Long-term measurements of OH reactivity: A new metric for air quality

Poor air quality has been reported as the greatest environmental risk to public health in the UK (DEFRA, 2017), has recently been linked to dementia (Carey et al., 2018), and is estimated to cause over 40,000 premature deaths in the UK each year (Royal College of Physicians, 2016). Policies designed to address issues such as air quality and climate rely on accurate knowledge of atmospheric composition, requiring understanding of the emission rates, concentrations, and chemistry of trace VOCs in the atmosphere. However, it is only possible to identify and measure the concentrations of a small fraction of the vast array of VOCs present in the atmosphere (Goldstein and Galbally, 2007), which hinders our ability to provide accurate predictions of air quality and climate. Despite this challenge, it is possible to quantify the presence of unmeasured species, and the extent to which they contribute to the production of ozone and SOA (Yang et al., 2016), through measurements of the rate at which OH radicals are consumed in the atmosphere, since almost all species emitted into the atmosphere react with OH (Heard and Pilling, 2003; Stone et al., 2012).

Measurements of the total OH loss rate in the atmosphere can be used to define the OH reactivity, which is the pseudo-first-order rate coefficient describing the loss (kOH) and the inverse of the chemical lifetime of OH (TOH = 1/kOH). Comparison between measurements of OH reactivity and calculations based on observations of OH sinks, which include CO, NO, NO2 and VOCs, and laboratory measurements of OH radical kinetics, provides a means to determine the comprehensiveness of the observed sinks (Yang et al., 2016; Fuchs et al., 2017), which enables assessment of the potential contribution of unmeasured species to air quality and climate (Kirchner et al., 2001; Yang et al., 2016).

While several instruments have been developed to measure OH reactivity, including work in the Leeds group which has demonstrated significant impacts of large VOCs (>=C9) and biogenic emissions in London (Ingham et al., 2009; Stone et al., 2016; Whalley et al., 2016), these instruments tend to be limited to short-term intensive measurements. The capability to make long-term OH reactivity measurements would enhance our abilities to monitor changing trends in pollutant emissions, to assess emissions inventories, and to provide more accurate air quality and climate forecasts. Long term measurements of OH reactivity would provide an exciting new metric for air quality for use by policy makers.