Nighttime SOA yield


I have some questions about the SOA yield approach.

  1. I want to know what’s the role of the “RXN counter”.
    From what I know, the RXN counter is only used to count the rxn moles, and to calculate the amount of SOA species by multiplying the RXN counter with SOA yield. (produce of SOA = SOA yield x rxn counter)
    Is this correct?

  2. Nighttime SOA yield.
    In CMAQ, there are two types of SOA yields (high-NOx, low-NOx) for SOA precursors.
    What I’m guessing is that these SOA yields are NOT for nighttime chemistry but to take features for area (like urban, and rural) into account.
    If this is correct, why is there no nighttime SOA yield?

What I think is that the SOA yield represents all of probability of SOA production, so it contains dominant oxidation pathway for precursors. But, the dominant oxidation pathway differs on which oxidant (like O3, NO3, OH) the precursors react with.
As you know, NO3 radical is dominant oxidant during nighttime.

So, how does the SOA yield approach consider the nighttime chemistry in CMAQ?

  1. Yes, reaction counters allow for the calculation of SOA via RXNCOUNTER*yield. Reaction counters track how much precursor has reacted.
  2. SOA yields vary as a function of time of day because the chemical pathways leading to SOA vary during the day. See the schematic from the v5.3 release notes for a summary of pathways in CMAQ: CMAQ/ at master · USEPA/CMAQ · GitHub. All those pathways are active at all times of day to varying degrees. You can find the manuscripts documenting SOA development here: Many of the development papers include plots of typical diurnal variation for each system.

I appreciate your taking the time to answer my question.