Developing Observable Proxies to Infer Hydroxyl Radical Spatiotemporal Variability

The hydroxyl radical (OH) is an important constituent of atmospheric composition as the primary oxidant, but its lifetime is less than a second, precluding direct measurement of the spatiotemporal variability in OH throughout the atmosphere. Thus, we seek to use more readily observable species that may define OH variability in the remote atmosphere. In the remote atmosphere, formaldehyde (HCHO) is produced primarily from the reaction of methane with OH and lost via photolysis. Therefore we first examine the potential for HCHO variability to reflect OH variability. Second, we consider a convolved set of variables known to be key drivers in OH production: the rate of ozone photolysis to produce O(¹D) (J_O3), the concentrations of water vapor (H₂O) and carbon monoxide (CO), and the sum of nitric oxide and nitrogen dioxide, NO_x (NO_x = NO + NO₂).

We conduct a correlation analysis with directly measured OH to assess the viability of either HCHO or the convolved proxy using observations from the NASA Atmospheric Tomography (ATom) aircraft mission. Segregating the data by 1 km altitude bins, separately over the Atlantic and Pacific oceans, we find that both proxies capture spatial variability in OH, especially in the mid-troposphere (r² ≈ 0.7). We compare these observed relationships with those simulated by the chemistry-climate model GFDL AM3 sampled along the flight tracks. Through this work, we gain deeper insights into the drivers of atmospheric oxidation that can be applied to evaluate chemistry-climate models used to project future atmospheric composition.