Development of a sensitive long path absorption photometer to quantify peroxides in aerosol particles (Peroxide-LOPAP)

[摘要] A new off-line instrument to quantify peroxides in aerosol particles usingiodometry in long path absorption spectroscopy has been developed and iscalled peroxide long path absorption photometer (Peroxide-LOPAP). The newanalytical setup features important technical innovations compared tohitherto published iodometric peroxide measurements. Firstly, the extraction,chemical conversion and measurement of the aerosol samples are performed in aclosed oxygen-free (~ 1 ppb) environment. Secondly, a 50-cm opticaldetection cell is used for an increased photometric sensitivity. The limit ofdetection was 0.1 μM peroxide in solution or0.25 nmol m⁻³ with respect to an aerosol sample volume of 1 m³.The test reaction was done at a constant elevated temperature of40 °C and the reaction time was 60 min.

Calibration experiments showed that the test reaction with all reactiveperoxides, i.e. hydrogen peroxide (H₂O₂), peracids and peroxideswith vicinal carbonyl groups (e.g. lauroyl peroxide) goes to completion andtheir sensitivity (slope of calibration curve) varies by only ±5%.However, very inert peroxides have a lower sensitivity. For example,tert-butyl hydroperoxide shows only 37% sensitivity compared toH₂O₂ after 1 h. A kinetic study revealed that even after 5 h only85% of this inert compound had reacted.

The time trends of the peroxide content in secondary organic aerosol (SOA)from the ozonolysis and photo-oxidation of α-pinene in smog chamberexperiments were measured. The highest mass fraction of peroxides with34% (assuming a molecular weight of 300 g mol⁻¹) was found infreshly generated SOA from α-pinene ozonolysis. Mass fractionsdecreased with increasing NO levels in the photo-oxidation experiments. Adecrease of the peroxide content was also observed with aging of the aerosol,indicating a decomposition of peroxides in the particles.

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