Spatiotemporal Controls on Observed Daytime Ozone Deposition Velocity Over Northeastern U.S. Forests During Summer

Abstract

Spatiotemporal variability in ozone dry deposition is often overlooked despite its implications for interpreting and modeling tropospheric ozone concentrations accurately. Understanding the influences of stomatal versus nonstomatal deposition processes on ozone deposition velocity is important for attributing observed changes in the ozone depositional sink and associated damage to ecosystems. Here, we aim to identify the stomatal versus nonstomatal deposition processes driving observed variability in ozone deposition velocity over the northeastern United States during June–September. We use ozone eddy covariance measurements from Harvard Forest in Massachusetts, which span a decade, and from Kane Experimental Forest in Pennsylvania and Sand Flats State Forest in New York, which span one growing season each, along with observation‐driven modeling. Using a cumulative precipitation indicator of soil wetness, we infer that high soil uptake during dry years and low soil uptake during wet years may contribute to the twofold interannual variability in ozone deposition velocity at Harvard Forest. We link stomatal deposition and humidity to variability in ozone deposition velocity on daily timescales. The humidity dependence may reflect higher uptake by leaf cuticles under humid conditions, noted in previous work. Previous work also suggests that uptake by leaf cuticles may be enhanced after rain, but we find that increases in ozone deposition velocity on rainy days are instead mostly associated with increases in stomatal conductance. Our analysis highlights a need for constraints on subseasonal variability in ozone dry deposition to soil and fast in‐canopy chemistry during ecosystem stress.

Sensitivity of Ozone Dry Deposition to Ecosystem‐Atmosphere Interactions: A Critical Appraisal of Observations and Simulations

“…Our modeling analysis, as supported by multiple observational data sets, indicates that limitation in stomatal conductance due to soil moisture deficits (as opposed to LAI changes) is the primary driver of V d,O3 variability during large‐scale drought events. On basis of data over one forest site, Clifton et al () notice enhanced non‐stomatal O 3 deposition in dry years associated with greater soil uptake; this effect is unlikely to influence observed V d,O3 variability during sub‐continental to continental scale drought events.…”

Dry Deposition of Ozone Over Land: Processes, Measurement, and Modeling

“…Several field studies report increases in inferred nonstomatal uptake over vegetation with relative humidity, providing evidence that aqueous surface chemistry on leaves may be important at ecosystem scales (Altimir et al, 2006;Clifton et al, 2019;Lamaud et al, 2009;Q. Li, Gabay et al, 2018;Neirynck & Verstraeten, 2018;Rannik et al, 2012;L.…”

“…While there are fairly consistent increases in v d over vegetation after rain and dew across field studies (Table 2), whether observed responses truly indicate changes in cuticular uptake remains an open question. For example, there may be changes in g s after rain (e.g., Clifton et al, 2019) or emissions of highly reactive species that influence the observed ozone flux (e.g., Altimir et al, 2006;Clifton et al, 2019;Turnipseed et al, 2009).…”

“…Massman (2004) compiles resistances to ozone dry deposition to soil (r soil ) inferred from observations, suggesting 100 s m −1 for dry soil and 500 s m −1 for wet soil. Later site-specific work defines similar empirical models (Bassin et al, 2004;Clifton et al, 2019;Fares et al, 2012Fares et al, , 2014Mészáros, Horváth, et al, 2009), but studies more directly isolating r soil support stronger dependencies on soil moisture (Fumagalli et al, 2016) or surface relative humidity Stella et al, 2019). Whether simple models accurately capture r soil magnitude and variability on a variety of scales is uncertain.…”

“…Future work should evaluate model performance not only in terms of climatological seasonal and diel cycles of v d but also spatiotemporal variability and relationships with meteorological and biophysical parameters. For example, observations suggesting a weak dependence on LAI (Clifton et al, 2017(Clifton et al, , 2019Mahrt et al, 1995;Wolfe et al, 2015) may imply the strong LAI sensitivity in many models (Charusombat et al, 2010;Schwede et al, 2011;Silva & Heald, 2018) is exaggerated. However, whether observations generally suggest a weak dependence on vegetation density is unknown; some work suggests a strong dependence (e.g., MacPherson et al, 1995).…”