the sampled air masses was investigated using the Stochastic Time-Inverted Lagrangian Transport (STILT) model and combined with an NH3 emission inventory to obtain model-predicted NHx (=NH3+pNH4) enhancements. Enhancements represent the increase in NH3 mixing ratios within the last 24 h due to emissions within the model footprint. Comparison of these NHx enhancements with measured NHx from the Twin Otter shows that modelled values are a factor of 1.6 to 4.4 lower for the three major valleys in the region. Among these, the underestimation is largest for Cache Valley, an area with intensive agricultural activities. We find that one explanation for the underestimation of wintertime emissions may be the seasonality factors applied to NH3 emissions from livestock. An investigation of inter-valley exchange revealed that transport of NH3 between major valleys was limited and PM2.5 in Salt Lake Valley (the most densely populated area in Utah) was not significantly impacted by NH3 from the agricultural areas in Cache Valley. We found that in Salt Lake Valley around two thirds of NHx originated within the valley, while about 30 % originated from mobile sources and 60 % from area source emissions in the region. For Cache Valley, a large fraction of NOx potentially leading to PM2.5 formation may not be locally emitted but mixed in from other counties.