Five-channel TD-CEAS measurements of gaseous and particulate organic nitrates with NO/NO₂ interference correction under high-NO_x conditions

Abstract. Organic nitrates (ONs) are important temporary reservoirs of atmospheric NO_x and, for sufficiently low-volatility species, contributors to secondary organic aerosol formation. However, online measurements of particle-phase ONs remain limited, hindering quantitative constraints on ON abundance and gas–particle partitioning. Here we present a five-channel thermal dissociation cavity-enhanced absorption spectrometer (TD–CEAS) for in situ, time-resolved measurements of NO₂ and operationally defined ON classes in both the gas and particle phases. The instrument combines a room-temperature channel for ambient NO₂ with thermal dissociation channels operated at 250 and 450 °C to quantify total peroxy nitrates (ΣPNs) and total alkyl nitrates (ΣANs), respectively. Gas–particle separation is achieved using paired inlet/filter configurations, and gas- and particle-phase ΣPNs and ΣANs are retrieved by channel differencing. The 1σ (1 s) detection limits are 49 pptv for gΣPNs, 49 pptv for pΣPNs, 48 pptv for gΣANs, and 68 pptv for pΣANs. Laboratory characterization included temperature-dependent dissociation measurements, cross-validation of PAN against GC–ECD (R²=0.988; slope = 0.987), and an operational calibration for particulate ΣANs using 2-ethylhexyl nitrate (recovery slope=1.036±0.028; method detection limit =0.029 µg NO₂). Dedicated interference experiments showed that NO and NO₂ can introduce substantial nonlinear biases in ΣPN measurements; these effects were parameterized using a multiple nonlinear regression model. The instrument was deployed at an urban site in Guangzhou during September–October 2025 and provided 6 min measurements of NO₂, gas- and particle-phase ΣPNs, and gas- and particle-phase ΣANs under high-NO_x conditions. During the October intensive period, corrected gas-phase ΣPNs covaried well with independently measured PAN (R²=0.83), and PAN accounted for 78 % of daytime gΣPNs. This five-channel TD–CEAS provides a framework for continuous observations of ON phase partitioning and reactive nitrogen processing in polluted urban atmospheres.