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 c hemistry-transport model MOGUNTIA. This meteorologically simplified model a llows the inclusion of a rather detailed scheme to describe NMHC oxidation chemistry. Comparing model results calculated with and without NMHC oxidati on chemistry indicates that NMHC oxidation adds 40-60% to surface carbon mo noxide (CO) levels over the continents and slightly less over the oceans. F ree 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. O rganic nitrate formation during NMHC oxidation, and their transport and dec omposition affect the global distribution of NOx and thereby O-3 production . The impact of the short-lived NMHC extends over the entire troposphere du e to the formation of longer-lived intermediates like CO, and various carbo nyl and carboxyl compounds. NMHC oxidation almost doubles the net photochem ical production of O-3 in the troposphere and leads to 20-80% higher O-3 co ncentration in NOx-rich boundary layers, with highest increases over and do wnwind of the industrial and biomass burning regions. An increase by 20-30% is calculated for the remote marine atmosphere. At higher altitudes, small er, but still significant increases, in O-3 concentrations between 10 and 6 0% are calculated, maximizing in the tropics. NO from lightning also enhanc es the net chemical production of O-3 by about 30%, leading to a similar in crease in the global mean OH radical concentration. NMHC oxidation decrease s the OH radical concentrations in the continental boundary layer with larg e NMHC emissions by up to 20-60%. In the marine boundary layer (MBL) OH lev els can increase in some regions by 10-20% depending on season and NOx leve ls. However, in most of the MBL OH will decrease by 10-20% due to the incre ase in CO levels by NMHC oxidation chemistry. The large decreases especiall y over the continents strongly reduce the marked contrasts in OH concentrat ions between land and ocean which are calculated when only the background c hemistry is considered. In the middle troposphere, OH concentrations are re duced 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 globa l tropospheric chemistry. Convection of hydrocarbon oxidation products: hyd rogen peroxides and carbonyl compounds, especially acetone, is the main sou rce of HOx in the upper troposphere. Convective transport and addition of N O from lightning are important for the O-3 budget in the free troposphere.