Impacts of Global Emissions of CO, NOx, and CH4 on China Tropospheric Hydroxyl Free Radicals

Using the global chemistry and transport model MOZART, the simulated distributions of tropospheric hydroxyl free radicals (OH) over China and its sensitivities to global emissions of carbon monoxide (CO), nitrogen oxide (NO_x), and methane (CH₄) were investigated in this study. Due to various distributions of OH sources and sinks, the concentrations of tropospheric OH in east China are much greater than in west China. The contribution of NO + perhydroxyl radical (HO₂) reaction to OH production in east China is more pronounced than that in west China, and because of the higher reaction activity of non-methane volatile organic compounds (NMVOCs), the contributions to OH loss by NMVOCs exceed those of CO and take the dominant position in summer. The results of the sensitivity runs show a significant increase of tropospheric OH in east China from 1990 to 2000, and the trend continues. The positive effect of double emissions of NO_x on OH is partly offset by the contrary effect of increased CO and CH₄ emissions: the double emissions of NO_x will cause an increase of OH of 18.1%--30.1%, while the increases of CO and CH₄ will cause a decrease of OH of 12.2%--20.8% and 0.3%--3.0%, respectively. In turn, the lifetimes of CH₄, CO, and NO_x will increase by 0.3%--3.1% with regard to double emissions of CH₄, 13.9%--26.3% to double emissions of CO and decrease by 15.3%--23.2% to double emissions of NO_x.