A Drone Cutting Across the Plume

A pollutant drifts through a still layer of air. With $x$ and $y$ measured in metres, the concentration at the point $(x,\,y)$ is

$C(x,\,y) = 10\ln\!\left(x^{2} + y^{2}\right) + x^{3} - 3xy^{2}$

measured in parts per million (ppm).

A sensor drone is at the point $P$ and flies in a straight line toward the point $Q$, holding a constant speed:

At the instant the drone passes through $P$, how fast is the concentration it reads changing, per second?

Round your answer to 2 decimal places and include the sign.