Comment on acp-2021-405

<strong class="journal-contentHeaderColor">Abstract.</strong> The NASA Atmospheric Tomography (ATom) mission built a photochemical climatology of air parcels based on in situ measurements with the NASA DC-8 aircraft along objectively planned profiling transects through the middle of the Pacific and Atlantic oceans. In this paper we present and analyze a data set of 10 s (2 km) merged and gap-filled observations of the key reactive species driving the chemical budgets of O<span class="inline-formula">₃</span> and CH<span class="inline-formula">₄</span> (O<span class="inline-formula">₃</span>, CH<span class="inline-formula">₄</span>, CO, H<span class="inline-formula">₂</span>O, HCHO, H<span class="inline-formula">₂</span>O<span class="inline-formula">₂</span>, CH<span class="inline-formula">₃</span>OOH, C<span class="inline-formula">₂</span>H<span class="inline-formula">₆</span>, higher alkanes, alkenes, aromatics, NO<span class="inline-formula">_<i>x</i></span>, HNO<span class="inline-formula">₃</span>, HNO<span class="inline-formula">₄</span>, peroxyacetyl nitrate, other organic nitrates), consisting of 146 494 distinct air parcels from ATom deployments 1 through 4. Six models calculated the O<span class="inline-formula">₃</span> and CH<span class="inline-formula">₄</span> photochemical tendencies from this modeling data stream for ATom 1. We find that 80 %–90 % of the total reactivity lies in the top 50 % of the parcels and 25 %–35 % in the top 10 %, supporting previous model-only studies that tropospheric chemistry is driven by a fraction of all the air. In other words, accurate simulation of the least reactive 50 % of the troposphere is unimportant for global budgets. Surprisingly, the probability densities of species and<span id="page13730"/> reactivities averaged on a model scale (100 km) differ only slightly from the 2 km ATom data, indicating that much of the heterogeneity in tropospheric chemistry can be captured with current global chemistry models. Comparing the ATom reactivities over the tropical oceans with climatological statistics from six global chemistry models, we find excellent agreement with the loss of O<span class="inline-formula">₃</span> and CH<span class="inline-formula">₄</span> but sharp disagreement with production of O<span class="inline-formula">₃</span>. The models sharply underestimate O<span class="inline-formula">₃</span> production below 4 km in both Pacific and Atlantic basins, and this can be traced to lower NO<span class="inline-formula">_<i>x</i></span> levels than observed. Attaching photochemical reactivities to measurements of chemical species allows for a richer, yet more constrained-to-what-matters, set of metrics for model evaluation.