This interactive choropleth map visualizes annual PM2.5 particle concentrations across US counties from 2000 to 2018. A slider and play button animate through years, updating county fill colors via d3.scaleSequential with interpolateYlGnBu. The visualization uses d3.geoPath with geoIdentity projection, TopoJSON county and state boundaries, and React state management for temporal navigation. County-level data comes from a CSV file, with hover tooltips showing exact values.