# Can CMAQ simulate the further impact of aerosols on O3 through their impact on photolysis? **URL:** \ **Category:** CMAQ\ **Created:** [May 22, 2023, 9:59am UTC](https://forum.cmascenter.org/t/can-cmaq-simulate-the-further-impact-of-aerosols-on-o3-through-their-impact-on-photolysis/4131 "2023-05-22T09:59:17Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![bandaoshutiao](https://avatars.discourse-cdn.com/v4/letter/b/9fc348/32.png) [@bandaoshutiao](https://forum.cmascenter.org/u/bandaoshutiao)\ **Post date:** [May 22, 2023, 9:59am UTC](https://forum.cmascenter.org/t/can-cmaq-simulate-the-further-impact-of-aerosols-on-o3-through-their-impact-on-photolysis/4131/1 "2023-05-22T09:59:17Z")
Hi If possible, which part of the code or file controls this part of the content. By the way, if I want a certain aerosol substance to have a zero impact on O3 (or for all substances) through this effect, how should I set it? Thanks ---
**Author:** ![lizadams](https://avatars.discourse-cdn.com/v4/letter/l/49beb7/32.png) [@lizadams](https://forum.cmascenter.org/u/lizadams)\ **Post date:** [May 22, 2023, 3:57pm UTC](https://forum.cmascenter.org/t/can-cmaq-simulate-the-further-impact-of-aerosols-on-o3-through-their-impact-on-photolysis/4131/2 "2023-05-22T15:57:12Z")
This post may describe a method that you can use. > [@Running CMAQ 5.2.1 without aerosol species](https://forum.cmascenter.org/t/running-cmaq-5-2-1-without-aerosol-species/2482): > > Hi there, I am trying to run a case with only biogenic emissions using CMAQ5.2.1. We generate biogenic emissions using MEGAN3.0, which include many gas-phase species, but no aerosol species. This seems to be a problem, because I get the error shown below. Value for CTM\_ZERO\_PCSOA not defined;returning default: FALSE \>\>---\>\> WARNING in subroutine AERO\_EMIS\_INIT on PE 014 Cannot define index for SO4 emissions: aerospc%emis=' ' in AERO\_DATA.F. Check if AE namelist is …