Rethinking of the adverse effects of NOx-control on the reduction of methane and tropospheric ozone – Challenges toward a denitrified society

ABSTRACT

Nitrogen oxides (NOx = NO + NO2) are key precursors of tropospheric ozone (O3) together with volatile organic compounds (VOC) and carbon monoxide (CO). Since O3 has positive radiative forcing and is harmful to human health, the reduction of anthropogenic emissions of NOx is thought to be beneficial from the perspectives of climate change and air pollution in principle. However, there have been discussions contending that the reduction of NOx emissions is not necessarily beneficial for the mitigation of climate change and improvement of air quality, since 1) it decreases the atmospheric mixing ratio of hydroxyl radicals (OH), which increases the atmospheric lifetime of methane (CH4), and 2) O3 formation is VOC-limited in urban areas and the decrease of NOx emission would increases urban O3 by facilitating the NO titration effect. In order to scrutinize such discussion, literature review have been made on the temporal variations of the increasing rate of tropospheric CH4 in the last 30 years, and on urban/rural O3 issues related to the NOx-limited/VOC-limited regime. Based on the review, it may be concluded that the variation of emissions of CH4 itself paly a dominant role, and the variation of consumption rate by OH play a minor role for the recent variation of CH4. It has been suggested that NOx and NMVOC should be reduced simultaneously in order to avoid the adverse effect on climate change mitigation. From the review on policy-related discussion of NOx-limited and VOC-limited O3 formation, the increase of O3 by the decrease in NOx emissions has generally been seen in winter and nighttime when photochemical production is minimal, and the higher percentile or diurnal maximum mixing ratios of O3 in summer tends to decrease with the decrease in NOx emissions. We suggested that the NOx-limited/VOC-limited approach is not appropriate as a long-term policy guideline for ozone control, since it is unreasonable that NOx reduction is not recommended when ambient NOx levels are high, while further NOx reduction is recommended only when the VOC/NOx ratio gets high after NOx control has been achieved based on other policy principle. Simultaneous reduction of NOx and NMVOC would be beneficial for reducing global, regional, and urban O3 to alleviate climate change and human health impacts. The ultimate reduction of anthropogenic emissions of NOx can be envisioned toward a denitrified (de-NOx) society along with a decarbonized (de-CO2) society.