Eta-CMAQ air quality forecasts for O₃ and related species using three different photochemical mechanisms (CB4, CB05, SAPRC-99): comparisons with measurements during the 2004 ICARTT study

Abstract. A critical module of air quality models is the photochemical mechanism. In this study, the impact of the three photochemical mechanisms (CB4, CB05, SAPRC-99) on the Eta-Community Multiscale Air Quality (CMAQ) model's forecast performance for O₃, and its related precursors has been assessed over the eastern United States with observations obtained by aircraft (NOAA P-3 and NASA DC-8) flights, ship and two surface networks (AIRNow and AIRMAP) during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) study. The results show that overall none of the mechanisms performs systematically better than the others. On the other hand, at the AIRNow surface sites, CB05 has the best performance with the normalized mean bias (NMB) of 3.9%, followed by CB4 (NMB=−5.7%) and SAPRC-99 (NMB=10.6%) for observed O₃≥75 ppb, whereas CB4 has the best performance with the least overestimation for observed O₃<75 ppb. On the basis of comparisons with aircraft P-3 measurements, there were consistent overestimations of O₃, NO_z, PAN and NO_y and consistent underestimations of CO, HNO₃, NO₂, NO, SO₂ and terpenes for all three mechanisms although the NMB values for each species and mechanisms were different. The results of aircraft DC-8 show that CB05 predicts the H₂O₂ mixing ratios most closely to the observations (NMB=10.8%), whereas CB4 and SAPRC-99 overestimated (NMB=74.7%) and underestimated (NMB=−25.5%) H₂O₂ mixing ratios significantly, respectively. For different air mass flows over the Gulf of Maine on the basis of the ship data, the three mechanisms have relatively better performance for O₃, isoprene and SO₂ for the clean marine or continental flows but relatively better performance for CO, NO₂ and NO for southwesterly/westerly offshore flows. The results of the O₃-NO_z slopes over the ocean indicate that SAPRC-99 has the highest upper limits of the ozone production efficiency (ε_N) (5.8), followed by CB05 (4.5) and CB4 (4.0) although they are much lower than that inferred from the observation (11.8), being consistent with the fact that on average, SAPRC-99 produces the highest O₃, followed by CB05 and CB4, across all O₃ mixing ratio ranges