Development, characterization and first deployment of an improved online
reactive oxygen species analyzer
Abstract. Inhalation of atmospheric particles is linked to human diseases. Reactive oxygen species (ROS) present in these atmospheric aerosols may play an important role. However, the ROS content in aerosols and their formation pathways are still largely unknown. Here, we have developed an online and offline ROS analyzer using a 2’,7’-dichlorofluorescin (DCFH) based assay. The sensitivity of the ROS analyzer was characterized using a suite of model organic compounds. The instrument detection limit determined as three times the noise is 1.3 nmol L−1 for offline analysis and 2 nmol m−3 of sampled air when the instrument is operated online at a fluorescence response time of approximately 8 min, while the offline method detection limit is 9 nmol L−1 to 13 nmol L−1. Potential interferences from gas phase O3 and NOx, matrix effects of particulate SO42− and NO3− were tested, but not observed. Fe3+ had no influence on the ROS signal while soluble Fe2+ reduced it if present at high concentrations in the extracts. Both online and offline methods were applied to identify the ROS content of different aerosol types, i.e., ambient aerosols as well as fresh and aged aerosols from wood combustion emissions. The stability of the ROS was assessed by comparing the ROS concentration measured by the same instrumentation online in-situ with offline measurements. We also analyzed the evolution of ROS in specific samples by conducting the analysis after storage times of up to four months. The ROS were observed to decay with increasing storage duration. From their decay behavior, ROS in secondary organic aerosol (SOA) can be separated into short- and long-lived fractions, with an average half-life of ~ 1.7 h and ~ 432 d, respectively. All these measurements showed consistently that, on average 60 ± 20 % of the ROS were very reactive and disappeared during the filter storage time. This demonstrates the importance of a fast online measurement of ROS.