The effect of updating scavenging and conversion rates on cloud droplets and ice particles in the TM global chemistry transport model

In this technical report we document the updates which have been made to the heterogeneous reaction data used online in the TM model for the calculation of Henry’s Law Co-efficients. The hydration step which needs to be included for the correct partitioning of aldehydes is now included explicitly which results in increases in the fraction transferred into the aqueous phase. In addition, wet deposition of all aldehydes is now included in the model. This update reduces the resident [HCHO] by between ~2-10% as a result of a ~65% increase in the amount lost through wet deposition, where the largest loss occurs in the tropics between 20N-20S. The subsequently reduction in [HO2] results in less in-situ formation of the peroxy-species H2O2, CH3O2H and ROOH, which contributes to the ~3-5% decrease in the tropospheric burdens for these species,. For methylglyoxal, the wet deposition term becomes the second most important sink process in the model once included.

A second update is associated with the introduction of parameterisations for a more accurate description of the size (effective radius)of ice particles and the corresponding reactive surface area in the model. Moreover, the reactive surface area from cloud droplets is also explicitly introduced into the calculation of the heterogeneous conversion of N2O5 into HNO3 on the surface of such particles. For TM4, this significantly reduces the loss of N2O5 compared to the original formulation, which subsequently increases both [O3] and [NO3]. This is associated, in part, with an erroneous calculation of the Liquid Water Content in TM4. For TM5 the effects are more modest, where the heterogeneous conversion rate of N2O5 into HNO3 decreases by ~50%.

Finally the gas phase conversion of N2O5 into HNO3 involving water vapour is introduced into the modified CBM4 mechanism and tested online. This reaction has previously been identified as a large source of uncertainty between the different chemical mechanisms adopted in global CTM’s. Analysis of the chemical budget shows that in the presence of heterogeneous conversion routes the influence of this additional conversion term is rather negligible.

Comparisons against a host of different in-situ measurements shows that adopting the updates does not degrade the performance of the model, where improvements are made to the mid-tropospheric ozone burdens and HCHO in the lower troposphere.

It is recommended that the new heterogeneous reaction data and the parameterisations for the calculation of the micro-physical properties of ice particles are adopted across all versions of the TM model.