Significance: Dry deposition is a key sink of atmospheric particles, which impact human and ecosystem health, and the radiative balance of the planet. However, the deposition parameterizations used in climate and air-quality models are poorly constrained by observations. Dry deposition of submicron particles is the largest uncertainty in aerosol indirect radiative forcing. Our particle flux observations indicate that dry deposition velocities are an order of magnitude lower than models suggest. Our updated, observation-driven parameterizations should reduce uncertainty in modeled dry deposition. The scheme increases modeled accumulation mode aerosol number concentrations, and enhances the combined natural and anthropogenic aerosol indirect effect by −0.63 W m ⁻² , similar in magnitude to the total aerosol indirect forcing in the Intergovernmental Panel on Climate Change report.

Dry deposition of ozone is an important sink of ozone in near‐surface air. When dry deposition occurs through plant stomata, ozone can injure the plant, altering water and carbon cycling and reducing crop yields. Quantifying both stomatal and nonstomatal uptake accurately is relevant for understanding ozone's impact on human health as an air pollutant and on climate as a potent short‐lived greenhouse gas and primary control on the removal of several reactive greenhouse gases and air pollutants. Robust ozone dry deposition estimates require knowledge of the relative importance of individual deposition pathways, but spatiotemporal variability in nonstomatal deposition is poorly understood. Here we integrate understanding of ozone deposition processes by synthesizing research from fields such as atmospheric chemistry, ecology, and meteorology. We critically review methods for measurements and modeling, highlighting the empiricism that underpins modeling and thus the interpretation of observations. Our unprecedented synthesis of knowledge on deposition pathways, particularly soil and leaf cuticles, reveals process understanding not yet included in widely used models. If coordinated with short‐term field intensives, laboratory studies, and mechanistic modeling, measurements from a few long‐term sites would bridge the molecular to ecosystem scales necessary to establish the relative importance of individual deposition pathways and the extent to which they vary in space and time. Our recommended approaches seek to close knowledge gaps that currently limit quantifying the impact of ozone dry deposition on air quality, ecosystems, and climate.

Significance: Wildfire emissions in the western United States have had increasingly larger impacts on air quality, health, and climate forcing in recent decades. However, our understanding of how wildfire plume composition evolves remains incomplete. Particularly, the evolution of carbonaceous material, including fine particle mass (PM _2.5 ) and light-absorbing brown carbon, has remained elusive because of limited knowledge about the interplay between primary emissions and subsequent chemical and physical transformations that convert material between vapors and particles. Using a comprehensive analysis of in situ wildfire smoke measurements in combination with simulation chamber experiments, we quantitatively assess primary versus secondary organic particulate matter and brown carbon light absorption in authentic wildfires plumes. These results improve our fundamental knowledge of wildfire plume composition and evolution.

Despite increasing incidence of wildfires in the United States, wildfire smoke is poorly characterized, with little known about particle composition and emission rates. Chemistry in transported plumes confounds interpretation of ground and aircraft data, but near-field observations can potentially disentangle the effects of oxidation and dilution on aerosol mass and chemical composition. We report the organic aerosol (OA) emission ratios from aircraft observations near the fire source for the 20 wildfires sampled during the Western Wildfire Experiment: Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) study of summer 2018. We observe no changes in submicron non-refractory OA mass concentration, relative to CO which accounts for simple dilution, between 0.5 and up to 8 hours of aging. However, static OA excess mixing ratios hide shifts in the aerosol chemical composition that suggest near-balanced, simultaneous oxidation-driven condensation and dilution-driven evaporation. Specifically, we observe significant increases in the extent of oxidation, evident by an increase in oxidation marker f44, and loss of the biomass burning marker f60, as the smoke ages through chemistry and dilution. We discuss the competing effects of oxidative chemistry and dilution-driven evaporation on the evolution of the chemical composition of aerosols in wildfire smoke over time.

The House Observations of Microbial and Environmental Chemistry (HOMEChem) study is a collaborative field investigation designed to probe how everyday activities influence the emissions, chemical transformations and removal of trace gases and particles in indoor air. Sequential and layered experiments in a research house included cooking, cleaning, variable occupancy, and window-opening. This paper describes the overall design of HOMEChem and presents preliminary case studies investigating the concentrations of reactive trace gases, aerosol particles, and surface films. Cooking was a large source of VOCs, CO₂, NO_x, and particles. By number, cooking particles were predominantly in the ultrafine mode. Organic aerosol dominated the submicron mass, and, while variable between meals and throughout the cooking process, was dominated by components of hydrocarbon character and low oxygen content, similar to cooking oil. Air exchange in the house ensured that cooking particles were present for only short periods. During unoccupied background intervals, particle concentrations were lower indoors than outdoors. The cooling coils of the house ventilation system induced cyclic changes in water soluble gases. Even during unoccupied periods, concentrations of many organic trace gases were higher indoors than outdoors, consistent with housing materials being potential sources of these compounds to the outdoor environment. Organic material accumulated on indoor surfaces, and exhibited chemical signatures similar to indoor organic aerosol.