Estimates of submicron particulate matter (PM₁) concentrations for 1998-2022 across the contiguous USA: leveraging measurements of PM₁ with nationwide PM_2·5 component data.

Abstract

Excess health risk estimates of exposure per unit mass concentration of fine particulate matter (PM_2·5) still exhibit a wide range, potentially due to variations in aerosol size and composition. Submicron particulate matter (PM₁) was recently reported to exert stronger health impacts than PM_2·5 from studies in China, but an absence of long-term PM₁ data in the USA has prohibited such investigations despite a wealth of cohorts. This study aims to fill this data gap and estimate PM₁ concentrations over 1998-2022 across the USA. We estimated biweekly gapless ambient PM₁ concentrations and their uncertainties at 1 km² resolution across the contiguous USA over the 25-year period of 1998-2022, from hybrid estimates of PM_2·5 chemical composition that merged information from satellite retrievals, air quality modelling, and ground-based monitoring. The mass fractions of PM_2·5 components with diameters below 1 μm were constrained by observations for four major components and from established scientific understanding for the other components. PM₁ concentrations exhibited pronounced spatial variation across the contiguous USA with enhancements observed in the east, major urban and industrial areas, and areas affected by wildfires; low concentrations are prevalent over the arid west. The main components of population-weighted mean (PWM) PM₁ in 2022 (6·1 μg/m³) were organic matter (47%), sulphate (22%), nitrate (12%), black carbon (8%), and ammonium (7%). The biweekly PM₁ estimates were highly consistent with independent ground-based PM₁ measurements (slope=0·96, R²=0·78). The estimated 1-σ uncertainties of annual mean PM₁ for the 25 years over more than 8 million land pixels were less than 20% for 98% of data points, while 0·3% of the population of the contiguous USA was associated with uncertainties of more than 30% due to wildfires. The PWM PM₁ decreased significantly (p<0·0001) at a rate of -0·23 μg/m³ per year during 1998-2022, accounting for 86% of the overall reduction of PWM PM_2·5; the PWM PM₁/PM_2·5 ratio experienced simultaneous decrease (-0·0013 per year, p<0·0001). The dominance of PM₁ in PM_2·5 reduction and the decreasing PM₁/PM_2·5 ratio reflect the strong association of PM₁ with fossil fuel and other combustion sources and their responses to air quality regulations during the 25-year study period. The gradual coarsening of PM_2·5 calls for increasing urgency to separately assess health impacts of PM₁ versus PM_2·5, as supported by the quality of the derived PM₁ estimates. Future particulate matter monitoring programmes, health studies, and regulatory deliberations should consider PM₁ in addition to PM_2·5. National Institute of Environmental Health Sciences, National Institutes of Health.