  We developed a highly sensitive, reliable and portable automatic system
(H$^{3}$) to monitor the radon concentration of the underground experimental
halls of the Daya Bay Reactor Neutrino Experiment. H$^{3}$ is able to measure
radon concentration with a statistical error less than 10\% in a 1-hour
measurement of dehumidified air (R.H. 5\% at 25$^{\circ}$C) with radon
concentration as low as 50 Bq/m$^{3}$. This is achieved by using a large radon
progeny collection chamber, semiconductor $\alpha$-particle detector with high
energy resolution, improved electronics and software. The integrated radon
monitoring system is highly customizable to operate in different run modes at
scheduled times and can be controlled remotely to sample radon in ambient air
or in water from the water pools where the antineutrino detectors are being
housed. The radon monitoring system has been running in the three experimental
halls of the Daya Bay Reactor Neutrino Experiment since November 2013.
