Surface atmosphere dynamics of reactive trace gases and water-soluble aerosol components above agricultural grassland and tropical rainforest

Abstract

The interaction between biosphere and atmosphere in the cycling of gas and aerosol species is of key importance in considering overall emission and deposition rates of nutrients and pollutants. Understanding of the biosphere-atmosphere processes that govern these cycles is critical to modelling overall global concentrations of atmospheric aerosols and trace gases, which in turn is vital to developing predictions for overall global climate and international pollution burdens. However, to understand these processes, more measurements over a variety of different ecosystems are required, preferably measurements which are taken in real time, which are of high temporal resolution, and record a variety of species simultaneously and at potentially low background concentrations. This thesis presents work in which the Gradient of Aerosols and Gases Online Registration (GRAEGOR), an instrument which employs a modified form of the aerodynamic gradient method (AGM) to determine fluxes from measured concentrations, was used to determine concentrations and fluxes of the trace gases NH3, HCl, HONO, HNO3 and SO2 and the water-soluble aerosol species NH+4, Cl-, NO--2 , NO-3 and SO2-4 above agricultural grassland and tropical rainforest. Measurements of the suite of trace gases and aerosols were conducted from May 2016 to June 2016 at the Easter Bush agricultural grassland site (Midlothian, United Kingdom). From these measurements, full time scale and diel profiles for concentrations, fluxes and deposition velocities for each species were developed. Through the use the conservative exchange fluxes of tot-NH-4 and tot-NO-3 , it was found that a ground source of HNO3 existed after a fertilisation event, which after scavenging by volatilised NH3 formed ammonium nitrate aerosol. Diel cycle variation in HONO concentrations showed a background concentration of HONO during midday, contrary to expectations regarding the chemical behaviour of HONO. This suggests a potential daytime source of HONO at the site. A link between the deposition velocities for Cl-, NO-3 and SO2-4 and a proxy for aerosol size provided evidence for the modelled link between increasing deposition velocity with increasing aerosol size. A comparison between the HONO concentrations measured by the GRAEGOR and the HONO concentrations measured by the Long Path Absorption Photometer (LOPAP) was also conducted, which found that the GRAEGOR records a higher concentration of HONO in comparison to the LOPAP, suggesting the presence of artefact factors within the GRAEGOR when measuring HONO. A modified form of a correction factor was developed to account for this HONO artefact. A similar comparison for NH3 recorded by the GRAEGOR and NH3 recorded by the Quantum Cascade Laser (QCL) was also conducted, finding that the QCL overestimated NH3 concentrations. Measurements of trace gases and aerosols above tropical rainforest were carried out from October 2017 to November 2017 at the Amazon Tall Tower Observatory (ATTO) site (Amazonas, Brazil). This was during the tropical dry season. Measurements of HONO concentration found that values remained above the detection limit of the instrument, even during daytime. Calculations of HONO flux found small but significant emissions of the trace gas in the early morning, suggesting formation of HONO below canopy during the evening followed by venting of the gas to above the canopy during the morning. It was found that local, regional and global sources of biomass burning led to periods of elevated SO2 concentrations, with an associated increase in the dry deposition of SO2 and associated SO2-4 containing aerosol. Emissions of all measured aerosols, particularly Cl_, were observed throughout the campaign, which may be related to emissions of primary biological aerosol particles (PBAPs). Bi-directional exchange of NH3 was measured during the campaign at ATTO. In combination with ancillary measurements of leaf wetness data, a novel parameterisation of NH3 emission and deposition to tropical rainforest was developed. This parameterisation was able to accurately simulate the bi-directional pattern of observed NH3 fluxes at the rainforest site. Based on the observed pattern of NH3 emissions occurring during periods where measured leaf wetness was low, it was concluded that emissions were driven by stomatal exchange of leaf NH3 with the atmosphere. This study has demonstrated that observed HONO concentrations above agricultural grassland are sometimes not consistent with predicted chemical pathways based on HONO photodissociation, and that there exists a potential source of HONO that affects overall daytime concentrations. Similarly, emissions of HONO have been demonstrated to exist from tropical rainforest, with a proposed pathway from soil emissions to the atmosphere. Furthermore, this study has conducted simultaneous measurements of the individual components of the NH3-HNO3-NH4NO3 triad, noting apparent formation of NH4NO3 from urea application to agricultural grassland. Finally, bi-directional exchange of NH3 from the rainforest has been demonstrated to occur during the tropical dry season, particularly during the warm, dry periods at the canopy level that are characteristic of the hours immediately following noon.