Updated comment.

  !

  !   with w(i,j) the faction of source interval i with length dx(i)

  !   that overlaps with target interval j.

  !

  !   Similar, if f is a volume mixing ratio in [ppbv] and x and y

  !   are pressure coordinates in [Pa], then the result is:

  !

  !        g(j)     = sum_i  f(i)  *  dx(i) * w(i,j)

  !      [ppbv*Pa]          [ppbv]   [Pa]

  !

  !   The user could then afterwards convert the result to proper units,

  !   for example [(kg tr)/m2] using the mole masses of the the tracer and of air,

  !   and the gravity acceleration.

  !

  !   A coordinate x with this units can be computed from pressure [Pa]

  !   devided by the gravitiy constant g (~9.8 [m/s2]):

  !