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.

The Mediterranean basin is a region of particular interest for studying atmospheric aerosols due to the large variety of air masses it receives, and its sensitivity to climate change. In this study we use the newest collection (C006) of aerosol optical depth from MODIS-Aqua, from which we also derived the fine-mode fraction and Ångström exponent over the last 12 years (i.e., from 2002 to 2014), providing the longest analyzed dataset for this region. The long-term regional optical depth average is 0.20 ± 0.05, with the indicated uncertainty reflecting the inter-annual variability. Overall, the aerosol optical depth exhibits a south-to-north decreasing gradient and an average decreasing trend of 0.0030 per year (19% total decrease over the study period). The correlation between the reported AOD observations with measurements from the ground AERONET stations is high (= 0.76-0.80 depending on the wavelength), with the MODIS-Aqua data being slightly overestimated. Both fine-fraction and Ångström exponent data highlight the dominance of anthropogenic aerosols over the northern, and of desert aerosols over the southern part of the region. Clear intrusions of desert dust over the Eastern Mediterranean are observed principally in spring, and in some cases in winter. Dust intrusions dominate the Western Mediterranean in the summer (and sometimes in autumn), whereas anthropogenic aerosols dominate the sub-region of the Black Sea in all seasons but especially during summer. Fine-mode optical depth is found to decrease over almost all areas of the study region during the 12-year period, marking the decreasing contribution of anthropogenic particulate matter emissions over the study area. Coarse-mode aerosol load also exhibits an overall decreasing trend. However, its decrease is smaller than that of fine aerosols and not as uniformly distributed, underlining that the overall decrease in the region arises mainly from reduced anthropogenic emissions.