Investigation of atmospheric trace gas composition and air-sea flux detection potential over Halifax (NS) Harbour using OP-FTIR spectroscopy

Abstract

In a world where air quality and the future of the earth are being heavily influenced by anthropogenic pollution, it is vital to protect air quality and improve our understanding of the relationship between the ocean and atmospheric trace gas composition. Tropospheric trace gas concentrations as well as vertical concentration differences (an input to the calculation of ocean-air flux) are derived over a period of one month (August – September, 2021), based on available long Open-Path Fourier Transform InfraRed (OP-FTIR) spectroscopic measurements at the air-sea interface in ambient air in Halifax, NS. Average concentrations and measurement errors are reported, as well as possible explanations driving the temporal behaviour of key air-quality related trace gases (ammonia, formic acid, carbonyl sulfide, carbon monoxide, and methanol), as well as the greenhouse gas carbon dioxide. The concentration results show ship plume events as well as diurnal cycles. The concentration differences suggest an ocean source of carbonyl sulfide and formic acid, and an ocean sink of carbon dioxide and ammonia, which is consistent with prior knowledge of air-sea gas exchange. This is the first report of the flux-related concentration difference of these gases in a marine environment by this technique. Prior to these spectral retrievals, the sensitivity to different parameter choices in fitting OP-FTIR spectra using the Multiple Atmospheric Layer Transition (MALT) Non-Linear Least Squares (NLLS) algorithm was investigated, with a focus on the FOV parameter, which is related to the optical configuration of the spectrometer and spectral line broadening.