Wildfire particulate matter from Canadian forest fires significantly impacted the air quality in the northeastern United States during the summer of 2023. Here, we used real-time and time-integrated instrumentation to characterize the physicochemical properties and radiative effects of wildfire particulate matter reaching the metropolitan areas of New Jersey/ New York during this extreme incident. The radiative forcing of −352.4 W/m² derived here based on the measured optical properties of wildfire particulate matter explains, to some extent, the ground level temperature reduction of about 3 °C observed in New Jersey/ New York City during this incident. Such negative radiative forcing in densely populated megacities may limit natural ventilation, increase the residence time of wildfire particulate matter and background air pollutants, exacerbating public health risks. This study highlights the importance of radiative effects from wildfire particulate matter in densely populated areas and their potential implications for climate, air quality and public health.

Here, the light absorption of brown carbon (BrC) emitted by wood combustion and denuded from volatile organic carbon (VOC) at 300 °C is elucidated using a recently developed thermal decomposition platform coupled with a suite of real-time aerosol instrumentation and time-integrated sampling systems. The BrC particle size distribution, morphology and optical properties are closely controlled by increasing the combusted wood mass from 50 to 600 mg to emulate those measured for “real world” wildfire particulate matter (PM) emissions. Size-fractionation of such wildfire-like BrC reveals that the PM_0.1–2.5 fraction contains high molecular weight, carcinogenic polycyclic aromatic hydrocarbons (PAHs) and absorbs up to five times more light compared to the PM_0.1 fraction. Thus, increasing the combusted wood mass from 50 to 600 mg increases the PM_0.1–2.5 concentration by a factor of about eight and enhances the overall BrC mass absorption cross-section, MAC, up to a factor of two at a wavelength of 405 nm. Condensation of VOC on BrC reduces its MAC up to 40 %. Still, the particle size seems to largely determine the BrC light absorption, as large VOC-rich particles absorb more light compared to small VOC-lean ones. The size-resolved BrC MAC measured here can be interfaced with climate models to estimate the climate impact of wildfire PM emissions.