text: For a given background composition we may thus estimate 
  the fractional coverages for each available adsorbate for a 
  given type of doping. As an example, Fig. 3(a)-(c) shows the 
  fractional coverage of a Ni atom occupying a monovacancy, 
  divacancy I, and divacancy II, versus CO concentration in a 
  background of air at room temperature and 1 bar of pressure. 
  Due to the relatively small binding energy of N2 and H2O as 
  compared to O2 and CO, all Ni sites will be either empty or 
  occupied by O2 or CO. In particular, Ni in a monovacancy 
  (top panel of Fig. 3) will be completely oxidized for all rel- 
  evant CO concentrations. For the Ni occupied divacancy II 
  structures we ﬁnd the coverage of CO changes signiﬁcantly 
  around toxic concentrations (∼10 ppm). 
  In Fig. 3(d) we show the change in resistance calculated 
  from Eq. (7) as a function of CO concentration for Ni occu- 
  pying the three types of vacancies. The background reference 
  concentration of CO is taken to be C0 = 0.1 ppm. For the 
  monovacancy there is very little change in resistivity. This is 
  because most active sites are blocked by O2 at relevant CO 
  concentrations, as shown in the upper panel of Fig. 3. For Ni 
  in the divacancies there is, however, a change in resistance on 
  the order of 1Ω per site. For concentrations above ∼1 ppm, 
  the CO coverage of Ni in the divacancy II increases dramati- 
  cally and this leads to a signiﬁcant increase in resistance. 