Impact of non-methane hydrocarbons on tropospheric chemistry and the oxidizing power of the global troposphere: 3-dimensional modelling results

The impact of natural and anthropogenic non-methane hydrocarbons (NMHC) on tropospheric chemistry is investigated with the global, three-dimensional chemistry-transport model MOGUNTIA. This meteorologically simplified model allows the inclusion of a rather detailed scheme to describe NMHC oxidation chemistry. Comparing model results calculated with and without NMHC oxidation chemistry indicates that NMHC oxidation adds 40-60% to surface carbon monoxide (CO) levels over the continents and slightly less over the oceans. Free tropospheric CO levels increase by 30-60%. The overall yield of CO from the NMHC mixture considered is calculated to be about 0.4 CO per C atom. Organic nitrate formation during NMHC oxidation, and their transport and decomposition affect the global distribution of NOx and thereby O-3 production. The impact of the short-lived NMHC extends over the entire troposphere due to the formation of longer-lived intermediates like CO, and various carbonyl and carboxyl compounds. NMHC oxidation almost doubles the net photochemical production of O-3 in the troposphere and leads to 20-80% higher O-3 concentration in NOx-rich boundary layers, with highest increases over and downwind of the industrial and biomass burning regions. An increase by 20-30% is calculated for the remote marine atmosphere. At higher altitudes, smaller, but still significant increases, in O-3 concentrations between 10 and 60% are calculated, maximizing in the tropics. NO from lightning also enhances the net chemical production of O-3 by about 30%, leading to a similar increase in the global mean OH radical concentration. NMHC oxidation decreases the OH radical concentrations in the continental boundary layer with large NMHC emissions by up to 20-60%. In the marine boundary layer (MBL) OH levels can increase in some regions by 10-20% depending on season and NOx levels. However, in most of the MBL OH will decrease by 10-20% due to the increase in CO levels by NMHC oxidation chemistry. The large decreases especially over the continents strongly reduce the marked contrasts in OH concentrations between land and ocean which are calculated when only the background chemistry is considered. In the middle troposphere, OH concentrations are reduced by about 15%, although due to the growth in CO. The overall effect of these changes on the tropospheric lifetime of CH4 is a 15% increase from 6.5 to 7.4 years. Biogenic hydrocarbons dominate the impact of NMHC on global tropospheric chemistry. Convection of hydrocarbon oxidation products: hydrogen peroxides and carbonyl compounds, especially acetone, is the main source of HOx in the upper troposphere. Convective transport and addition of NO from lightning are important for the O-3 budget in the free troposphere.