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.

Accurate estimates for North American background (NAB) ozone (O 3 ) in surface air over the United States are needed for setting and implementing an attainable national O 3 standard. These estimates rely on simulations with atmospheric chemistry-transport models that set North American anthropogenic emissions to zero, and to date have relied heavily on one global model. We examine, for the first time, NAB estimates for spring and summer 2006 with two independent global models (GEOS-Chem and GFDL AM3). where it correlates with observed O 3 . At these sites, a 27-year GFDL AM3 simulation simulates observed O 3 events above 60 ppb and indicates a role for year-to-year variations in NAB O 3 in driving their frequency (contributing 50-60 ppb or more during some events). During summer over the eastern United States (EUS), when photochemical production from regional anthropogenic emissions peaks, NAB is largely uncorrelated with observed values and it is lower than at high-altitude sites (average values of ~20-30 ppb). We identify four processes that contribute substantially to model differences in specific regions and seasons: lightning NO x , biogenic isoprene emissions and chemistry, wildfires, and stratosphere-to-troposphere transport. Differences in model representation of these processes contribute more to uncertainty in NAB estimates than the choice of horizontal resolution within a single model. We propose that future efforts seek to constrain these processes with targeted analysis of multi-model simulations evaluated with observations of O 3 and related species from multiple platforms, and thereby reduce the error on NAB estimates needed for air quality planning.

The ozone (O3) dry depositional sink and its contribution to observed variability in tropospheric O3 are both poorly understood. Distinguishing O3 uptake through plant stomata versus other pathways is relevant for quantifying the O3 influence on carbon and water cycles. We use a decade of O3, carbon, and energy eddy covariance (EC) fluxes at Harvard Forest to investigate interannual variability (IAV) in O3 deposition velocities ( vd,O3). In each month, monthly mean vd,O3 for the highest year is twice that for the lowest. Two independent stomatal conductance estimates, based on either water vapor EC or gross primary productivity, vary little from year to year relative to canopy conductance. We conclude that nonstomatal deposition controls the substantial observed IAV in summertime vd,O3 during the 1990s over this deciduous forest. The absence of obvious relationships between meteorology and vd,O3 implies a need for additional long‐term, high‐quality measurements and further investigation of nonstomatal mechanisms.

Changing emissions can alter the surface O 3 seasonal cycle, as detected from northeastern U.S. (NE) observations during recent decades. Under continued regional precursor emission controls (>80% decreases in NE NO x by 2100), the NE surface O 3 seasonal cycle reverses (to a winter maximum) in 21st century transient chemistryclimate simulations. Over polluted regions, regional NO x largely controls the shape of surface O 3 seasonal cycles. In the absence of regional NO x controls, climate warming contributes to a higher surface O 3 summertime peak over the NE. A doubling of the global CH 4 abundance by 2100 partially offsets summertime surface O 3 decreases attained via NO x reductions and contributes to raising surface O 3 during December-March when the O 3 lifetime is longer. The similarity between surface O 3 seasonal cycles over the NE and the Intermountain West by 2100 indicates a NE transition to a region representative of baseline surface O 3 conditions.