Seminar Series

Single-Column Model Analysis of Available NIWA Observations to Determine the Self-Cleaning Capacity of the Atmosphere

Speaker

Laura Lopez-Comi Final Ph.D. Seminar

Institute

Time & Place

Thu, 24 Mar 2016 11:00:00 NZDT in Rutherford Room 531

All are welcome

Abstract

Because of the complexity constraining the hydroxyl radical (OH) in global models, a single-column model has been constructed to investigate how chemistry-climate model data biases affect OH concentrations. The single-column model has initially been set up for Lauder, a research station located in New Zealand and representative of the background conditions of southern mid-latitudes. By using observations, a bias-correction was applied to a few factors that are mostly driving the OH chemistry at this site, i.e. O3, H2O, CO, CH4, and temperature, inferring the concentrations of OH and other short-lived species. For testing purposes, a tropospheric steady-state model has been developed for Lauder to be compared with the single-column model. The result of this comparison shows that OH concentrations obtained from the single-column model are mostly consistent with those of the steady-state model, meaning that the single-column model passes a basic plausibility test of its functionality. In the sensitivity analyses using the single-column model, the contributions of O3, H2O, CO, CH4, and temperature are assessed, individually and in combination, to the budget of tropospheric OH at Lauder. Results indicate that OH responds approximately linearly to correcting biases in O3, H2O, CO, and CH4, except for temperature. Furthermore, the modelled OH obtained from driving the major forcings simultaneously shows an approximately linear relationship with the combination of the individual linear contributions. Therefore, the quantification of the individual contributions of biases in the major trace gases and temperature to OH chemistry allows for a bias-corrected calculation of OH in the troposphere at Lauder, especially for H2O and O3, which are the dominating factors controlling the OH abundance at this site. Sensitivity simulations taking the effect of clouds into account were also conducted using the single-column model. Results indicate that OH responds approximately linearly to changes in photolysis rates due to the presence of clouds. The impacts of liquid water and ice clouds were studied separately and in combination. The modelled OH responds plausibly to the presence of ice and liquid clouds corresponding to proportional changes in photolysis [jO(1D)]. The single-column model could also be applied to other clean environments of the Southern Hemisphere. However, its applicability would need to be reassessed for regions of the Northern Hemisphere where tropospheric chemistry is mainly driven by anthropogenic emissions of organic compounds that play an important role in the chemistry of O3 and OH.