Measurements and modelling of OH, HO2 and RO2 radicals and OH reactivity in the Tibetan Plateau

With an area of 2.5 million km² and an average altitude of about 4000 m above sea level, the Tibetan Plateau is the largest and highest plateau in the mid-latitudes of the Northern Hemisphere. The high altitude, low latitude, and large snow coverage leads to strong solar radiation at the surface and, as such, a high production potential for OH. Given the large area of the Plateau, understanding the chemistry controlling OH concentrations in this region is important to accurately predict the global lifetime of the greenhouse gas, methane.

Here we present the first field observations on the Tibetan Plateau of the hydroxyl radical, OH, and also HO₂ and RO₂ radicals, as well as OH reactivity, made in April-May 2019 at the Nam Co research station, which is located at 4730 m above sea level. The atmospheric pressure at Nam Co is ~ 0.57 atm and these represent the highest altitude ground-based measurements of OH and other radical species. Concentrations of OH radicals were measured directly using laser-induced fluorescence (LIF) spectroscopy at 308 nm, whereas HO₂ and RO₂ concentrations were measured via their chemical conversion to OH, followed by LIF. OH reactivity was measured using laser-flash photolysis followed by time-resolved LIF.

Previous photostationary steady state (PSS) calculations of the OH concentrations in this region, which consider O₃ photolysis and subsequent reaction of O(¹D) with H₂O as the primary source of OH, and CO and methane as the OH sinks, range from 3.7 to 11 x 10⁶ cm^-3 in January and from 1.4 to 3.0 x 10⁷ cm^-3 in July. (Lin et al., J. Geophys. Res., 113, D02309, doi:10.1029/2007JD008831, 2008) suggesting an extremely photo-active environment.

The average peak OH concentration was ~4 x 10⁶ cm^-3, which is at the lower end of the previously reported PSS predictions. The average peak HO₂ and total organic RO₂ concentrations observed were ~3 x 10⁸ cm^-3 (~20 pptv) and ~7 x 10⁸ cm^-3 (~ 50 ppt) respectively. On average, the measured OH reactivity was low, peaking before sunrise at ~2 s^-1 and displaying only a weak diurnal profile. However, the OH reactivity is up to 4 times greater than that calculated assuming only methane and CO as OH sinks. A box model utilising the Master Chemical Mechanism and constrained with in situ measurements of radical sources and sinks at the Nam Co site was used to calculate radical concentrations and OH reactivity for comparison with field measurements, and to examine chemical budgets.