Long-term field measurements of OH reactivity using laser flash photolysis coupled with time-resolved broadband UV absorption spectroscopy

Air quality is an important issue to human health and the climate. Poor air quality has been proven to be a risk factor in a wide range of cardiovascular and respiratory diseases, and it contributes to 6.9 million premature deaths worldwide [1]. Air quality also has complex links to climate change, with primary and secondary pollutants affecting the radiative forcing on Earth [2]. Therefore, accurate measurements of air pollutants, alongside understanding of their emissions and atmospheric sinks, is crucial information for informing policy on air quality.

Volatile organic compounds (VOCs), emitted from both anthropogenic and biogenic sources, impact air quality, as they are involved in processes that produce secondary pollutants like ozone and secondary organic aerosol. There is an estimated >100,000 VOCs found in ambient air [3], and they cannot all be measured using traditional direct techniques such as gas chromatography and mass spectrometry. As an alternative to measuring the total quantities of each individual VOC, techniques used to measure OH reactivity have been developed over the last 25 years. OH reactivity (k_OH), the inverse of the chemical lifetime of OH, provides a quantitative measure of the total reactive pollutant loading in an air mass. This information can be used to determine the extent to which measured OH sinks contribute to the OH loss rate and can be used to determine the total impact of VOCs. The measured OH reactivity can be compared to modelled OH reactivity to assess the completeness of models used to assess and predict air quality. Current instruments designed for measurements of OH reactivity are precise and accurate but are often technically challenging and expensive, which limits current OH reactivity measurements to short intensive field campaigns.

We have recently developed an instrument designed to make continuous long-term measurements of OH reactivity based on laser flash photolysis coupled with time-resolved broadband UV absorption spectroscopy. We will present measurements of OH reactivity made in Leeds, UK, using the UV absorption instrument, as well as results obtained using an instrument based on laser-induced fluorescence (LIF) spectroscopy [4] during an intercomparison exercise. The instruments both sampled ambient air from an urban site at the University of Leeds during February and March 2024. Results from the intercomparison will be presented, which indicated that OH reactivity varied between 0.5 and 45.5 s^-1, with a mean reactivity of 9.7 s^-1. It was found that the agreement between instruments was good and that the new instrument can successfully measure OH reactivity over a range of environmental conditions.

The instrument has been successfully deployed in the Birmingham Air Quality Supersite at the University of Birmingham, UK since November 2024. Initial measurements from this long-term campaign will be presented, alongside detailed chemical modelling using the Master Chemical Mechanism.

References:

[1]   World Health Organisation, https://www.who.int/news-room/fact-sheets/detail/ambient-(outdoor)-air-quality-and-health [Accessed 20/07/2024] (2022).

[2]   Intergovernmental Panel on Climate Change, Camb. Uni. Press, 923–1054 (2023).

[3]   A. H. Goldstein & I. E. Galbally, Env. Sci. & Tech. 41(5), 1514-1521 (2007)

[4]   D. Stone et al., Atmos. Meas. Tech, 9 2827–2844 (2016).