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

Area of Science:

• Environmental Health Sciences
• Atmospheric Chemistry
• Epidemiology

Background:

• Estimates of health risks from fine particulate matter (PM2.5) vary due to differences in aerosol size and composition.
• Submicron particulate matter (PM1) may have stronger health impacts than PM2.5, but long-term US data has been lacking.
• This study addresses the data gap by estimating PM1 concentrations across the USA from 1998-2022.

Purpose of the Study:

• To estimate biweekly ambient PM1 concentrations and their uncertainties across the contiguous USA from 1998-2022.
• To analyze the spatial and temporal trends of PM1 concentrations.
• To investigate the relationship between PM1 and PM2.5 concentrations and their components.

Main Methods:

• Developed hybrid estimates of PM2.5 chemical composition using satellite retrievals, air quality modeling, and ground monitoring.
• Estimated biweekly gapless ambient PM1 concentrations at 1 km² resolution.
• Constrained PM1 mass fractions using observational data for major components and scientific understanding for others.

Main Results:

• PM1 concentrations showed significant spatial variation, with higher levels in the east, urban/industrial areas, and wildfire-affected regions.
• Population-weighted mean (PWM) PM1 decreased significantly (-0.23 μg/m³ per year) from 1998-2022, accounting for 86% of the PM2.5 reduction.
• The PM1/PM2.5 ratio also decreased, indicating a shift towards coarser PM2.5, likely due to reduced emissions from combustion sources.

Conclusions:

• The significant reduction in PM1, a major component of PM2.5, reflects successful air quality regulations targeting fossil fuel and combustion sources.
• The decreasing PM1/PM2.5 ratio suggests a gradual coarsening of PM2.5, necessitating separate health impact assessments for PM1 and PM2.5.
• Future monitoring, health studies, and regulatory efforts should incorporate PM1 alongside PM2.5 for comprehensive environmental health protection.