Normalized CH4-O3 relation from Fiore et al (2008)
ch4_htap_pertAn object of class numeric of length 1.
Fiore, A.M., West, J.J., Horowitz, L.W., Naik, V. and Schwarzkopf, M.D., 2008. Characterizing the tropospheric ozone response to methane emission controls and the benefits to climate and air quality. Journal of Geophysical Research: Atmospheres, 113(D8).