Intensified heatwaves under global warming have influenced new particle formation (NPF) and related aerosol physicochemical properties. We show that aerosol optical hygroscopicity (f(RH)) was generally higher on NPF days than in non-event cases, likely due to enhanced secondary formation and subsequent growth of both pre-existing and newly formed particles with stronger photooxidation, specifically under persistent heatwaves. This would further impact the aerosol direct radiative forcing.