Evaluation Method for Nitrogen Oxide Emission Reduction Using Hypothetical Automobile Model: A Case in Guangdong Province

As a key precursor of tropospheric ozone and secondary particulate matter, nitrogen oxides (NOx) exert significant impacts on air quality. Traffic emissions represent a dominant source of near-surface NOx. The widespread adoption of new energy vehicles (NEVs) has progressively transformed the automobile fleet composition, leading to measurable reductions in NOx emissions. This study developed a NOx emission inventory model to quantify the impact of NEV penetration on emission trends in Guangdong (2013–2022), under the assumption that the emission shares of internal combustion engine vehicles (ICEVs) and NEVs have no significant change in adjacent years. Results demonstrate that total vehicular NOx emissions peaked in 2019 at 55.69 × 104 tons (a 16.6% increase from 2018), followed by a consistent decline. ICEVs exhibited a declining emission share from 0.037 × 104 tons/year in 2013 to 0.022 × 104 tons/year in 2019—a 40.5% reduction, attributable to progressive technological advancements. Following a marginal increase (2019–2021), the emission share declined significantly to 0.019 × 104 tons/year in 2022. In contrast, NEVs contributed to emissions reduction, with maximal mitigation observed in 2021 (−0.241 × 104 tons). ICEVs initially demonstrated emission reductions (2014–2017), succeeded by a transient increase (11.7 × 104 tons through 2021) before resuming decline in 2022. The NEV-driven mitigation effect intensified progressively from 2018 to 2021, with modest attenuation in 2022.