Characteristics of peak concentrations of black carbon encountered by elementary school children.

The objectives of this study were to examine characteristics of peak concentrations, including frequency, duration, and relative magnitude, and estimate its contributions to overall daily exposure to BC by activity and microenvironment. We assessed daily personal exposures from August 2015 to January 2016 (75.2% of weekdays and 24.8% of weekend days; 64.1% of school days and 35.9% of holidays) among forty 10-12 years old children living in the Seoul metropolitan area. These children were equipped with a microaethalometer (BC monitor) and recorded a time-activity diary. Pre-administrated questionnaires and follow-up interviews also provided information on children's time-activity patterns. Owing to the absence of a generally accepted threshold, peaks were alternatively defined as BC concentrations higher TWA, the 95th percentile, and the 99th percentile. Peak concentrations made substantial contributions to total daily exposure to BC (peaks ≥ TWA: 60%, peaks ≥95th-percentile: 19%, and peaks ≥99th-percentile: 6%). Average peak levels higher than TWA and the 95th percentile differed significantly by activity and ME. Transportation and cooking led to frequent peak occurrences which disproportionately contributed to daily integrated exposure relative to time spent in these activities. Walking was characterized by occasional brief but high-magnitude peaks exceeding the 99th percentile, which produced the most intense potential dose (0.09% of daily time spent on walking accounted for 1.6% of daily potential dose). It might be attributed to encounters with high emissions sources such as passing/idling vehicles and environmental tobacco smoke. Trips by diesel vehicle produced frequently occurring and long-duration peaks above the 95th percentile that contributed 2% to total daily exposure (corresponding time: 0.3%). Charbroiling meat incurred sustained peaks as intense as those in trips by diesel vehicles. Peaks during commuting showed relatively high exposure intensity on weekdays, possibly because of increased surrounding traffic volume on these days, while those during cooking accounted for a more elevated residential contribution to daily integrated exposure.