Unveiling Atmospheric HO₂ Chemistry under Varying NO Conditions during the KORUS-AQ Campaign

Hydroperoxyl (HO2) radicals profoundly influence the formation of ozone and particulate matter through NOx-to-HOx cycles. Despite their importance, HO2-related processes remain insufficiently represented in current air quality models. Here, we provide comprehensive insights into HO2 chemistry using the Community Multiscale Air Quality (CMAQ) model together with airborne observations over South Korea during the KORUS-AQ campaign (1 May–9 June, 2016). Analysis of 20 flight scenarios suggests that modeled HO2 levels might be strongly linked to NO distributions. Under low-NO conditions, HO2 was simulated to be 6.0% higher than the observed levels, likely due to a missing pathway. To address this, we incorporated HO2 aerosol uptake into the CMAQ model by applying an uptake coefficient of 0.08, assuming no H2O2 production. Under high-NO conditions, a substantial underestimation of 9.90 ppt in modeled HO2 levels was observed. This discrepancy may be attributed to (i) underestimated isoprene, monoterpene, and CO emissions and (ii) overestimated NO levels. Finally, we further examined the impact of HO2 aerosol uptake on key atmospheric species (e.g., OH, HO2, H2O2, O3, and PM2.5 components) under different NO levels. Our results indicate that most of these species decrease in concentration due to a weakened oxidizing capacity. In contrast, nitrate concentrations increase in low-NO regions, likely driven by HNO3 partitioning facilitated by excess NH3. These findings pave the way for accurate simulations of HO2 radicals and advance our understanding of atmospheric HO2 chemistry.