Ozone and methane are chemically active climate-forcing agents affected by climate–chemistry interactions in the atmosphere. Key chemical reactions and processes affecting ozone and methane are presented. It is shown that climate-chemistry interactions have a significant impact on the two compounds. Ozone, which is a secondary compound in the atmosphere, produced and broken down mainly in the troposphere and stratosphre through chemical reactions involving atomic oxygen (O), NOx compounds (NO, NO₂), CO, hydrogen radicals (OH, HO₂), volatile organic compounds (VOC) and chlorine (Cl, ClO) and bromine (Br, BrO). Ozone is broken down through changes in the atmospheric distribution of the afore mentioned compounds. Methane is a primary compound emitted from different sources (wetlands, rice production, livestock, mining, oil and gas production and landfills).Methane is broken down by the hydroxyl radical (OH). OH is significantly affected by methane emissions, defined by the feedback factor, currently estimated to be in the range 1.3 to 1.5, and increasing with increasing methane emission. Ozone and methane changes are affected by NOx emissions. While ozone in general increase with increases in NOx emission, methane is reduced, due to increases in OH. Several processes where current and future changes have implications for climate-chemistry interactions are identified. It is also shown that climatic changes through dynamic processes could have significant impact on the atmospheric chemical distribution of ozone and methane, as we can see through the impact of Quasi Biennial Oscillation (QBO). Modeling studies indicate that increases in ozone could be more pronounced toward the end of this century. Thawing permafrost could lead to important positive feedbacks in the climate system. Large amounts of organic material are stored in the upper layers of the permafrost in the yedoma deposits in Siberia, where 2 to 5% of the deposits could be organic material. During thawing of permafrost, parts of the organic material that is deposited could be converted to methane. Furthermore, methane stored in deposits under shallow waters in the Arctic have the potential to be released in a future warmer climate with enhanced climate impact on methane, ozone and stratospheric water vapor. Studies performed by several groups show that the transport sectors have the potential for significant impacts on climate-chemistry interactions. There are large uncertainties connected to ozone and methane changes from the transport sector, and to methane release and climate impact during permafrost thawing.

A global 3-dimensional chemical transport model (CTM) has been used to study the impact of enhanced emission of pollutants in the Asian region on key chemical species like O₃, OH and nitrogen oxides. An extended set of chemical compounds are included in the model, of which 25 are transported. Diurnal variations are calculated, including the diurnal cycle of photodissociation rates. The Asian region has been selected for this perturbation study since emissions are rapidly increasing in this region, and because there is a large potential for future increases. A 15-month reference run and 2 perturbation studies, both starting in May and running for 4 months are performed, one with increases in NO_x emission of a factor of 2 and one with a factor 4. Increases in the anthropogenic emissions of hydrocarbons and CO are also included. Results for the last month (August) are then compared with the results from the reference run for the same month. In the 3-D model calculations, the adopted perturbations lead to significant increases in surface ozone in the Asian region. It is interesting to notice that enhanced short term ozone values during episodes are becoming more pronounced in the cases with high emissions. Significant ozone increases are also obtained in the upper troposphere due to efficient convective transport. The impact is noticible over a larger region in the free troposphere than in the surface layer due to longer chemical lifetime of ozone, and an efficient transport. A doubling of NO_x emission, gives significant ozone increases up to 30% in the upper troposphere. Calculations of the regionally positive radiative forcing from the increases in tropospheric ozone show values of about 0.5 W/m², which is 30–50% of the estimated negative radiative forcing due to the direct effect of sulphate aerosols in the region. DOI: 10.1034/j.1600-0889.1996.00003.x

Better measures of how global levels of OH vary from year to year will improve climate science.