Abstract: Objective　Dinitrogen pentoxide (N2O5) is a crucial reactive nitrogen species in the nocturnal atmosphere, serving as a primary driver of nocturnal atmospheric oxidative capacity and regional nitrogen cycling. Consequently, high-precision measurement of N2O5 is essential for elucidating nighttime atmospheric processes. Reliable calibration using a stable and controllable N2O5 source is essential for evaluating the performance of analytical techniques such as chemical ionization mass spectrometry (CIMS) and cavity ring-down spectroscopy (CRDS), particularly for assessing N2O5 sampling and transmission losses. However, generating N2O5 at controlled and reproducible concentrations for instrument calibration remains challenging because its generation involves the rapid reaction of NO2 with O3 to produce NO3, followed by the reaction of NO3 with NO2 to form N2O5. Therefore, the generated N2O5 may coexist with residual O3 and NO3, with excessive O3 potentially causing analytical interference and NO3 affecting the accuracy of N2O5 quantification. To address this challenge, this study aimed to develop an online dynamic N2O5 generation source based on the controlled gas-phase reaction of O3 and NO2 and to evaluate its stability in N2O5 generation and applicability in quantitative characterizing N2O5 sampling losses. Methods　The online dynamic N2O5 generation source was developed using a controlled gas-phase synthesis approach based on the reactions between O3 and NO2. A low-O3/high-NO2 generation strategy was adopted to promote N2O5 formation while minimizing the residual O3 concentration and increasing the N2O5/NO3 ratio. To enable stable and controllable generation of low-concentration O3, an ozone generator equipped with a mechanically adjustable shading structure was designed. The O3 output was regulated by adjusting the shading section, thereby providing a flexible means to control the concentration of O3 supplied to the N2O5 generation system. The effects of reaction residence time and NO2 input conditions on N2O5 generation were systematically investigated using a combination of box-model simulations and laboratory experiments. The optimal generation conditions were subsequently determined by considering N2O5 production together with the concentrations of residual O3 and NO3. Under the optimized conditions, the N2O5 output was continuously monitored to evaluate the temporal stability and repeatability of the generation source. Finally, the developed N2O5 source was coupled to a 6.35 mm perfluoroalkoxy (PFA) sampling tube, and N2O5 transmission loss in the PFA tube was evaluated under different sampling conditions by determining the corresponding first-order wall-loss rate constant. Results and discussion　The developed online N2O5 generation source achieved stable and controllable N2O5 generation under low-O3/high-NO2 conditions. The mechanically adjustable shading structure enabled effective regulation of the O3 output with a mass concentration of several tens of μg/m3. Under these conditions, the relatively high NO2 mass concentration favored N2O5 formation while limiting the amount of excess O3 remaining in the reaction mixture. Box-model simulations combined with laboratory experiments were used to optimize the reaction residence time and NO2 input conditions, yielding the operating conditions that support stable N2O5 production. Under the optimized conditions, the concentration of N2O5 generated could remain stable during continuous operation, with concentration fluctuations within 1%. The developed source was subsequently coupled to a 6.35 mm PFA sampling tube to evaluate N2O5 transmission loss under sampling conditions. The corresponding first-order wall-loss rate constant was determined to be (0.018 ± 0.002) s−1, which is comparable to previously reported values, indicating that the developed N2O5 generation source can provide sufficiently stable N2O5 concentration for the quantitative characterization of sampling losses and the evaluation of N2O5 transmission efficiency in sampling systems. Conclusions　An online dynamic N2O5 generation source was successfully developed using a controlled gas-phase reaction system involving O3 and NO2. The combination of a mechanically adjustable O3 generation structure and a low-O3/high-NO2 synthesis strategy enabled stable and controllable N2O5 production while minimizing residual O3 and increasing the N2O5/ NO3 ratio. Under the optimized operating conditions, the generated N2O5 concentration exhibited excellent temporal stability, with fluctuations maintained within 1% during continuous operation. Application of the developed source to a 6.35 mm PFA sampling tube yielded a first-order N2O5 wall-loss rate constant of (0.018 ± 0.002) s−1, demonstrating the practicality of the source in quantitative characterizing N2O5 sampling losses. Overall, the developed source provides a practical and stable platform for controlled N2O5 generation and sampling-loss characterization, with potential applications in the calibration and performance evaluation of atmospheric N2O5 measurement systems, including CIMS, CRDS, and related techniques.

Key words: N2O5 dynamic generation source, low-concentration O3 regulation, low-O3/high-NO2 strategy, calibration application