# Data Dictionary - Global City Air Quality - daily PM2.5/NO2/O3 by city, cleaned Parquet

**Units:** vary by `pollutant` - read the per-row `unit` column (overall: µg/m³).  Times are UTC.

| column | type | description |
|---|---|---|
| `zone` | str | City |
| `ts` | timestamp (UTC) | Day |
| `pollutant` | str | Pollutant (PM2.5, NO2, O3, …) |
| `value` | float | Daily mean concentration |
| `unit` | str | Unit (µg/m³ or ppm) |
| `res_min` | int | Cadence in minutes (daily) |

*Source: OpenAQ contributors.*