Atmospheric Measurements of Biogenic and Anthropogenic Emissions by Broadband Cavity Enhanced Absorption Spectroscopy

This thesis describes the quantification of biogenic and anthropogenic trace gases using the highly sensitive spectroscopic technique of broadband cavity enhanced absorption spectroscopy (BBCEAS). This technique uses a high finesse optical cavity to make absorption measurements over extended path lengths within a compact instrument and over wavelength ranges that are sufficiently broad to enable several overlapping absorbers to be quantified simultaneously. Here, BBCEAS was applied to measure I₂ emissions in coastal regions, NO₂ in urban ambient air, and VOC oxidation products during experiments in an atmospheric simulation chamber. Much of the work used a novel, mobile, battery-powered BBCEAS system to measure gas concentrations in very close proximity to their emission sources. The dominant emission source of iodine into the atmosphere in coastal regions comes from intertidal macroalgal beds. Gas-phase iodine chemistry perturbs the HOₓ and NOₓ radical cycles, provides additional sink reactions for tropospheric ozone, and initiates nucleation of new aerosol particles. Results are presented from an extensive laboratory study of I₂ emissions from five species of temperate seaweeds. Time- and species-dependent I₂ emission rates were quantified in studies mimicking the progressive exposure of seaweeds to air around low tide. Seasonal differences in I₂ emission rates were investigated. By deploying the BBCEAS instrument from a boat, I₂ concentrations were also measured directly above Laminaria digitata and Ascophyllum nodosum seaweeds growing in their natural habitat. NOₓ emissions affect urban air quality directly and indirectly (the latter via formation of tropospheric ozone and secondary aerosol). BBCEAS was applied to measure NO₂ and the optical extinction from aerosol particles at locations around the Leicester University campus. BBCEAS results were compared with commercial NOₓ (chemiluminescence) and aerosol instrumentation. The mobile BBCEAS instrument was also deployed to investigate the dispersion of NO₂ from the roadside.