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

BACKGROUND

Excess health risk estimates of exposure per unit mass concentration of fine particulate matter (PM) still exhibit a wide range, potentially due to variations in aerosol size and composition. Submicron particulate matter (PM) was recently reported to exert stronger health impacts than PM from studies in China, but an absence of long-term PM data in the USA has prohibited such investigations despite a wealth of cohorts. This study aims to fill this data gap and estimate PM concentrations over 1998-2022 across the USA.

METHODS

We estimated biweekly gapless ambient PM concentrations and their uncertainties at 1 km resolution across the contiguous USA over the 25-year period of 1998-2022, from hybrid estimates of PM chemical composition that merged information from satellite retrievals, air quality modelling, and ground-based monitoring. The mass fractions of PM components with diameters below 1 μm were constrained by observations for four major components and from established scientific understanding for the other components.

FINDINGS

PM 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) PM in 2022 (6·1 μg/m) were organic matter (47%), sulphate (22%), nitrate (12%), black carbon (8%), and ammonium (7%). The biweekly PM estimates were highly consistent with independent ground-based PM measurements (slope=0·96, R=0·78). The estimated 1-σ uncertainties of annual mean PM 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 PM decreased significantly (p<0·0001) at a rate of -0·23 μg/m per year during 1998-2022, accounting for 86% of the overall reduction of PWM PM; the PWM PM/PM ratio experienced simultaneous decrease (-0·0013 per year, p<0·0001).

INTERPRETATION

The dominance of PM in PM reduction and the decreasing PM/PM ratio reflect the strong association of PM 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 calls for increasing urgency to separately assess health impacts of PM versus PM, as supported by the quality of the derived PM estimates. Future particulate matter monitoring programmes, health studies, and regulatory deliberations should consider PM in addition to PM.

FUNDING

National Institute of Environmental Health Sciences, National Institutes of Health.