The real-time method of assessing the contribution of individual sources on visibility degradation in Taichung

Visibility degradation caused by air pollution has become a serious environmental problem in megacities in Northeast Asia. In general, aerosol chemical compositions are measured by a conventional method of time integrated filter sampling for off-line analysis, which cannot represent temporal and spatial variations in the real atmosphere. The in situ air composition measuring equipment, OCEC carbon aerosol analyzer and a long-path visibility transmissometer-3 were used to collect hourly measurements of the soluble ions, organic/elemental carbon, and ambient visibility, respectively. During the observation, two types of weather conditions were identified: transport and stagnant. Because PM2.5 was identified as the predominant species of light extinction, the sources of PM2.5 were determined and investigated using a positive matrix factorization (PMF) analysis. The PMF outputs characterized the six main emission sources (marine/crustal aerosols, secondary nitrate, secondary sulfate, direct vehicle exhaust, coal/incinerator combustion, and local sewage emission) and reconstructed the PM2.5 mass concentrations of each pollutant source in two weather conditions. In addition, the light extinction (bext) was reconstructed using a multivariate linear regression analysis with hourly-reconstructed PM2.5 mass concentrations to determine the contributions of each source to bext. The primary results showed that the extinction coefficient was proportional to the PM2.5 with high value in stagnant weather conditions. The secondary sulfate was the most abundant source of bext contribution during the sampling period. In addition, the bext contributions of direct vehicle exhaust and coal/incinerator combustion significantly increased in the stagnant weather condition. According to the results of hourly measurements, this work further emphasized that the sources of direct vehicle exhaust and coal/incinerator combustion in PM2.5 were the important sources of visibility degradation in the stagnant weather conditions, which suggests that the pollutants derived from direct vehicle exhaust and coal/incinerator combustion should be controlled first to improve visibility in Taichung. •Hourly measurements of soluble ions, OC, EC and visibility were analyzed.•Six factors of PM2.5 were identified by PMF according to the hourly measurements.•Reconstructed PM2.5 of each pollutant source was estimated by PMF.•Source apportionment of visibility degradation was discussed by reconstructed PM2