Influence of meteorology and anthropogenic pollution on chemical flux divergence of the NO–NO<sub>2</sub>–O<sub>3</sub> triad above and within a natural grassland canopy

Abstract

Abstract. The detailed understanding of surface-atmosphere exchange fluxes of reactive trace gases is a crucial precondition for reliable modelling of processes in atmospheric chemistry. Plant canopies significantly impact the atmospheric budget of trace gases. In the past, many studies focused on taller forest canopies or crops, where the bulk plant material is concentrated in the uppermost canopy layer. However, within grasslands, a land-cover class that globally covers vast terrestrial areas, the canopy structure is fundamentally different, as the main biomass is concentrated in the lowest part of the canopy. This has obvious implications for aerodynamic in-canopy transport, and consequently also impacts on global budgets of key species in atmospheric chemistry such as nitric oxide (NO), nitrogen dioxide (NO 2 ) and ozone (O 3 ).This study presents for the first time a comprehensive data set of directly measured in-canopy transport times and aerodynamic resistances, chemical timescales, Damköhler numbers, trace gas and micrometeorological measurements for a natural grassland canopy (canopy height = 0.6 m). Special attention is paid to the impact of contrasting meteorological and air chemical conditions on in-canopy transport and chemical flux divergence. Our results show that the grassland canopy is decoupled throughout the day. In the lowermost canopy layer, the measured transport times are fastest during nighttime, which is due to convection during nighttime and a stable stratification during daytime in this layer. The inverse was found in the layers above. During periods of low wind speed and high NO x (NO+NO 2 ) levels, the effect of canopy decoupling on trace gas transport was found to be especially distinct. The aerodynamic resistance in the lowermost canopy layer (0.04-0.2 m) was around 1000 s m −1 , which is as high as values determined previously for the lowest metre of an Amazonian rain forest canopy. The aerodynamic resistance representing the bulk canopy was found to be more than 3-4 times higher than in forests. Calculated Damköhler numbers (ratio of transport and chemical timescales) suggest a strong flux divergence for the NO-NO 2 -O 3 triad within the canopy during daytime. During that time, the timescale of NO 2 uptake by plants ranged from 90 to 160 s and was the fastest relevant timescale, i.e. faster than the reaction of NO and O 3 . Thus, our results reveal that grassland canopies of similar structure exhibit a strong potential to retain soilemitted NO due to oxidation and subsequent uptake of NO 2 by plants. Furthermore, photo-chemical O 3 production was observed above the canopy, which was attributed to a deviation from the NO-NO 2 -O 3 photostationary state by a surplus of NO 2 due to oxidation of NO, by e.g. peroxy radicals. The O 3 production was one order of magnitude higher during high NO x than during low NO x periods and resulted in an underestimation of the O 3 deposition flux measured with the EC method.Published by Copernicus Publications on behalf of the European Geosciences ...

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

“…The contribution of NO-NO 2 -O 3 chemistry to observed ozone fluxes can be estimated with ozone, NO, and NO 2 fluxes at two heights (Finco et al, 2018;Fitzjarrald & Lenschow, 1983;Lenschow & Delany, 1987;Vilá-Guerau de Arellano et al, 1993). An empirical technique only requiring flux measurements at one height has also been used (e.g., Plake, Sörgel, et al, 2015;Stella et al, 2012;Vuolo et al, 2017). This empirical technique assumes flux divergences of NO, NO 2 , and ozone are logarithmic with height, but the theoretical basis for this assumption is lacking.…”

“…In a shaded forest canopy, photolysis of NO 2 (i.e., R2) is expected to be low. In general, the effects of R1 on ozone fluxes are usually small (Kramm et al, ; Plake, Sörgel, et al, ; Rannik et al, ; Stella et al, ; Vuolo et al, ) because there is typically much more ozone than NO x (=NO + NO 2 ). However, some studies suggest R1 accounts for a nonnegligible influence on observed ozone fluxes (Dorsey et al, ; Finco et al, ; Lamaud et al, ; Neirynck & Verstraeten, ; Rummel et al, ), especially at night when there is relatively high NO x .…”

Large Eddy Simulation for Investigating Coupled Forest Canopy and Turbulence Influences on Atmospheric Chemistry

“…Observed ozone fluxes measured near the canopy top are commonly interpreted as dry deposition only but may be influenced by fast chemical losses of ozone such as the NO + O 3 reaction (Finco et al., 2018; Kramm et al., 1995; Plake et al., 2015). Negative NO‐O 3 $${I}_{\mathrm{s}}$$ means that the influence of NO on the ozone flux is even weaker than under the assumption of well‐mixed conditions.…”

Nitrogen oxides and ozone fluxes from an oilseed-rape management cycle: the influence of cattle slurry application

“…NO x , and especially NO 2 , are toxic gases for humans, exposure to which increases risks of various respiratory diseases. The World Health Organization gives guidelines for NO 2 exposure limits, both annual means (40 µg m −3 ) and 1 h mean (200 µg m −3 ) (Prüss-Üstün et al, 2016). For O 3 , only a short-term threshold is given (100 µg m −3 for the 8 h mean) because there are fewer studies on long-term exposure.…”