Quantitative detection of trace gas species in the atmosphere using LED Broadband Cavity Enhanced Absorption Spectroscopy

A broadband version of cavity enhanced absorption spectroscopy (BBCEAS) has been applied to measure the ambient concentrations of reactive trace gases, particularly nitrogen dioxide (NO2) in the polluted urban environment and molecular iodine (I2) in the marine boundary layer. The spectrometer’s light source, a light emitting diode (LED), enables absorption spectra of gas samples to be acquired over 50 nm bandwidths at visible and nearultraviolet wavelengths. Differential optical absorption spectroscopy (DOAS) is then applied to unambiguously identify and quantify the various absorbers present in the samples. Especially difficult operating conditions were encountered during the instrument’s first field deployment onboard the research vessel RRS Discovery. As a result, the instrument did not achieve the I2 detection limits it had demonstrated in laboratory testing and was unable to detect any I2 signal in the mid‐Atlantic Ocean during the Reactive Halogens in the Marine Boundary Layer Experiment. However, iodine was detected around many night‐time low tides during the BIOFLUX II campaign at the Mace Head Atmospheric Research Station in County Galway, Ireland. The maximum I2 concentration of 608 pptv (parts per trillion by volume) correlated with the campaign’s lowest tide, and is three times greater than any previous report of atmospheric I2 concentrations. The BIOFLUX observations support current understanding that coastal I2 derives from seaweed (particularly kelp species) exposed to air around low tide. LED‐BBCEAS measurements of NO2 made on the University campus during the Leicester Air Quality Measurement Project are compared with data from two chemiluminescence instruments. The need to closely co‐locate instruments during comparisons of the various techniques is discussed, and evidence of a possible interference in the chemiluminescence measurements has been found.