FUNCTION DUSTEM_LOGN_VDIST, x, par
; generates a volume normalized log-normal law 
; returns distribution in nr of grains : dn/da
; x : grain size 
; par(0) : centroid of log-normal
; par(1) : sigma of log-normal
; par(2) : VOLUME normalization

 np = n_elements(x)
 x0 = par(0)
 sigma = par(1)
 y = exp(- 0.5 * ( alog(x/x0) / sigma )^2 ) / x
 vy = x^4 * y
 xm = par(0) * exp( -par(1)^2 ) ; x of max in dn/da
 print,'(W) DUSTEM_LOGN_VDIST: dn/da max @',xm*1e7,' nm'
 dx = alog(x(1:np-1)) - alog(x(0:np-2)) 
 yi = TOTAL( (vy(1:np-1) + vy(0:np-2))*0.5*dx )
 y = par(2) * y / yi
RETURN, y
END