Seminar

Abstract

OH radicals play a central role in the chemistry of the troposphere. They are mainly responsible for the chemical degradation of many trace gases and they initiate chemical reactions that may eventually lead to photochemical formation or depletion of tropospheric ozone. Owing to the short chemical OH lifetime, concentrations of OH are mainly controlled by chemistry rather than transport. Thus, field measurements of OH can be used to test predicted concentrations calculated by atmospheric chemistry models. Even more rigorous tests have become feasible since in addition the chemical OH lifetime can be measured in ambient air. The ratio of the measured concentration and lifetime of OH equals the chemical loss rate of OH which can be compared with the production rate from all known OH sources (e.g. photolysis of ozone, HONO etc. plus radical recycling by HO₂ + NO --> OH + NO₂). For a short-lived species like OH, both rates should be balanced. Recent field experiments in one of the most polluted regions worldwide in the heavily populated Pearl-River Delta (PRD) in Southern China showed that the OH budget was indeed balanced for conditions at NO > 1ppb, but a significant OH source was missing in the afternoon at NO < 1ppb. In the latter case, the observed OH concentrations exceeded the prediction by a box model (modified RACM) by a factor of 3-5 [Hofzumahaus et al., 2009]. It should be noted that the measurement site was characterized by a high load of anthropogenic and biogenic VOCs that included up to a few ppb of isoprene in the afternoon. It is interesting that a similar strong model underprediction of measured OH has been reported in isoprene-rich air in the pristine atmosphere over the Amazonian rainforest [Lelieveld et al., 2008] and Borneo [Pugh et al., 2009], and in forested regions of the US [Tan et al., 2001; Ren et al., 2008]. All studies including the one at PRD conclude that the high levels of observed OH are likely caused by a so far unknown cycling of OH. The challenging questions are: Which kind of reactions cause such efficient OH cycling? What does it mean to our understanding of the trace gas degradation and photochemical ozone production which is normally linked with radical cycling through NO reactions?

References:

Hofzumahaus, A., et al., Amplified trace gas removal in the troposphere. Science 324, 1702 (2009)
Lelieveld, J. et al. Atmospheric oxidation capacity sustained by a tropical forest. Nature 452, 737-740 (2008)
Ren, X. R. et al.. HOx chemistry during INTEX-A 2004: Observation, model calculation, and comparison with previous studies. J. Geophys. Res. 113, D05310 (2008)
Tan, D. et al. HOx budgets in a deciduous forest: Results from the PROPHET summer 1998 campaign. J. Geophys. Res. 106, 24407-24427 (2001)
Pugh, T. A. M. et al., Simulating atmospheric composition over a South-East Asian tropical rainforest: Performance of a chemistry box model, Atmos. Chem. Phys. Discuss. 9, 19243-19278 (2009)