R. von Kuhlmann

Development and intercomparison of condensed isoprene oxidation mechanisms for global atmospheric modelling

A new condensed isoprene oxidation mechanism forglobal atmospheric modeling (MIM) was derived from ahighly detailed master chemical mechanism (MCM). In abox model intercomparison covering a wide range ofboundary layer conditions the MIM was compared withthe MCM and with five other condensed mechanisms, someof which have already been used in global modelingstudies of nonmethane hydrocarbon chemistry. Theresults of MCM and MIM were generally in goodagreement, but the other tested mechanisms exhibitedsubstantial differences relative to the MCM as well asrelative to each other. Different formation yields,reactivities and degradation pathways of organicnitrates formed in the course of isoprene oxidationwere identified as a major reason for the deviations.The relevance of the box model results for chemistrytransport models is discussed, and the need for avalidated reference mechanism and for an improvedrepresentation of isoprene chemistry in global modelsis pointed out.

On the impact of heterogeneous chemistry on ozone in the tropopause region

We examine the impact of heterogeneous chemistry involving liquid aerosol and ice particles on net ozone (O3) production rates under conditions representative of the midlatitude upper troposphere (UT) and lowermost stratosphere (LS). We demonstrate that heterogeneous effects are controlled by nitrogen oxides (NOx) and by the location of the air masses relative to the tropopause (TP). The net effect of heterogeneous chemistry is to decrease net O3 production below the TP (via heterogeneous HO2 loss) and to cause O3 destruction above the TP (via heterogeneous chlorine (Cl) activation). In the UT, gas phase chemistry due to non-methane hydrocarbons (NMHCs) can become as important for O3 chemistry as heterogeneous reactions, and removal of HO2 by particles can become more important than changes of hydrogen oxides (HOx) through heterogeneous bromine (Br) chemistry. In the humid LS, Cl activation can become sufficiently large, so that O3 depletion occurs at all conceivable values of NOx. Such cold and humid conditions occur frequently enough to reduce the average ozone production rates in the midlatitude LS by more than 10%.

Global Photochemical Model Evaluation using GOME Tropospheric Column Data

The focus of our TROPOSAT-related work has been mainly on comparisons of tropospheric NO2 with output from our global chemistry-transport model MATCH-MPIC. Global monthly NO2 distributions from MATCH-MPIC were compared with GOME NO2 columns in von Kuhlmann (2001). A closer focus on NO2 over Asia has been taken by a joint PhD student with the University of Bremen (T. Kunhikrishnan); the first part of this study has been aimed at improving our understanding of NOX in the troposphere over Asia (especially India), and at developing an improved estimate of the NOX emissions rate from this region, through the use of sensitivity runs with MATCH-MPIC along with GOME tropospheric NO2 columns. We first verified that the retrieval assumptions from GOME (stratospheric zonal symmetry and negligible troposphere NO2 in the Pacific reference sector) hold well for MATCH-MPIC. We then showed that the model simulates the mean NO2 amounts in the region fairly well, and that the NOX lifetime of about 15 hours computed by the model, agrees well with the lifetime derived from the decay of NO2 off the west coast of India. Taken together, these imply that the current total magnitude of NOX emissions in MATCH-MPIC is reasonable. While 60–70 % of the NOX in the lower troposphere over India comes from Indian emissions, in the upper troposphere the NOX comes mainly from outside the region during the winter months and from convective transport and lightning during the summer monsoon.