The high-resolution version of TM5-MP for optimised satellite retrievals: Description and Validation
Authors: Williams J. E. , Boersma K. F. , Le Sager P. , Verstraeten W. W. 
Published: July 2016

Abstract. We provide a comprehensive description of the high-resolution version of the TM5-MP global Chemistry-Transport Model which is employed to provide highly resolved vertical profiles of nitrogen dioxide (NO2), formaldehyde (CH2O), and sulphur dioxide (SO2) for use in satellite retrievals from platforms such as the Ozone Monitoring Instrument (OMI) and the Sentinel-5 Precursor, the TROPOspheric Monitoring Instrument (TROPOMI). Comparing seasonal differences in the global distributions of 222Rn we show that there are generally differences of &amp;pm;20 %, with larger increases occurring near specific coastal and decreases over specific tropical ocean regions. Analyzing vertical profiles of 222Rn above strong nitrogen oxide (NOx) source regions, we show that there are differences in the strength of the convective activity of around ~ 2–10 % (~ 10–20 %) at 1º × 1º constrained below 700 hPa (200 hPa) in the NH (tropics). We analyze the global distribution and chemical budget terms for tropospheric ozone (O3), the reactive NOx and N-reservoir species from simulations performed at 1º × 1º horizontal resolution. Compared to simulations at 3º × 2º, we show that although the impact on photolysis rates may be important regionally, changes in the seasonal means representative of the boundary layer are of the order of a few percent, in spite of the higher spatial variability of meteorological data fields from ERA-Interim. Surface concentrations of O3 in high-NOx regions decrease by between ~ 5–10 % at 1º × 1º as a result of a reduction in NOx recycling terms and an increase in the titration term of O3 involving NO. At 1º × 1º the net global stratosphere-troposphere exchange of ozone decreases by ~ 7 %, dominated by substantial reductions in the Northern Hemisphere somewhat offset by increases in down-welling which occurs in the Southern Hemisphere. By comparing NO, NO2, HNO3 and PAN profiles against a host of measurements made across large regional domains, we show that TM5-MP captures the vertical distribution of NOx and long-lived NOx reservoirs at background locations, but exhibits a too high NO/NO2 ratio in the Free Troposphere, with changes at 1º × 1º being limited to a few percent. Surface mixing ratios in both NO and NO2 are generally under-estimated in both low and high NOx scenarios. For Europe, we show that there is a negative bias in NO concentrations at the surface across the whole domain, with lower biases at 1º × 1º at only ~ 20 % of sites. For NO2, biases are more variable, with lower (higher) biases occurring at ~ 35 % (~ 20 %) of sites. For nitric acid (HNO3) there is a seasonal cycle present at the surface in TM5-MP which is not observed, leading to a low bias during wintertime and a high bias during summertime. For the shorter-lived NOx reservoirs, no significant changes occur with the exception of N2O5 due to ~ 10 % perturbations in the NO3 radical. For CH2O, the impact of higher resolution on the chemical budget terms is rather modest, with changes less than 5 %. The simulated vertical distribution of CH2O agrees reasonably well with measurements in pristine locations, but CH2O vertical column densities are generally underestimated relative to satellite measurements in polluted regions. For SO2, the performance at 1º × 1º is principally governed by the quality of the emission inventory, with limited improvements in the site specific biases with most showing no significant improvement. For the vertical column, improvements near strong source regions occur which reduce the biases in the integrated column.