High time-resolution measurements of atmospheric pollutants were conducted in the Yokohama coastal area to estimate the contributions of the primary emissions and secondary formation to formaldehyde (HCHO) concentrations using three approaches: the emission ratio method, multiple linear regression analysis, and photochemical age-based parameterization method. Based on the emission ratio method employing the HCHO/CO ratio, the average daytime contribution of the primary emissions was 49%. Based on the multiple linear regression analysis, CO and total peroxy nitrates (PNs) were used as indicators of the primary emissions and secondary formation, respectively. The coefficient of determination between the observed and calculated values was 0.85, indicating good agreement, and the average daytime contribution of the primary emissions was 48%, which was consistent with the emission ratio method. Although the potential influence of the PNs thermal decomposition was considered, insufficient data limited a robust evaluation. For the photochemical age-based parameterization method, five underlying assumptions were assessed, revealing that this approach was unsuitable for application in the Yokohama coastal area. Even on August 1, 2019, when a photochemical oxidant alert was issued, the primary emissions accounted for approximately 40% of the daytime HCHO concentrations.