An evaluation of chemistry's role in the winter-spring ozone maximum found in the northern midlatitude free troposphere

We employ the Geophysical Fluid Dynamics Laboratory global chemical transport model to investigate the contribution of photochemistry to the winter-spring ozone maximum in the northern hemisphere (NH) midlatitude free troposphere (770-240 mbar; 30 degrees N-60 degrees N). Free tropospheric ozone mass slowly builds up in the winter and early spring, with net chemistry and transport playing comparable roles. Winter and early spring conditions are favorable to net ozone production for two reasons: (1)Winter conditions (cold, low Sun angle, and dry) reduce HOx and lower the level of NOx needed for chemical production to exceed destruction (balance point); and (2) throughout the winter and early spring, NOx, because of its longer chemical lifetime, increases above normally net-destructive levels in the remote atmosphere. Interestingly, net production in the midlatitude NH free troposphere maximizes in early spring because relatively high NOx and low balance point conditions are present at a time when increasing insolation is speeding up photochemistry. Conceptually, the net ozone production is associated with an annual atmospheric "spring cleaning" in which high levels of NOx are removed via OH oxidation. Further, we find that human activity has a major impact on both the levels of tropospheric ozone and the role of chemistry in the NH midlatitude, where anthropogenic NOx emissions dominate. In that region, modern ozone levels have increased by similar to 20% in the winter and similar to 45% in the spring, winter-spring chemistry has switched from net destructive to net productive, the winter-spring balance between transport and chemistry has switched from transport dominance in preindustrial times to the present parity, and the preindustrial February maximum has progressed to March-April. Estimated 2020 levels of NOx emissions were found to lead to even greater net production and to push the O-3 spring maximum later into April-May.