CO oxidation on Pd(100), -(111), -(110), and Pt(110) single crystals was studied at steady-state conditions at low (≤2 × 10−3 Torr) and high (2−88 Torr) pressures at various reactant compositions. At low pressures the reaction fell into two regimes, one with a CO-dominant surface where the CO2 formation rate is low, and a second with an O-dominant surface where the reaction rate is high. Within this second regime, the reaction is collision-limited with no oxygen inhibition. Under high-pressure reaction conditions, three reaction regimes are evident: (i) a CO-inhibited metallic regime displaying a low CO2 formation rate; (ii) an oxygen-rich metallic regime with a high CO2 formation rate; and (iii) a high-temperature regime where the CO2 formation rate is either mass transfer limited on a metallic surface or limited by the reduced reactivity of the oxidized surface. The superior activity of Pt group metal oxides compared to the reduced metal, as proposed recently, was not observed in this study.

The CO oxidation reaction on Rh(111) was studied both at low pressures (e2 × 10 -4 Torr) under steadystate conditions and at high pressures (0.01-88 Torr) in a batch reactor at various gaseous reactant compositions. Surface CO and O coverages were determined using polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and X-ray photoelectron spectroscopy (XPS). CO titration experiments were also carried out on surfaces with known oxygen coverages. Both CO and O inhibition were evident at low pressures so that only within a relatively narrow temperature range were the reaction conditions optimized such that the CO conversion reached ∼20% of the CO flux to the surface. For high pressures and with stoichiometric or slightly oxidizing reactant ratios (O 2 /CO e 2), the reaction fell into three regimes: (i) a CO-inhibited low temperature regime where the reaction rate was determined by CO desorption; (ii) a mass transfer limited regime at high temperatures; and (iii) a transient, high-rate regime lying between regimes (i) and (ii) where the reaction was not completely controlled by mass transfer limitation. For all reaction conditions investigated (when O 2 /CO e 2), the surface oxygen coverage did not exceed ∼0.5 monolayer. With very oxidizing reactants (O 2 /CO g 5), the reactivity of the Rh surface decreased dramatically at high temperatures due to oxidation. Furthermore, the so-called "superior oxide reactivity" for CO oxidation that has been proposed in several recent studies is not evident in this investigation.

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Abstract. Extreme particulate matter (PM) air pollution of January 2013 in China was found to be associated with an anomalous eastward extension of the Siberian High (SH). We developed a Siberian High position index (SHPI), which depicts the mean longitudinal position of the SH, as a new indicator of the large-scale circulation pattern that controls wintertime air quality in China. This SHPI explains 58 % (correlation coefficient of 0.76) of the interannual variability of wintertime aerosol optical depth (AOD) retrieved by MODIS over North China (NC) during 2001–2013. By contrast, the intensity-based conventional Siberian High index (SHI) shows essentially no skill in predicting this AOD variability. On the monthly scale, some high-AOD months for NC are accompanied with extremely high SHPIs; notably, extreme PM pollution of January 2013 can be explained by the SHPI value exceeding 2.6 times the standard deviation of the 2001–2013 January mean. When the SH extends eastward, thus higher SHPI, prevailing northwesterly winds over NC are suppressed not only in the lower troposphere but also in the middle troposphere, leading to reduced southward transport of pollution from NC to South China (SC). The SHPI hence exhibits a significantly negative correlation of −0.82 with MODIS AOD over SC during 2001–2013, although the robustness of this correlation depends on that of satellite-derived AOD. The suppressed northwesterly winds during high-SHPI winters also lead to increased relative humidity (RH) over NC. Both the wind and RH changes are responsible for enhanced PM pollution over NC during the high-SHPI winters.