Methods and issues of reducing reactive loss impacts in ambient VOC source apportionments

With the increase in ozone (O₃) concentrations in multiple cities or regions worldwide in recent years, accurate source apportionment methods of its key precursor VOCs have been acquired increasing attention. Chemical reactive losses of ambient VOCs have been a long-term issue yet to be resolved in the VOC source analyses research. Thus, we systematically assessed the common methods and existing issues in ways to reduce losses and loss impacts in source apportionment studies by reviewing relevant publications and suggests research directions for improved VOC source apportionments. Compared to any other mathematical models, positive matrix factorization (PMF) is now a main VOC source apportionment approach. The issue in using any apportionment tool is the processing of the data to be analyzed to reduce the impacts of reactive losses. Calculating the initial concentrations of VOC species based on photochemical age has become a major method to reduce reactive loss effects in PMF, except for selecting low-reactivity species or nighttime data into the analysis. The initial concentration method only considers daytime reactions with hydroxyl (•OH) radicals at present. However, the •OH rate constants vary with temperature, and that has not been considered. Losses from reactions with O₃ and NO₃ radicals remain to be included. Therefore, the accuracy of the currently photochemical age estimation is uncertain. Beyond developing accurate quantitative methods for chemical losses, source apportionment methods of the consumed VOCs and the accurate quantification of source contributions to O₃ and secondary organic aerosol (SOA) are important directions for future studies.