Figure 4. Field measurements at the SMEAR II station.

Hyytiälä, Finland, 29–31 March 2011. (a) Time series of the estimated concentrations of O,O-C₁₀H₁₅(O₂)₂O₂ radicals (in black) from ozonolysis of terpenes and HO-C₁₀H₁₅(OO)(OOH)O₂ radicals (in red) from the OH radical reaction as well as the corresponding organic nitrate (in olive). Full lines show the measurements by the nitrate-CI-APi-TOF mass spectrometer, the dashed red line stands for the corrected HO-C₁₀H₁₅(OO)(OOH)O₂ radical trace assuming an enhanced sensitivity by a factor of 40 based on the comparison of results using nitrate or acetate ionization in the laboratory experiments. (b) Global radiation is taken as a proxy for OH radicals. Enhanced NO concentration especially in morning hours of 29 March yielded enhanced organic nitrate concentrations.