Meteorological variations impeded the benefits of recent NOx mitigation in reducing atmospheric nitrate deposition in the Pearl River Delta region, Southeast China

The trends and variability of atmospheric nitrogen deposition in the Pearl River Delta (PRD) region for the period 2008–2017 were investigated by integrating ground- and satellite-based observations and a chemical transport model, in order to gauge the effects of emission reductions and meteorological variability. We show that dry deposition observation of oxidized nitrogen decreased at the rate of 2.4% yr⁻¹ for a moderate reduction in NOₓ emissions by 27% in the past decade, while reduced nitrogen presented an increase at the rate of 2.3% yr⁻¹ despite no regulated interventions for NH₃ emissions, which is likely related to changes in atmospheric gas-particle partitioning of NH₃ as reductions in SO₂ and NOₓ emissions. These results coincide with the trends in ground-level concentrations of oxidized and reduced nitrogen compounds in the atmosphere during 2008–2017. The changes in annual deposition fluxes of total oxidized and reduced nitrogen are not statistically significant trends and largely related with the inter-annual variability in their corresponding wet depositions, which reflects combined effects of variability in precipitation amount, and changes in atmospheric nitrogen compounds which dominates wet deposition of the oxidized and reduced forms. The meteorological conditions can mask 34% and 25% decrease in total oxidized and reduced nitrogen deposition on the decadal timescale, respectively. We conclude that meteorology-driven variability probably have masked the full response of oxidized nitrogen deposition to NOₓ emissions reduction. Our results also imply that persistent and integrated emission control strategies on NOₓ and NH₃ are needed to effectively reduce total nitrogen deposition fluxes towards the critical limit in the PRD region.