  Making precise measurements of pulsar dispersion measures (DMs) and applying
suitable corrections for them is amongst the major challenges in high-precision
timing programmes such as pulsar timing arrays (PTAs). While the advent of
wide-band pulsar instrumentation can enable more precise DM measurements and
thence improved timing precision, it also necessitates doing careful
assessments of frequency-dependent (chromatic) DMs that was theorised by Cordes
et al. (2016). Here we report the detection of such an effect in broadband
observations of the millisecond pulsar PSR J2241-5236, a high-priority target
for current and future PTAs. The observations were made contemporaneously using
the wide-band receivers and capabilities now available at the Murchison
Widefield Array (MWA), the upgraded Giant Metrewave Radio Telescope (uGMRT) and
the Parkes telescopes, thus providing an unprecedentedly large frequency
coverage from 80 MHz to 4 GHz. Our analysis shows the measurable changes in DM
that scale with the observing frequency ($\nu$) as $\rm \delta DM \sim \nu^{2.5
\pm 0.1}$. We discuss the potential implications of such a frequency dependence
in the measured DMs, and the likely impact on the timing noise budget, and
comment on the usefulness of low-frequency observations in advancing PTA
efforts.
