Tropospheric Ozone During the Last Interglacial

Abstract

Human activities have modified the atmosphere's composition since the Industrial Revolution in the 18th century, if not earlier (Ruddiman et al., 2020). While anthropogenic impacts on carbon dioxide and other long-lived greenhouse gases are well constrained (Rubino et al., 2019), much less is known about the human influence on atmospheric oxidation chemistry, and in particular, on tropospheric ozone (O 3 ).The most important photochemical cycle in the tropospheric O 3 budget involves nitrogen oxides (NO x = NO + NO 2 ) and volatile organic compounds (VOCs; here including methane and ethane). Photolysis of NO 2 produces atomic oxygen, which can react with molecular O 2 to form O 3 . This O 3 can also oxidize NO to form NO 2 and O 2 :(1)In low-NO x environments, this null photochemical cycle explains basic trends in ozone mixing ratios (Jacob, 1999). At steady state, the O 3 mixing ratio determined by this set of reactions, [O 3_ss ], can be expressed as:Here, J NO2 is the photodissociation rate of NO 2 , and k NO+O3 is the rate coefficient for Equation 3. Equation 4reveals that steady-state O 3 mixing ratios scale with the [NO 2 ]/[NO] ratio. The atmospheric oxidation of VOCs and carbon monoxide (CO) can increase the [NO 2 ]/[NO] ratio in low-NO x environments, elevating O 3 mixing ratios in both urban and remote atmospheres. The increasing use of fossil fuels over the late 20th century has led to the increases in the emission of NO x , VOCs, and CO into the atmosphere (Faïn et al., 2022;Rubino et al., 2019) and ultimately an increase in the observed tropospheric O 3 burden (Archibald et al., 2020).