The map shows the Sentinel-5P TROPOMI Aerosol Index over nine Southern California counties: Santa Barbara, Ventura, Los Angeles, Orange, San Diego, Imperial, Riverside, San Bernardino, and Kern. It runs from January 1 through February 28, 2025, a period that includes the weeks before, during, and after the January 2025 Los Angeles wildfires. Each of the 59 daily dates is one step of the time slider, and the color scale is the same on every date.
The Sentinel-5P TROPOMI Aerosol Index is the only analysis dataset. Pixels are used
only where qa_value > 0.8. For each county and date, the remaining
pixels are averaged with each pixel weighted by its area inside the county. The
result is the area-weighted county mean, ai_mean, which is the value the
map colors.
County boundaries come from the U.S. Census Bureau. The output is 531 county-day records, one for each of the nine counties on each of the 59 dates, hosted as a feature layer in ArcGIS Online. A county-day without enough valid pixels stays in the record and is drawn gray.
The Aerosol Index is a unitless indicator for the whole atmospheric column, not for the air at ground level. Positive values can indicate UV-absorbing aerosols such as smoke or dust. The map is not a measurement of surface PM2.5, and it does not estimate smoke concentration, exposure, health risk, or crop yield impact. The ends of the color scale, −1.5 and 1, are display limits, not health or pollution thresholds.
Sources
Aerosol Index data are the Sentinel-5P TROPOMI Aerosol Index Level 2 product, version 2, distributed by NASA GES DISC and described in the product readme (PDF). County boundaries are the 2024 U.S. Census Bureau cartographic boundary files.