This map of US counties visualizes county-level PM2.5 air quality data by year, with each county colored according to its pollution level and gray when no data is available. The visualization uses d3.geoPath and d3.geoIdentity for geographic projection, d3.scaleOrdinal with a color scale for data encoding, and TopoJSON from the US Atlas for county boundaries. React manages the component lifecycle, while the D3 data joins render SVG paths with tooltips showing county name, state, and measurement value. The underlying CSV data is loaded via d3.csv and filtered by year.