Abstract
Abstract. In a companion paper, we introduced the Chemistry of Atmosphere-Forest Exchange (CAFE) model, a vertically-resolved 1-D chemical transport model designed to probe the details of near-surface reactive gas exchange. Here, we apply CAFE to noontime observations from the 2007 Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX-2007). In this work we evaluate the CAFE modeling approach, demonstrate the significance of in-canopy chemistry for forest-atmosphere exchange and identify key shortcomings in the current understanding of intra-canopy processes. CAFE generally reproduces BEARPEX-2007 observations but requires an enhanced radical recycling mechanism to overcome a factor of 6 underestimate of hydroxyl (OH) concentrations observed during a warm (~29 °C) period. Modeled fluxes of acyl peroxy nitrates (APN) are quite sensitive to gradients in chemical production and loss, demonstrating that chemistry may perturb forest-atmosphere exchange even when the chemical timescale is long relative to the canopy mixing timescale. The model underestimates peroxy acetyl nitrate (PAN) fluxes by 50% and the exchange velocity by nearly a factor of three under warmer conditions, suggesting that near-surface APN sinks are underestimated relative to the sources. Nitric acid typically dominates gross dry N deposition at this site, though other reactive nitrogen (NOy) species can comprise up to 28% of the N deposition budget under cooler conditions. Upward NO2 fluxes cause the net above-canopy NOy flux to be ~30% lower than the gross depositional flux. CAFE under-predicts ozone fluxes and exchange velocities by ~20%. Large uncertainty in the parameterization of cuticular and ground deposition precludes conclusive attribution of non-stomatal fluxes to chemistry or surface uptake. Model-measurement comparisons of vertical concentration gradients for several emitted species suggests that the lower canopy airspace may be only weakly coupled with the upper canopy. Future efforts to model forest-atmosphere exchange will require a more mechanistic understanding of non-stomatal deposition and a more thorough characterization of in-canopy mixing processes.
Highlights
Forest-atmosphere exchange of hydrocarbons, ozone, oxidized nitrogen and other reactive species impacts both atmospheric composition and ecosystem productivity, with broad implications for air quality and climate (Goldstein et al, 2009; Isaksen et al, 2009; Fowler et al, 2009; Erisman et al, 1998)
The hot and cool periods are representative of the general meteorological trend observed during Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX)-2007, characterized by a hot and dry August followed by a sharp transition to cooler, more humid conditions in September (Bouvier-Brown et al, 2009a; Wolfe et al, 2009); neither period is representative of the extreme conditions sampled during the campaign
The hot period is typified by relatively high concentrations of biogenic VOC (BVOC), HOx, O3 and oxygenated hydrocarbons and lower levels of NO2 and acyl peroxy nitrates (APN = peroxy acetyl nitrate (PAN) + PPN + MPAN + . . . ); cold period data demonstrate the opposite trends
Summary
Forest-atmosphere exchange of hydrocarbons, ozone, oxidized nitrogen and other reactive species impacts both atmospheric composition and ecosystem productivity, with broad implications for air quality and climate (Goldstein et al, 2009; Isaksen et al, 2009; Fowler et al, 2009; Erisman et al, 1998). Recent work has indicated that the air within and just above the canopy is highly oxidizing during the daytime (Farmer and Cohen, 2008; Holzinger et al, 2005; Lelieveld et al, 2008). This oxidative photochemistry affects the net biosphere-atmosphere exchange of many species. As a substantial in-canopy sink for oxidants like ozone (O3), this chemistry could contribute to downward O3 fluxes (Goldstein et al, 2004; Kurpius and Goldstein, 2003; Fares et al, 2010a)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
