DOAS (Differential Optical Absorption Spectroscopy) is an open path optical

measuring technique applicable for a number of gases, including NO2 specifically,

absorbing light in the UV and visible spectral regions (Platt and Perner, 1983). The

technique is based on differential absorption, i.e. the difference between local

maximums and minima in the absorption spectrum of the probed gas. The DOAS

technique may also be used for NO, but a shorter path is required than for NO2.

Light from a broad-band xenon high-pressure lamp is transmitted up to several

kilometres through the atmosphere. The light is received and analysed by the use of a

fast scanning dispersive spectrometer to eliminate the influence of air turbulence. The

DOAS technique is used today in a large number of applications, the most common

being environmental monitoring of the main pollutants in urban areas, as a mean

concentration over some distance in urban air. The method has recently been approved

by US EPA for SO2, NO2 and O3.

The major advantage of DOAS is that several compounds can be measured at the

same time, i.a. HNO2, NO3 radicals, benzene, toluene etc. The DOAS technique is not

yet US EPA approved for these pollutants, but by application of suitable quality

control it will give reliable results. However, the scattering of light by fog, heavy rainor

snowfall may cause problems with data quality and availability. For aromatics

significant interference must be taken into account.


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