  This paper details the first application of a software tagging algorithm to
reduce radon-induced backgrounds in liquid noble element time projection
chambers, such as XENON1T and XENONnT. The convection velocity field in XENON1T
was mapped out using $^{222}\text{Rn}$ and $^{218}\text{Po}$ events, and the
root-mean-square convection speed was measured to be $0.30 \pm 0.01$ cm/s.
Given this velocity field, $^{214}\text{Pb}$ background events can be tagged
when they are followed by $^{214}\text{Bi}$ and $^{214}\text{Po}$ decays, or
preceded by $^{218}\text{Po}$ decays. This was achieved by evolving a point
cloud in the direction of a measured convection velocity field, and searching
for $^{214}\text{Bi}$ and $^{214}\text{Po}$ decays or $^{218}\text{Po}$ decays
within a volume defined by the point cloud. In XENON1T, this tagging system
achieved a $^{214}\text{Pb}$ background reduction of $6.2^{+0.4}_{-0.9}\%$ with
an exposure loss of $1.8\pm 0.2 \%$, despite the timescales of convection being
smaller than the relevant decay times. We show that the performance can be
improved in XENONnT, and that the performance of such a software-tagging
approach can be expected to be further improved in a diffusion-limited
scenario. Finally, a similar method might be useful to tag the cosmogenic
$^{137}\text{Xe}$ background, which is relevant to the search for neutrinoless
double-beta decay.
