A GLOBAL 3-DIMENSIONAL CHEMICAL-TRANSPORT MODEL .2. NITROGEN-OXIDES AND NONMETHANE HYDROCARBON RESULTS

In this paper, nitrogen oxide and total nonmethane hydrocarbons (NMHCs ) from the University of Oslo global three-dimensional chemical transp ort model (CTM) are presented and compared with data from both surface and free tropospheric observations. The model includes the most impor tant processes, including photochemical and thermal reactions, wet and dry deposition, natural and anthropogenic emissions, stratospheric co ntributions of NOy, and daily variations in the meteorological fields generated from the NASA Goddard Institute for Space Studies general ci rculation model. The model exhibits daily and seasonal variations in N MHC and NOy species, consistent with our understanding of tropospheric photochemistry. Generally good agreement between modeled mixing ratio s and observations is shown for NOy, PAN and total NMHCs over 3 orders of magnitude in both source and remote regions. For NOx, the model gi ves good agreement in rural and urban regions; however it tends to und er predict clean free tropospheric concentrations by about 20 parts pe r trillion by volume (50-100%). This is most likely due to subgrid bou ndary layer exchange processes, too efficient removal of NOx and/or re cent increases in NOx emissions which are not included in our 1980 emi ssions inventory (especially in east Asia). Seasonal cycles for NOx, P AN, NOy and NMHCs from a number of surface locations where these data are available are reproduced well. Speciation of NOy in the model is s hown to be consistent with observations at three sites where such data are available; Scotia, Pennsylvania; Mauna Loa, Hawaii; and Alert, No rthwest Territories where different NOy species dominate at each locat ion (NOx, HNO3, and PAN respectively). Several uses of the CTM are dem onstrated. Total tropospheric NOx, PAN, HNO3, and NOy are calculated w ith the model and shown to vary seasonally. The seasonal variations ar e consistent with enhanced lifetimes during winter. The model is also used to determine the fractional contributions of NOy from the major a nthropogenic source regions (Europe, Asia, and North America) at Mauna Loa. Future applications of the model will focus on the global ozone and nitrogen budgets and the sources of NOx to the remote free troposp here.