A new instrument for in-situ HONO measurements by Iterative Cavity enhanced DOAS

We present a new spectroscopic instrument to measure directly ambient in-situ HONO (nitrous acid) concentrations using the ICAD technique. HONO concentrations can be measured with an error of 400 ppt at 10s and 90 ppt at 1h time resolution. The advantage of the ICAD spectroscopic technique is that it does not require gas calibration and allows for simple long term operation with  high accuracy while not relying on the absolute stability of the light source intensity.

Atmospheric HONO concentrations are of interest since they significantly influence OH radical concentration and thus the tropospheric oxidation capacity. Also, HONO can give rise to the formation of highly mutagenic species in the human lung. HONO sources are still largely unknown in detail and discrepancies are observed between measured and modelled HONO concentrations. We present in-door and out-door in-situ observations of HONO and long-term stability tests.

The instrument provides simultaneous measurements of NO₂ with a measurement error of 600 ppt at 10s time resolution and 140 ppt at 1h time resolution (based on modified Allan deviation). With an overall power consumption of typically 40W, its robustness to vibrations and a 19” Rack housing size of 13,5 x 49 x 61 cm³, it is also suitable for mobile applications as is the commercially available NO₂/NO_X version of the instrument.