Several chemically reactive gases, including reactive nitrogen species (NO_x), carbon monoxide (CO), and the volatile organic compounds (VOCs), control, in part, the oxidizing capacity of the troposphere, as well as the abundance of ozone. These pollutants act as indirect greenhouse gases through their influence not only on ozone, but also on the lifetimes of CH₄ and other greenhouse gases. The emissions of NO_x and CO are dominated by human activities.   Carbon monoxide is identified as an important indirect greenhouse gas. Model calculations indicate that emission of 100 Mt of CO is equivalent in terms of greenhouse gas perturbations to the emission of about 5 Mt of CH₄. The abundance of CO in the Northern Hemisphere is about twice that in the Southern Hemisphere and has increased in the second half of the 20th century along with industrialization and population.

    The reactive nitrogen species NO and NO₂, (whose sum is denoted NO_x), are key compounds in the chemistry of the troposphere, but their overall radiative impact remains difficult to quantify. The importance of NO_x in the radiation budget is because increases in NO_x concentrations perturb several greenhouse gases; for example, decreases in methane and the HFCs and increases in tropospheric ozone. Deposition of the reaction products of NO_x fertilises the biosphere, thereby decreasing atmospheric CO₂. While difficult to quantify, increases in NO_x that are projected to the year 2100 would cause significant changes in greenhouse gases.