| Summary: | PM<sub>2.5</sub> pollution and visibility impairment has attracted wild public attention in urban China during the past decade. Field observation was carried out in Tianjin, China, during heating (HP) and non-heating periods (NHP). The IMPROVE (Interagency Monitoring of Protected Visual Environments) method was localized to better understand the quantitative impact of PM<sub>2.5</sub> composition on extinction coefficient (B<sub>ext</sub>). B<sub>ext</sub> of organic mass (OM) was classified into that of primary organic aerosol (POA) and secondary organic aerosol (SOA). B<sub>ext</sub> of Rayleigh scattering was calculated based on observation data directly, instead of a fixed value in the original IMPROVE method. The mass extinction efficiency (MEE) of EC was also updated based on literature research. The estimation bias of reconstructed B<sub>ext</sub> was reduced from 4.8 Mm<sup>−1</sup> to 0.8 Mm<sup>−1</sup> with the localized algorithm. It was found that the secondary components contribute more than 40% of B<sub>ext</sub>, and ammonium sulfate (AS) and ammonium nitrate (AN) are the dominant components. The different formation pathway leads to significant difference in secondary inorganic components during HP and NHP. Based on the positive matrix factorization (PMF) model and localized IMPROVE method, secondary aerosols were identified as the main source contributing to both PM<sub>2.5</sub> concentration (48.2%) and B<sub>ext</sub> (44.3%) during HP, followed by vehicular emission and coal combustion, biomass burning with municipal incinerators, fugitive dust, and steel processing.
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