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