Table 2. Second-order, cross-term sensitivities of the reactivity L-CH4 relative to critical species (NOx, O3, CH4, CO) calculated in pair combinations. Results here are from ATom-1 and the Pacific basin (54°S-60°N). The single +10% sensitivities calculated for individual species, S(X+10%), are shown in italics along the diagonal. The doubly signed (i.e., ++ or –) off-diagonal values represent the additional change in sensitivity due to the coupling of the two species: S(X+10% & Y+10%) – S(X+10%) – S(Y+10%). When the 2nd-order is small (only the O3 x CO term here), the coupled perturbation is simply the sum of the two individual ones. Atom-1 results for other reactivities (P-O3, L-O3) and the Atlantic basin are given in Supplemental Table S3.