Source region definitions for the HTAP3-OPNS multi-model exercise

A NetCDF file containing definitions of the source regions used in the HTAP3-OPNS multi-model exercise. The file contains gridded data at 0.1 degree resolution. There is one variable per source region, with values of 1 where the region is defined, and 0 elsewhere. The regions are modified based on the HTAP2 Tier1 and Tier2 regions, with information north of the Arctic Circle filled in with data from https://www.gadm.org. This is version 4 of the region definitions, which corrects a bug in version 3 where the grid cells in the EMEP_East region east of 180 degrees west were incorrectly assigned to the EMEP_West region. The boundary between the EMEP_West and EMEP_East regions is set at 45 degrees East for compatibility with regional model simulations using the CAMS simulation domain, and the land-based source regions have been extended by 3 degrees into the oceans to make sure that all land-based emissions are fully covered by each source region.