Dry and Wet Atmospheric Deposition of Organic and Inorganic Nitrogen in Hong Kong: Method Development and Year-long Monitoring

Abstract

Atmospheric deposition of nitrogen (N) is a significant environmental issue affecting ecosystems across China. However, most previous studies have focused primarily on bulk and/or wet deposition of inorganic nitrogen (IN), while dry deposition of N has been largely overlooked. Notably, organic nitrogen (ON) has often been under-evaluated in past N deposition observations due to the lack of sensitive and direct measurement methods. This study presents a year-round, simultaneous observation of both dry and wet depositions of IN and ON at a coastal site in Hong Kong, with particular emphasis on method development for ON and its contribution to N deposition.

Water-insoluble organic nitrogen (WION), often neglected due to analytical challenges, was quantified using a separation technique involving suction filtration and analysis of the filtration filter by an aerosol nitrogen analyzer. This method effectively retained the majority of WION in water samples, and a linear correction was established to account for positive artifacts of water-soluble total nitrogen (WSTN) on filtration filters. Additionally, a freeze-drying preconcentration method was developed to concentrate water samples of N deposition, while quantitatively retaining both IN and ON. This approach uses two quartz filters to sandwich a freeze-dried water sample. Preliminary trials with IN and water-soluble ON (WSON) standards, as well as selected rainwater samples, confirmed that this freeze-drying approach offers high recovery and reproducibility. After preconcentration, rainwater samples can be effectively analyzed using a thermal evolution-based aerosol nitrogen analyzer combined with multivariate curve resolution (MCR) of thermograms for simultaneous quantification of IN and WSON. The analytical methods developed in this study provide a robust framework for assessing atmospheric N deposition and its ecological impacts.

The year-long monitoring of dry and wet deposition revealed a high annual total TN deposition (wet plus dry) of 52.8 kg N ha^-1 year^-1 in Hong Kong, making it one of the global hotspots for N deposition. Of this total, dry and wet depositions contributed 11.3 kg N ha^-1 year^-1 kg N ha^-1 year^-1 (21%) and 41.5 kg N ha^-1 year^-1 (79%), respectively. As a subtropical region, Hong Kong experiences high annual precipitation, resulting in wet deposition being the dominant pathway for N input. For dry deposition monitoring, two surrogate surfaces—quartz filter and water—were employed to compare their collection efficiencies. Water was found to collect significantly more dry N deposition, likely due to the dissolution and retention of gaseous WSTN. This finding highlights the substantial contribution of gas-phase N deposition to the aquatic ecosystem in Hong Kong. Long-term, simultaneous observations of dry and wet deposition are essential to fully assess the ecological risks posed by N deposition in Hong Kong. ON, accounting for nearly one-quarter of total N deposition, is a critical chemical component. Both WSON and WION exhibited distinct seasonal patterns from those of IN, implying complex sources and deposition mechanisms for ON. Source analysis using tracers revealed secondary formation, biomass burning, and soil dust emissions as major sources of WSON.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Jianzhen YU (Supervisor)