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Wildfire emissions affect downwind air quality and human health. Predictions of these impacts using models are limited by uncertainties in emissions and chemical evolution of smoke plumes. Using high‐time‐resolution aircraft measurements, we illustrate spatial variations that can exist within a plume due to differences in the photochemical environment. Horizontal and vertical crosswind gradients of dilution‐corrected mixing ratios were observed in midday plumes for reactive compounds and their oxidation products, such as nitrous acid, catechol, and ozone, likely due to faster photochemistry in optically thinner plume edges relative to darker plume cores. Gradients in plumes emitted close to sunset are characterized by titration of O₃in the plume and reduced or no gradient formation. We show how crosswind gradients can lead to underestimated emission ratios for reactive compounds and overestimated emission ratios for oxidation products. These observations will lead to improved predictions of wildfire emissions, evolution, and impacts across daytime and nighttime.

Small cumulus clouds over the western United States were measured via airborne instruments during the wildfire season in summer of 2018. Statistics of the sampled clouds are presented and compared to smoke aerosol properties. Cloud droplet concentrations were enhanced in regions impacted by biomass burning smoke, at times exceeding 3,000 cm⁻³. Images and elemental composition of individual smoke particles and cloud droplet residuals are presented and show that most are dominantly organic, internally mixed with some inorganic elements. Despite their high organic content and relatively low hygroscopicity, on average about half of smoke aerosol particles >80 nm diameter formed cloud droplets. This reduced cloud droplet size in small, smoke‐impacted clouds. A number of complex and competing climatic impacts may result from wide‐spread reductions in cloud droplet size due to wildfires prevalent across the region during summer months.

Cloud condensation nuclei (CCN) are mediators of aerosol‐cloud interactions, which contribute to the largest uncertainty in climate change prediction. Here, we present a machine learning (ML)/artificial intelligence (AI) model that quantifies CCN from model‐simulated aerosol composition, atmospheric trace gas, and meteorological variables. Comprehensive multi‐campaign airborne measurements, covering varied physicochemical regimes in the troposphere, confirm the validity of and help probe the inner workings of this ML model: revealing for the first time that different ranges of atmospheric aerosol composition and mass correspond to distinct aerosol number size distributions. ML extracts this information, important for accurate quantification of CCN, additionally from both chemistry and meteorology. This can provide a physicochemically explainable, computationally efficient, robust ML pathway in global climate models that only resolve aerosol composition; potentially mitigating the uncertainty of effective radiative forcing due to aerosol‐cloud interactions (ERF_aci) and improving confidence in assessment of anthropogenic contributions and climate change projections.

Wildfires in the western United States are large sources of particulate matter, and the area burned by wildfires is predicted to increase in the future. Some particles released from wildfires can affect cloud formation by serving as ice‐nucleating particles (INPs). INPs have numerous impacts on cloud radiative properties and precipitation development. Wildfires are potentially important sources of INPs, as indicated from previous measurements, but their abundance in the free troposphere has not been quantified. The Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen campaign sampled free tropospheric immersion‐freezing INPs from smoke plumes near their source and downwind, along with widespread aged smoke. The results indicate an enhancement of INPs in smoke plumes relative to out‐of‐plume background air, but the magnitude of enhancement was both temperature and fire dependent. The majority of INPs were inferred to be predominately organic in composition with some contribution from biological sources at modest super cooling, and contributions from minerals at deeper super cooling. A fire involving primarily sagebrush shrub land and aspen forest fuels had the highest INP concentrations measured in the campaign, which is partially attributed to the INP characteristics of lofted, uncombusted plant material. Electron microscopy analysis of INPs also indicated tar balls present in this fire. Parameterization of the plume INP data on a per‐unit‐aerosol surface area basis confirmed that smoke is not an efficient source of INPs. Nevertheless, the high numbers of particles released from, and ubiquity of western US wildfires in summertime, regionally elevate INP concentrations in the free troposphere.

