Rapid and high-precision cavity-enhanced spectroscopic measurement of HONO and NO₂: Application to emissions from heavy-duty diesel vehicles in chassis dynamometer tests and in mobile monitoring

Nitrous acid (HONO) is crucial in atmospheric chemistry as it is a major precursor for hydroxyl radicals (OH), the dominant atmospheric oxidant. Hydroxyl radicals are essential in the formation of secondary air pollutants like ozone and particulate matter. This study presents a newly developed Incoherent Broadband Cavity Enhanced Absorption Spectroscopy (IBBCEAS) system for precise and rapid measurements of HONO and nitrogen dioxide (NO₂) emissions. The instrument's optical cavity (formed by two mirrors separated by 96 cm and with reflectivity of 0.99955 at 378 nm) resulted in an effective optical path length of 1.4 km. With an integration time of 5 s, the 1σ measurement precisions for HONO and NO₂ were 0.19 ppb and 0.48 ppb with overall measurement uncertainties of 10 % and 7 %, respectively. Comparative analysis of the IBBCEAS and a commercial cavity-attenuated phase shift (CAPS) systems under non-emission conditions demonstrated excellent agreement (slope = 1.01 and R² = 0.98). The instrument was applied to study HONO and NO₂ emissions from heavy-duty vehicles in chassis dynamometer tests and mobile monitoring. Chassis dynamometer tests revealed that HONO and NO₂ emissions depend strongly on vehicle speed and driving conditions. We find a HONO/NO_X ratio of 1.01 × 10⁻² across the entire China-World Transient Vehicle Cycle (C-WTVC) driving cycle. Mobile monitoring in urban areas shows emission characteristics similar to those observed in chassis dynamometer tests. Frequent acceleration-deceleration patterns of diesel vehicles under congested traffic conditions lead to higher HONO and NO₂ emissions compared to driving under steady speed conditions. Improving traffic flow conditions will help reduce HONO and NO₂ emissions.