Ozone deterioration over North China plain caused by light absorption of black carbon and organic carbon

Previous studies have revealed that the light absorption effect of black carbon (BC) aerosol could influence ozone concentrations via absorbing sunlight and modulating photolysis rates, the so-called aerosol-photolysis interaction (API). However, few studies have quantified the API effect of organic carbon (OC) aerosol, which has recently been found to contribute to about 10%–40% of the total light absorption at 300 nm wavelength. This study tried to quantify and compare the OC and BC aerosols' influence on ozone concentrations through a case study over North China Plain (NCP) in Oct. 2018. By modulating OC's absorption index based on field measurements, we quantified the OC and BC API effect on surface ozone by the WRF-Chem model. The API effect and its uncertainties of OC aerosols were evaluated using the high and low absorption values that have been reported in previous studies. Results showed that, in terms of the pollution periods in NCP, removing all the OC aerosol from the atmosphere could increase the daytime maximum daily average 8-h (MDA8) O3. In this episode, the average (extreme) MDA8 O3 increased by 0.7–2.4 ppb (1.1–3.7 ppb), accounting for 1.5%–5.3% (2.4%–8.8%) of ozone concentration, comparable to that by BC of 2.0 ppb (3.3 ppb), accounting for 4.4% (7.8%), which means the OC's API effect was 35%–120% of BC's API effect on surface ozone in this case. The results also indicated a fact that, though OC has a lower light absorption rate than BC, the API efficiency of OC on surface ozone when reducing aerosols could be compensated by a higher portion of OC aerosols and higher light absorption efficiency due to much more OC was distributed on higher altitude (especially 1–3 km) than BC aerosols, which result in larger ozone changes in both the low troposphere and surface.