  We investigate the behavior of microscopic heavy particles settling in
homogeneous air turbulence. The regimes are relevant to the airborne transport
of dust and droplets: the Taylor-microscale Reynolds number is Re = 289 - 462,
the Kolmogorov-scale Stokes number is St = 1.2 - 13, and the Kolmogorov
acceleration is comparable to the gravitational acceleration (i.e., the Froude
number Fr = O(1)). We use high-speed laser imaging to track the particles and
simultaneously characterize the air velocity field, resolving all relevant
spatio-temporal scales. The role of the flow sampled by the particles is
spotlighted. In the present range of parameters, the particle settling velocity
is enhanced proportionally to the velocity scale of the turbulence. Both
gravity and inertia reduce the velocity fluctuations of the particles compared
to the fluid; while they have competing effect on the particle acceleration,
through the crossing trajectories and inertial filtering mechanisms,
respectively. The preferential sampling of high-strain/low-vorticity regions is
measurable, but its impact on the global statistics is moderate. The inertial
particles have large relative velocity at small separations, which increases
their pair dispersion; however, gravity offsets this effect by causing them to
experience fluid velocities that decorrelate faster in time compared to
tracers. Based on the observations, we derive an analytical model to predict
the particle velocity and acceleration variances for arbitrary St, Fr, and Re.
This agrees well with the present observations and previous simulations and
captures the respective effects of inertia and gravity, both of which play
crucial roles in the transport.
