The reward and penalty for ozone pollution control caused by natural sources and regional transport: A case study in Guangdong province.

Ground-level O pollution in the Pearl River Delta region (PRD) is closely related to anthropogenic, natural emissions and regional transport. However, the interactions among different sources and natural intervention in modulating anthropogenic management have not been comprehensively assessed. Here, the WRF-CMAQ-MEGAN modeling system was utilized to simulate an O episode over PRD. The integrated source apportionment method (ISAM) and brute-force top-down combined with factor separation approach (BF-TD-FSA) were applied to quantify source contributions, impacts of individual or multiple sources on O, and decouple interactions among various emissions; additionally, based on ISAM, O isopleths visualized MDA8 O response of different source types to anthropogenic perturbations. ISAM concluded considerable MDA8 O contributions of regional transport in PRD/NPRD (non-PRD areas in Guangdong province) (38.8 %/35.7 %), followed by anthropogenic (32.7 %/24.8 %), BVOC (biogenic volatile organic compounds, 23.8 %/20.3 %) and SNO (soil NO, ∼4 %) emissions. Compared to concentrated anthropogenic contributions, widespread natural contributions were observed across their source areas and along the transport pathways. The BF-TD also revealed that regional transport had the largest impact (>90 μg m) on MDA8 O while anthropogenic and BVOC emissions greatly affected downwind PRD (64.5 and 7.7 μg m). Negative synergy between anthropogenic and natural emissions (especially BVOC emission) suggested potential natural-induced intensification of anthropogenic impact during O management. The MDA8 O isopleths further demonstrated significant BVOC-induced reward and regional transport-induced penalty for anthropogenic NO (ANO) emission control benefits, leading to additional maximum MDA8 O decrease and increase by -27.5 and 13.8 μg m in polluted high-emission grids. The natural-induced reward effect could impose loose requirements on anthropogenic reduction (decreased by 13.3 %-17.7 %) if there were no regional transport-induced control penalty. It is advisable to prioritize ANO control and seek collaboration on air quality management with neighboring provinces to maximize the natural-induced control reward and achieve desired targets with minimal human efforts.