REPOSITORY > RESULTS

Abstract

The atmospheric mixing state (or “stability”) is of considerable interest because it is among
the largest factors influencing air pollution variability, which has become a major global
environmental issue. Diurnal timescale changes in the atmospheric mixing state, associated
with daytime convection and the formation of nocturnal inversion layers, are known to
strongly influence air quality in urban settings, which are home to 54% of the global
population. However, the atmospheric mixing state in regions of complex topography (e.g.
basin/valley environments) can also change on longer timescales (e.g. synoptic), as evident
by persistent temperature inversion (PTI) conditions in non-summer months. PTI events
can cause periods of high pollution concentrations that persist for many consecutive days.
Common conventional meteorological techniques (e.g. Pasquill-Gifford schemes), more
sophisticated stability classification methods (e.g. Richardson Number, Monin-Obukhov
similarity), and even more recently-developed diurnal atmospheric stability classification
techniques based on near-surface observations of the natural radioactive noble gas radon
(222Rn), are not capable of reliably identifying synoptic timescale changes of atmospheric
stability in complex terrain for separate analysis.
Consequently, the emphasis of this research was placed on developing a novel radonbased
technique to identify synoptic timescale changes in the atmospheric mixing state,
and combining this with the existing diurnal radon based technique, to better characterise
and understand the comparative atmospheric mixing states of two complex terrain
environments in Slovenia (Ljubljana, an urban subalpine basin and Ajdovščina, a sub-
Mediterranean town within Vipava Valley) on diurnal, synoptic, and seasonal timescales.
This Ph.D. thesis consists of three main investigations.
Firstly, a novel radon-based tool is developed for identifying PTI conditions in
topographic basin/valley environments, which is demonstrated to be more reliable in its
characterization of meteorology and air quality during PTI events than the conventional
pseudo-vertical temperature gradient method. Secondly, a contemporary diurnal radonbased
technique was used to investigate relative changes in summer atmospheric stability
at two selected sites. The obtained results showed that radon-based stability classification
can consistently distinguish between subtle changes in atmospheric mixing state at the two
observation sites with strongly contrasting topographic settings. Thirdly, PTI conditions
identified by the new radon-based tool were assigned a new (separate) mixing class within
the framework of the contemporary diurnal radon-based stability classification method.
The combined diurnal and synoptic timescale radon-based mixing classification technique
separated, for the first time, “uncontrollable” influences on pollution variability (i.e.
meteorology, geographic setting, topographic effects, and cross boundary pollution) from
controllable influences (e.g. source distributions and strength) in the urban Ljubljana
Basin. The technique developed is universally applicable to non-coastal urban centers
throughout Europe, as well as to other continental mid- to high-latitude regions (e.g.
central North America).
Results of this Ph.D. study have demonstrated that radon-based methods of
atmospheric mixing state classification in urban regions (of either simple or complex
topographic settings) is simpler and more effective than pure meteorological approaches.
Furthermore, they have provided a novel and consistent tool with which to account for
both diurnal and synoptic timescale changes in the atmospheric mixing state of complex
terrain, and how this impacts air quality. Based on the consistency and accuracy of novel
radon-based atmospheric classification method developed, it is expected that it will prove
to be a powerful tool for improved assessment of pollution mitigation measures, evaluating
the performance of urban pollution models, and informing public health studies.