Background/Aim: Previous reports have used time-resolved personal monitor to assess the relationship between acute exposure to traffic-related air pollutants (TRAP) and changes in DNA methylation of genes in the arginase (ARG)-nitric oxide synthase (NOS) pathways. We aim to use a data cleaning process to reduce potential artifactual errors from personal measurement and assess its impact on associations between exposure to black carbon (BC), a marker for TRAP exposure, and DNA methylation in ARG-NOS pathways in children with asthma.Methods: Ninety daily buccal cell samples (collected in 5 days) were collected from 18 children with physician-diagnosed asthma. Previous 24-hour personal BC was recorded using microaethalometers (AE51, Aethlabs). DNA methylation in promoter regions of NOS1 (6 sites), NOS2 (16 sites) and ARG1 (8 sites) was assessed by bisulfite pyrosequencing. A combination of two published cleaning methods was applied to the raw measurements. After multivariate analysis of variance (MANOVA), statistically significant associations between percent methylation and BC were analyzed by linear-mixed effect models adjusted for day-of-week, age, gender, and body mass index. Results: The mean (± standard deviation) of raw and cleaned BC were 0.82 µg/m3 (± 0.6) and 1.63 µg/m3 (± 1.36), respectively. We found a significant decrease in methylation of position 1 in NOS3 with an interquartile increase in log-transformed BC in raw BC data (-5.8%, 95% Confidence Interval (CI): –9.4, –2.2) and in cleaned BC data (- 5.5%, 95% CI: –8.9, –2.1), respectively. Increased raw BC, but not cleaned BC, was associated with a decrease in methylation of ARG1, in contrast to the hypothesized effects. No significant associations were found in methylation of NOS1 and BC.Conclusions: Compared with cleaned data, raw BC appeared to underestimate personal exposure and was associated with unexpected increase in methylation of ARG1 genes, although associations with decreased methylation in NOS3 were similar.