Simultaneous determination of gaseous nitric acid and particulate nitrate in ambient air using continuous samplers and chemiluminescent online analysers.

A sensitive and fast method for the simultaneous determination of gaseous nitric acid (HNO3) and particulate nitrate (NO3-) in ambient air is presented. HNO3 is continuously sampled using a wet diffusion denuder and analysed online by a newly developed continuous flow analyser, while NO3- bound to aerosol particles is collected in parallel using a continuous aerosol sampler and analysed by the previously developed continuous flow analyser. Chemiluminescence detection in both CFAN and CFAO enables sensitive determination of NO3- directly in sampler concentrates without preconcentration, with limits of detection (LOD = 3 S/N) of 5.1 nM and 12.8 nM, respectively. The corresponding LODs are 0.13 μg m-3 (49.7 ppt) for HNO3 and 0.10 μg m-3 for NO3-. The developed method enables continuous online measurement of the HNO3/NO3- distribution in ambient air, with chemiluminescence signal recorded at 1 s intervals. The optimised method was applied to the determination of HNO3 and NO3- in urban air in Brno during winter and summer campaigns conducted between 2022 and 2023. Results obtained using the developed method were compared with those from a reference method based on sampling on filters and dry diffusion denuders coated with sodium fluoride.