Abstract: The primary particle concentration in China keeps continuously decreasing in recent years, thus, the secondary generated aerosols have become the main component of atmospheric particulate matter. As an important source of secondary aerosols, new particle formation has received increasing attention since the beginning of this century. In the chemical process of new particle formation, gaseous sulfuric acid is an extremely important gas precursor. Due to its extremely low concentration and extremely high instrument sensitivity required for detection, the accurate quantification of gaseous sulfuric acid is always the biggest difficulty in the study of new particle formation chemical process. The existing quantification method for gaseous sulfuric acid is mainly based on photochemistry methods, which uses ultra-violet radiation to photolyze water vapor to generate OH radicals and then react with excess sulfur dioxide to finally generate gaseous sulfuric acid. So, the concentration of gaseous sulfuric acid is determined by the intensity of ultraviolet light and the concentration of water vapor, in which the mainly uncertainty is the intensity of ultraviolet light due to its direct measurement highly affected by the uneven distribution of light intensity inside the reaction chamber of the calibration instrument. Besides, the scattering and absorption of background aerosols during the filed measurement can also amplify the uncertainty of the direct measurement of ultraviolet light. Therefore, this study establishes and compares two indirect measurement methods for ultraviolet radiation determination based on ozone and nitrous oxide actinometry, respectively. The results show that the main error of the ozone-based indirect measurement method comes from the absorption cross section of oxygen, with a theoretical error of > 60% and a repeated laboratory experiment error is around 12%. While the main errors of the nitrous oxide-based indirect measurement method come from the secondorder rate constant kn, the uncertainties of the measurement instruments for nitrogen oxide and the mass flow controllers, with a theoretical error of < 16% and a repeated laboratory experiment error of < 1%. During the field measurement of gaseous sulfuric acid, for the indirect measurement method based on nitrous oxide, the measurement error of gaseous sulfuric acid is around 24% under the condition of calibration carried out once a day for half a month, which is close to or better than that of foreign observation results. This indicated that the nitrous oxide-based actinometry method in the quantification of ultra-violet radiation intensities has quite good accuracy and stability, and can be effectively applied to the quantitative measurement of gaseous sulfuric acid.

Key words: ultra-violet radiation, actinometry, gaseous sulfuric acid