Understanding the Loss of Tropospheric Chlorine Atoms

In the pursuit of improved understanding surrounding tropospheric chemistry, and thus the ability to
introduce timely and effective air quality policies, the role of atomic chlorine remains a poorly
defined area. Research has indicated that chlorine could have significant roles in the control of the
budgets of ozone (O3) and nitrogen oxides (NOx), and the determination of the lifetime of greenhouse
gases such as methane (CH4). Its overall significance in these processes, however, remains poorly
understood owing to large uncertainties in global concentrations. Understanding the sources and
sinks of Cl-atoms is therefore vital if we are to accurately evaluate its importance as a tropospheric
oxidant.

This thesis details the development of a Chlorine Comparative Reactivity Method (Cl-CRM)
instrument to directly measure the total Cl loss rate. The system was tested and calibrated using
several single and mixed hydrocarbon standards, with a working range of 12 – 400 s-1
and an uncertainty of 30% identified. Nitrogen oxide (NO) is identified as a significant source of
interference in the setup which currently prevents the field deployment of the instrument. Methods
to minimise this interference are explored, and potential reactor re-designs proposed to minimise
photolytic losses within the system.

Seasonal thermodynamic partitioning of chloride between the gas and particulate phase at an urban
background site in the UK was also investigated. The thermodynamic model ISORROPIA II is found
to underestimate particulate chloride by a factor of 2 in the summer, however this discrepancy was
attributed to charge imbalance owing to a lack of concurrent metal ion measurements. Larger
discrepancies were observed in the winter dataset for which the source is unknown. In contrast to
previous studies, we did not find any evidence that the temperature dependence of the HCl effective
equilibrium constant function used in the model is too large.