Filter-PUF Tandem Exhaust Sampling Corrects Underestimation of Vehicular Nitrated Phenols

Conventional filter-only exhaust sampling misses gas-phase and semivolatile nitrated phenols (NPs), biasing vehicular emission estimates low. We deployed a tandem quartz filter-polyurethane foam (filter-PUF) sampler downstream of dilution to quantify 19 NPs from in-use diesel (China III–VI) and gasoline vehicles (China IV–V), and found that filter-only protocols underestimate total NPs (sum of filter and PUF retained under specified dilution) by 66–76% (diesel) and 66–71% (gasoline). Corrected fuel-based NP emission factors declined sharply with tighter standards (up to 97% lower from China-III to -VI), and the mixture was dominated by 4-nitrophenol and its methylated derivatives (∼50–63%). Applying the corrected factors to China’s 2023 fleet yields vehicular NP emissions of 528 Mg, which is comparable in magnitude to previous estimates for biomass burning (∼670 Mg, based on filter-only data) and potentially larger than that for residential coal (∼178 Mg, based on filter-only data). This suggests that vehicular emissions may rank similarly to, or even surpass, these sources when accounting for nonfilter-retained NPs. Recognizing and correcting this filter-only artifact with a drop-in filter-PUF fix enables more accurate inventories and will improve assessments of NPs’ impacts on urban air quality, HONO budgets, and brown carbon.