Potential error in PSD unit in EPM Aerosol::Coagulate

[lrobion]

I think the 1D Aerosol object used in the EPM for soot and SO4 has unit problem in the Aerosol::Coagulate function:

My understanding of the code is that the Aerosol.pdf represents dn/d(ln(r)), however it seems like it is used incorrectly when computing the volume of aerosol per bin:

APCEMM/Code.v05-00/src/AIM/Aerosol.cpp

Lines 170 to 174 in adf714b

	{
	v[iBin] = pdf[iBin] * bin_VCenters[iBin] * 1.0E+06;
	/* Unit check:
	* [#/cm^3] * [m^3] * [cm^3/m^3] = [cm^3/cm^3] */
	}

v is supposed to hold the volume concentration per bin which we should get by multiplying the number of particles per bin by the mean volume of a particle in that bin. However the code above does not convert the pdf to a number of particles, it is instead still dn/d(ln(r)).

The Grid_Aerosol version of this calculation is done in Grid_Aerosol::Volume() called by Grid_Aerosol::Coagulate. Looking at that version it correctly multiplies the pdf by the bin size to recover the number of particles:

APCEMM/Code.v05-00/src/AIM/Aerosol.cpp

Lines 1244 to 1255 in adf714b

	for (iBin = 0; iBin < nBin; iBin++)
	{
	ratio = log(bin_Edges[iBin + 1] / bin_Edges[iBin]);
	for (jNy = 0; jNy < Ny; jNy++)
	{
	for (iNx = 0; iNx < Nx; iNx++)
	{
	volume[iBin][jNy][iNx] = ratio * bin_VCenters[iBin][jNy][iNx] *
	pdf[iBin][jNy][iNx];
	/* Unit check: [m^3] * \
	* [#/cm^3] \
	* = [m^3/cm^3] */

The log difference is instead simplified to a log(ratio) here but the calculation is mathematically the same. There is a test on this function that does not catch it so we should also investigate that.

Not 100% sure about this so I need someone else's eyes on this. @sdeastham @marcoslogrono