4. Probability densities of critical species
The sensitivity analysis here helps us identify the critical species where a bias in the modeled distribution will produce a large bias in the budgets of O3 and CH4. The main species singled out here (NOx, O3, CO, CH4) are well known and CCMs often use these as standard measurement metrics. A surprise was the emergence of CH3OOH as being important for P-O3, and a global climatology of this species should be a priority measurement. Although we have long known that H2O and T are important factors (e.g., Table 2 of Holmes et al., 2013), these quantities have often been relegated to the physical climate system and not often though of a major source of model error in chemical system. Thus, when we are diagnosing the future tropospheric O3 or CH4 from the multi-model comparisons (Stevenson et al., 2013; Voulgarakis et al., 2013; Young et al., 2013, 2018; Griffiths et al., 2021), we need to document biases in T and H2O.
Comparing T with mean profiles is straightforward, but H2O is more difficult, even with profiles, because of the 3 orders of magnitude change over the troposphere. Thus, we recommend that relative humidity over liquid water (RHw, %) be used to detect bias. We show histograms of probability densities (PDs) of RHw from ATom-1234 in Figure 1c. We select the three tropical basin (C. Pacific, E. Pacific, Atlantic) and lower troposphere (0-6 km) because this is where most of the reactivity occurs (see Figure 1b). The ATom data for RHw show a clear bimodal distribution with extensive tropic regions having narrow PD with RHw < 10% and the bulk showing a broad PD about 80%. The E. Pacific has high seasonality with ATom-14 (the more reactive periods) lacking air with RHw < 20%, consistent with the high reactivities noted above. PDs and for O3, NOx and CO, as well as the 0-12 km PD for RHw, are shown in Figures S34-S37. These gases show a wide range of seasonality, especially in the Pacific and provide a challenge for the models.