Wildfires are a major source of gas‐phase ammonia (NH₃) to the atmosphere. Quantifying the evolution and fate of this NH₃is important to understanding the formation of secondary aerosol in smoke and its accompanying effects on radiative balance and nitrogen deposition. Here, we use data from the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) to add new empirical constraints on the e‐folding loss timescale of NH₃and its relationship with particulate ammonium (pNH₄) within wildfire smoke plumes in the western U.S. during summer 2018. We show that the e‐folding loss timescale of NH₃with respect to particle‐phase partitioning ranges from ∼24 to ∼4000 min (median of 55 min). Within these same plumes, oxidation of nitrogen oxides is observed concurrent with increases in the fraction ofpNH₄in each plume sampled, suggesting that formation of ammonium nitrate (NH₄NO₃) is likely. We find wide variability in how close ourin situmeasurements of NH₄NO₃are to those expected in a dry thermodynamic equilibrium, and find that NH₄NO₃is most likely to form in fresh, dense smoke plumes injected at higher altitudes and colder temperatures. In chemically older smoke we observe correlations between both the fraction ofpNH₄and the fraction of particulate nitrate (pNO₃) in the aerosol with temperature, providing additional evidence of the presence of NH₄NO₃and the influence of injection height on gas‐particle partitioning of NH₃.

The Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) deployed the NSF/NCAR C‐130 aircraft in summer 2018 across the western U.S. to sample wildfire smoke during its first days of atmospheric evolution. We present a summary of a subset of reactive oxidized nitrogen species (NO_y) in plumes sampled in a pseudo‐Lagrangian fashion. Emissions of nitrogen oxides (NO_x = NO + NO₂) and nitrous acid (HONO) are rapidly converted to more oxidized forms. Within 4 h, ∼86% of the ΣNO_yis in the form of peroxy acyl nitrates (PANs) (∼37%), particulate nitrate (pNO₃) (∼27%), and gas‐phase organic nitrates (Org N_(g)) (∼23%). The averagee‐folding time and distance for NO_xare ∼90 min and ∼40 km, respectively. Nearly no enhancements in nitric acid (HNO₃) were observed in plumes sampled in a pseudo‐Lagrangian fashion, implying HNO₃‐limited ammonium nitrate (NH₄NO₃) formation, with one notable exception that we highlight as a case study. We also summarize the observed partitioning of NO_yin all the smoke samples intercepted during WE‐CAN. In smoke samples intercepted above 3 km above sea level (ASL), the contributions of PANs andpNO₃to ΣNO_yincrease with altitude. WE‐CAN also sampled smoke from multiple fires mixed with anthropogenic emissions over the California Central Valley. We distinguish samples where anthropogenic NO_xemissions appear to lead to an increase in NO_xabundances by a factor of four and contribute to additional PAN formation.

Reactive nitrogen (N_r) within smoke plumes plays important roles in the production of ozone, the formation of secondary aerosols, and deposition of fixed N to ecosystems. The Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) field campaign sampled smoke from 23 wildfires throughout the western U.S. during summer 2018 using the NSF/NCAR C‐130 research aircraft. We empirically estimateN_rnormalized excess mixing ratios and emission factors from fires sampled within 80 min of estimated emission and explore variability in the dominant forms ofN_rbetween these fires. We find that reduced N compounds comprise a majority (39%–80%; median = 66%) of total measured reactive nitrogen (ΣN_r) emissions. The smoke plumes sampled during WE‐CAN feature rapid chemical transformations after emission. As a result, within minutes after emission total measured oxidized nitrogen (ΣNO_y) and measured totalΣNH_x(NH₃ + pNH₄) are more robustly correlated with modified combustion efficiency (MCE) than NO_xand NH₃by themselves. The ratio of ΣNH_x/ΣNO_ydisplays a negative relationship with MCE, consistent with previous studies. A positive relationship with total measuredΣN_rsuggests that both burn conditions and fuel N content/volatilization differences contribute to the observed variability in the distribution of reduced and oxidizedN_r. Additionally, we compare our in situ field estimates ofN_rEFs to previous lab and field studies. For similar fuel types, we findΣNH_xEFs are of the same magnitude or larger than lab‐based NH₃EF estimates, andΣNO_yEFs are smaller than lab NO_xEFs.