Development of a novel instrument for long-term measurements of OH reactivity

Improving air quality is one of the main challenges in achieving a sustainable future. The mitigation strategies for air quality and climate change require accurate knowledge of both the amount of trace gases present in the atmosphere and chemical processes involving these trace gases. The primary removal mechanism of trace species such as methane (CH₄), volatile organic compounds (VOCs), and NO_x (NO_x = NO + NO₂) in the atmosphere is the reaction with hydroxyl radical (OH), which leads to the formation of secondary pollutants such as ozone (O₃) and secondary organic aerosol (SOA). Thus, understanding the behavior of OH in the atmosphere is critical to understanding the lifetimes of many trace species and the reaction pathways leading to the production of secondary pollutants. Although it is not possible to quantify all species in the atmosphere that react with OH, it is possible to quantify their impacts on air quality and climate through measurements of OH reactivity (k_OH) (Kovacs and Brune, 2001). OH reactivity is the total pseudo-first-order coefficient describing the loss of OH, which is the inverse of the OH chemical lifetime, and defined as , where is the rate coefficient for reaction of OH with species X_i.
Measurements of k_OH have been made successfully in the field using several techniques (Sadanaga et al., 2004; Sinha et al., 2008; Stone et al., 2016), but long-term continuous measurements have proved challenging, particularly in high NOx environments (Fuchs et al., 2017). In this work we describe the development of a novel instrument based on laser flash photolysis coupled with time-resolved broadband UV absorption spectroscopy to make long-term measurements in a wide range of environments. In the field configuration, the instrument has a limit of detection (LOD) of k_OH around 1.5 s^-1 and LOD of [OH] around 5 × 10¹⁰ molecules cm^-3.  We will present details about the instrument development, characterisation and the field intercomparison with a laser-induced fluorescence (LIF) instrument.