A proposed two-step mechanism for the formation of nitrogen dioxide is given below.
Step 1 (fast): NO(g) + O3(g) → NO2(g) + O2(g)
Step 2 (slow): NO2(g) + O(g) → NO(g) + O2(g)
Which species is most appropriately classified as an intermediate in this mechanism?
a) O3(g)
b) NO2(g)
c) NO(g)
d) O2(g)