Comparison of tropospheric NO3 radical measurements by differential optical absorption spectroscopy and matrix isolation electron spin resonance

Despite the importance of NO3 in the nighttime atmosphere only two techniques, Differential Optical Absorption Spectroscopy (DOAS) and Matrix Isolation Electron Spin Resonance (MIESR) have been applied to its detection in ambient air to date. Here we report the results of the first intercomparisons of these techniques in the atmosphere carried out at rural sites in Germany, at Deuselbach in 1983 and near Berlin in 1998. The simultaneously measured NO3 mixing ratios, which were in the range from 9 to 20 ppt and at one measurement near 100 ppt, were in good agreement within the error limits. A regression analysis yields a linear relationship between the DOAS and MIESR data with a correlation coefficient of R = 0.99 and a slope of 0.83 +/- 0.03 (1 sigma error) at a negligible intercept of 0.33 +/- 0.73 ppt (1 sigma error). The deviation from unity is within the total systematic error of both measurement techniques. This result shows the reliability of the two techniques over the past 15 years.