Coronado's air quality frequently measures worse than in San Diego cities across the Coronado Bridge, according to the environmental nonprofit Emerald Keepers.

The group, led by Board President Dave Landon, published its latest weekly ocean water and air quality report for Sept. 10–16 on Monday, Sept. 21, in the Coronado Times. The report is part of a monitoring effort Emerald Keepers has run since 2023 using PurpleAir monitors installed on homes around the city to track the local Air Quality Index, or AQI.

The group said it does not yet know why Coronado's readings run higher than those on the mainland. By publishing the data weekly, Emerald Keepers said, it hopes to identify patterns and potential causes.

The county's Department of Environmental Health tests Coronado's ocean water on a regular schedule. That data, sourced from the California Water Boards and PurpleAir, feeds into the weekly reports alongside the AQI readings.

Specific AQI numbers and water testing results for the Sept. 10–16 period were not available in the published report text. The full data, including charts, is posted at emeraldkeepers.org.

Beach closure overlapped with report period

The Sept. 10–16 window overlapped with a Coronado shoreline closure that San Diego County health officials announced on Tuesday, Sept. 8. The county shut down the shoreline through North Beach because of sewage contamination from the Tijuana River.

As KPBS reported, the county's Department of Environmental Health said the beach would stay closed until sampling confirmed the area was safe for water contact. The Tijuana Slough, Imperial Beach and Silver Strand shorelines also remained closed at that time.

Whether the Coronado closure was still in effect during the full report period could not be confirmed from available county records.

The broader Tijuana sewage crisis has weighed heavily on the region. As we reported Sept. 17, the contamination costs the area up to $503 million a year.

The county reminded beachgoers that contaminated water can cause illness. Updated advisories are available at sdbeachinfo.com or by calling the county's 24-hour hotline at 619-338-2073.

Emerald Keepers publishes its water and air quality reports weekly in the Coronado Times.