Single-column model analysis of available NIWA observations to determine the self-cleaning capacity of the atmosphere.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
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. By using a single-column model, only the fast photochemical processes of the overhead column are considered, hence neglecting the interaction with circulation dynamics present in chemistry-climate models. As a starting point to this work, the single-column model has been set up for Lauder, a research station located in New Zealand representative of the background conditions of the southern mid-latitudes. By using long-term observations and other available data (e.g. re-analysis data), 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 for Lauder has been developed 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 contribution of O3, H2O, CO, CH4, and temperature to the budget of tropospheric OH at Lauder has been assessed, individually and in combination. Results indicate that OH responds approximately linearly to correcting biases in O3, H2O, CO, and CH4, except for temperature. The individual sensitivity coefficients show directly related OH responses to relative changes in O3 and photolysis [jO(1D)], which range from around 0 % to 25 %, and between 20 % and 50 % respectively. The response of OH is also directly related to the applied relative changes in H2O, varying from around 5 - 10 % to 50 %. The OH sensitivity to correcting CH4 and CO biases is inversely related to the relative changes applied to these two chemical species, which range from about -17 % to -35 %, and between -30 % and -50 % respectively. The assessment of the effects of temperature in OH indicates a non-linear response of OH to temperature biases, but these effects are found to be small. 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. Additional analyses of long-term time series of OH at Lauder under clear-sky conditions provide evidence of short-term variations of OH but a significant long-term trend (5.4 ± 2.7 % at the 95 % confidence interval) was only found at 5 - 7.5 km in the troposphere. This trend in OH is mainly caused by an increase in humidity in the re-analysis data at these altitudes. 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 clouds corresponding to proportional changes in jO(1D) that vary between 0 % - 10 %, 0 % - 12 %, and 0 % - 20 % due to ice, liquid water, and the combination of ice + liquid water clouds respectively. Moreover, the vertical distribution of clouds seems to have more in uence on photolysis and OH, rather than the change in cloud water content. Because of the large uncertainty of the impact of clouds on photolysis and OH in global models, and the lack of suitable observations for clouds at Lauder to constrain the single-column model, their impact on OH has been quantified separately from the effect of bias-correcting the major forcings. By using a single-column model, only instantaneous changes in the chemistry of the overhead column caused by correcting biases in the major species to OH chemistry have been considered. The advantage is that it allows straight forward control of all impact parameters and enables separation of long- and short-lived effects. The single-column model could also be applied to other clean environments using the same methods conducted in this work. However, its applicability would need to be reassessed for regions where tropospheric chemical conditions become more complex, i.e. organic compounds and NOx that play a key role in the chemistry of O3 and OH.