The combination of Cavity Enhanced Absorption Spectroscopy (CEAS) with broad-band light sources (e.g. Light-
Emitting Diodes, LEDs) lends itself to the application of cavity enhanced DOAS (CE-DOAS) to perform sensitive and
selective point measurements of multiple trace gases with a single instrument. In contrast to other broad-band CEAS
techniques, CE-DOAS relies only on the measurement of relative intensity changes, i.e., does not require knowledge of
the light intensity in the absence of trace gases and aerosols (I0). We have built a prototype LED-CE-DOAS instrument
in the blue spectral range (420-490nm) to measure nitrogen dioxide (NO2), glyoxal (CHOCHO), iodine monoxide (IO),
water (H2O) and oxygen dimers (O4). Aerosol extinction is retrieved at two wavelengths by means of observing water
and O4 and measuring pressure, temperature and relative humidity independently. The instrument components are
presented, and the approach to measure aerosol extinction is demonstrated by means of a set of experiments where
laboratory generated monodisperse aerosols are added to the cavity. The aerosol extinction cross section agrees well with
Mie calculations, demonstrating that our setup enables measurements of the above gases in open cavity mode.
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