Emission, deposition and chemical conversion of atmospheric trace substances in and above vegetation canopies

Ryder, James. 2010 Emission, deposition and chemical conversion of atmospheric trace substances in and above vegetation canopies. University of Manchester, Department for Earth and Environmental Sciences, PhD Thesis, 241pp.

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Measurements and modelling of surface / atmosphere exchange of trace gases and aerosols are used to study the processes controlling the exchange and to derive parameterisation for inclusion into chemical transport models (CTMs) that underpin predictions of air quality, climate change, ecosystem effects and long-range transboundary air pollution. Many of the compounds of interest in this context are chemically reactive and their deposition / emission interacts with chemical conversion processes. As a result, vegetation exchange may deviate from the fluxes measured or predicted at a reference height above the canopy. However, this process is rarely taken into account, - little is known about the role of chemical conversions and source/sink distributions within vegetation canopies. This study researches two aspects of the processes – the physical and chemical sources and sinks of biogenic volatile organic compounds (BVOCs) in a tropical forest canopy and the processes that are responsible for the complex behaviour of fluxes of inorganic aerosol and its associated gas phase components. Firstly, we report the physical measurements and modelling results from comprehensive in-canopy measurements that were conducted during July 2008 as part of the ACES/OP3 campaign at Danum Valley (Sabah, Malaysia). In-canopy profiles of BVOCs, meteorological parameters and turbulence statistics were collected to provide a comprehensive description of chemical composition and transport within the rainforest canopy. Significant concentrations of isoprene and monoterpene were observed during daylight hours. Considerable day-to-day variation in the concentration and dispersion of compounds can be partly explained by measurements of in-canopy turbulence and measured photosynthetically active radiation (PAR). Measured in-canopy turbulence is low (friction velocity < 0.4 m s-1), and PAR is influenced by fast changing cloud cover. The application of an Inverse Lagrangian Transport source/sink analysis demonstrates that the bulk of the isoprene and monoterpene is emitted from the uppermost levels of the trees and both emissions are light dependent. The measurements show that 30% of the isoprene is converted to (longer-lived) first order oxidation products within the canopy, which are transported further down into the canopy. Larger concentrations of methanol observed close to the ground suggest that this compound is partly sourced from understory vegetation or, more likely, leaf litter. To study the effect of partitioning of the NH3-HNO3-NH4NO3 system on bulk chemical and size-resolved aerosol fuxes, a new coupled 1D size-segregated emission, deposition, transport and chemistry model is developed, and applied to two existing extensive measurement datasets of existing measurement campaigns of gas and aerosol fluxes above contrasting canopies: a calluna vulgaris heathland and a Douglas Fir forest. The model predicts evaporation of NH4NO3 near and within the canopy which induces unexpected ‘apparent’ fluxes at the measurement height that are consistent with the measurements: deposition velocities (Vd) of NH4+ and NO3- aerosol that exceeds the inert values by a factor of, for Speuld, between ~2.6 to 7.3 for NH4+as well as bi-directional behavior of size-segregated aerosol fluxes, with apparent emission of the smallest and apparent fast deposition of the larger accumulation mode particles, with a Vd that increases strongly with particle size. By converting slowly depositing aerosol into fast depositing gases, the process increases the total nitrogen (N) deposition received by the canopy by an average of 31% and 30% over heathland and forest compared with model runs without chemistry, respectively. This demonstrates the need to account for chemical interactions for the precise estimation of N dry deposition inputs derived from measurements and model predictions. Through comparing model runs with and without chemistry inside the canopy, it is estimated that for the forest canopy 50% of the evaporation takes place within the canopy itself. The heathland value is still 30%, emphasizing the importance to consider chemical reactions even within short canopies, which so far have received little attention. However, due to the lack of in-canopy measurements in the heathland canopy, these model predictions are more poorly constrained.

Item Type: Publication - Thesis (PhD)
UKCEH and CEH Sections/Science Areas: Billett (to November 2013)
NORA Subject Terms: Atmospheric Sciences
Date made live: 18 Nov 2019 12:45 +0 (UTC)

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