I am currently using a box model based on CMAQ 5.3.1, where most of the source code is identical to that of CMAQ.
During a one-month winter simulation for Seoul, South Korea, I observed that PM25AC exhibited sudden drops at certain time periods. This issue resulted in noisy behavior in PM2.5 and other PM-related components in the model output.
To investigate this issue, I reviewed PMDIAG_DATA.F, but I was unable to determine the exact factors influencing PM25AC over time.
To address this problem, I would like to gain a deeper understanding of how PM25AC is calculated in CMAQ 5.3.1 and what factors influence its variation over time.
Could you provide insights into the relevant calculation processes within CMAQ? Additionally, any suggestions on diagnosing and resolving this issue would be greatly appreciated.
Thank you for sharing your results via this forum. It is always interesting to see how the algorithms are behaving in isolated test cases.
PM25AC (the fraction of the accumulation mode that would be captured as PM2.5) is dependent on the size distribution parameters and, to some degree, on the chemical composition of the accumulation mode aerosol.
First, it’s good to verify that PM25AT is usually 1.0 and always greater than PM25AC. This is what we would expect for the Aitken mode – it should usually be captured completely as PM2.5.
You can find the calculation for fPM25 in aero_subs.F. The subroutine is called aero_inlet. The wet geometric mean diameter, standard deviation, and bulk density of each aerosol mode are input to this routine. It calculates the Stokes diameter from the Aerodynamic diameter input and then uses error functions to calculate the integral fraction of each mode that is below 2.5 um .
So the size distribution parameters (i.e. geometric mean diameter, standard deviation) are dependent by design on the emissions and any other microphysical processes happening during the simulation. As the accumulation mode grows in size, the PM25AC ratio will decrease, as expected. As new emissions are added, which are generally smaller in size, the bulk ratio will grow closer to 1.0.
I do get nervous when I see the ratio of PM25AT lower than that of PM25AC. What this probably means is that the standard deviation of PM25AT has gotten large enough that a few percent of the distribution is beyond the PM2.5 cutoff, while the accumulation mode standard deviation is smaller and indicates a narrower distribution that is within the cutoff. It is hard to tell without knowing the values of the diameter and standard deviation parameters.
