In the paper, an accurate and sensitive cavity attenuated phase shift spectroscopy (CAPS) sensor was used to monitor the
atmospheric visibility. The CAPS system mainly includes a LED light source, a band-pass filter, an optical resonant
cavity (composed of two high mirror, reflectivity is greater than 99.99%), a photoelectric detector and a lock-in amplifier.
The 2L/min flow rate, the optical sensor rise and fall response time is about 15 s, so as to realize the fast measurement of
visibility. An Allan variance analysis was carried out evaluating the optical system stability (and hence the maximum
averaging time for the minimum detection limit) of the CAPS system. The minima (~0.1 Mm-1) in the Allan plots show
the optimum average time (~100s) for optimum detection performance of the CAPS system. During this period, the
extinction coefficient was correlated with PM2.5 mass (0.88), the extinction coefficient was correlated with PM10 mass
(0.85). The atmospheric visibility was correlated with PM2.5 mass (0.74). The atmospheric visibility was correlated with
PM10 mass (0.66).