Influence of meteorology and anthropogenic pollution on chemical flux divergence of the NO–NOsub2/sub–Osub3/sub triad above and within a natural grassland canopy

摘要

The detailed understanding of surface–atmosphere exchange fluxes of reactivetrace gases is a crucial precondition for reliable modelling of processes inatmospheric chemistry. Plant canopies significantly impact the atmosphericbudget of trace gases. In the past, many studies focused on taller forestcanopies or crops, where the bulk plant material is concentrated in theuppermost canopy layer. However, within grasslands, a land-cover class thatglobally covers vast terrestrial areas, the canopy structure is fundamentallydifferent, as the main biomass is concentrated in the lowest part of thecanopy. This has obvious implications for aerodynamic in-canopy transport,and consequently also impacts on global budgets of key species in atmosphericchemistry such as nitric oxide (NO), nitrogen dioxide (NO₂) and ozone(O₃).This study presents for the first time a comprehensive data set of directlymeasured in-canopy transport times and aerodynamic resistances, chemicaltimescales, Damk?hler numbers, trace gas and micrometeorologicalmeasurements for a natural grassland canopy (canopy height = 0.6 m).Special attention is paid to the impact of contrasting meteorological and airchemical conditions on in-canopy transport and chemical flux divergence. Ourresults show that the grassland canopy is decoupled throughout the day. Inthe lowermost canopy layer, the measured transport times are fastest duringnighttime, which is due to convection during nighttime and a stablestratification during daytime in this layer. The inverse was found in thelayers above. During periods of low wind speed and high NO_x(NO+NO₂) levels, the effect of canopy decoupling on trace gastransport was found to be especially distinct. The aerodynamic resistance inthe lowermost canopy layer (0.04–0.2 m) was around 1000 s m?1, whichis as high as values determined previously for the lowest metre of anAmazonian rain forest canopy. The aerodynamic resistance representing thebulk canopy was found to be more than 3–4 times higher than in forests.Calculated Damk?hler numbers (ratio of transport and chemical timescales)suggest a strong flux divergence for the NO–NO₂–O₃ triad within thecanopy during daytime. During that time, the timescale of NO₂ uptake byplants ranged from 90 to 160 s and was the fastest relevant timescale, i.e.faster than the reaction of NO and O₃. Thus, our results reveal thatgrassland canopies of similar structure exhibit a strong potential to retainsoil-emitted NO due to oxidation and subsequent uptake of NO₂ by plants.Furthermore, photo-chemical O₃ production was observed above the canopy,which was attributed to a deviation from the NO–NO₂–O₃photostationary state by a surplus of NO₂ due to oxidation of NO, bye.g. peroxy radicals. The O₃ production was one order of magnitudehigher during high NO_x than during low NO_x periodsand resulted in an underestimation of the O₃ deposition flux measuredwith the EC method.