Remote sensing of nitrogen dioxide across Cork city using a Low- Cost Long Path DOAS system

Differential Optical Absorption Spectroscopy (DOAS) is a long-standing remote sensing technique used to measure many absorbing trace gases. To attain the necessary sensitivity to weak absorptions from trace gases, conventional DOAS systems normally have kilometre- long path lengths and large telescopes to maximise the collected light. Such systems are expensive and largely limited to research applications. The work describes a low – cost, long path DOAS system with a 1.5 km path length in Cork city, Ireland specifically to measure nitrogen dioxide (NO₂) for air quality monitoring. The system consists of a temperature-stabilised 0.8 W blue LED (peak wavelength at 435 nm), 5 cm diameter transmitting and receiving telescopes and a compact spectrometer. A band pass filter (420 nm – 460 nm) is used to eliminate stray light. The system is much smaller than conventional DOAS configurations and is relatively straightforward to assemble. The outlook for using long path DOAS as an affordable approach to monitor the contribution to NO₂ to urban air quality is discussed.