Readme file for monthly back-trajectory data  from FLEXPART-ERA5
(total + based on ozone percentiles over Western North America (WNA))

* The filename: YYYY-MM_NH.nc (Example file: 2001-02_NH.nc)
* The data is saved as NetCDF format. It contains monthly averaged back-trajectory data (referred to as "retroplume"). The unite is s m3 kg-1.
* The data is saved at a 1-degree x 1-degree resolution for longitude (-180° to 179°) and latitude (0° to 89°N) in the Northern Hemisphere. 
* The values are stored as percentiles of ozone levels over Western North America (WNA), calculated relative to a baseline ozone value from 2004-2014. This baseline helps reduce sampling errors associated with varying numbers of observations across different years. Nonetheless, no significant sensitivity was observed with the choice of baseline years.
* The percentiles are divided into the following ranges: 0-5 (cleanest ozone level over WNA), 5-33, 33-50, 50-66, 66-95, 95-100 (highest ozone level over the WMA), and total (0-100; all ozone level).
* The back-trajectory data are available for each longitude and latitude point, across five vertical layers: 0-300m, 300m-3km, 3-8km, 8-13km, and 13-20km.
* The number of back-trajectory data points (e.g., count_{each percentile range}_all; e.g., 0-5 for the clean ozone percentile) within each percentile range is also saved in the file.