Seasonal variation of the concentrations of nitrogenous species and their nitrogen isotopic ratios in aerosols at Gosan, Jeju Island: Implications for atmospheric processing and source changes of aerosols

Atmospheric aerosol samples (n = 84) were collected at Gosan site, Jeju Island, South Korea between April 2003 and April 2004 for the measurements of total nitrogen (TN) and its isotopic ratio (δ¹⁵N) as well as nitrogen species (NH₄⁺ and NO₃^-). Measurements were also conducted for remained nitrogen (remained N) and removed nitrogen (removed N) on HCl fume treatment. A pronounced seasonal variation was found in the δ¹⁵N of TN, remained N (mostly NH₄ ⁺), and removed N (mostly NO₃^-). The highest mean δ¹⁵N values of TN (+16.9‰ ±4.5‰) and remained N (+20.2‰ ±5.2‰) are detected in summer (June-August) whereas the lowest mean δ¹⁵N values (+12.9‰ ±3.4‰ and +11.3‰ ±5.1‰, respectively) are in winter (December-February). These trends can partly be explained by an enhanced contribution of ¹⁵N-enriched emissions from agricultural straw burning in China in a harvest season (summer and autumn). The mean δ¹⁵N of removed N showed an opposite trend: the lowest (+8.9‰ ±3.7‰) in warm season (March-August) and the highest (+14.1‰ ±3.7‰) in cold season (September-February). These results can be explained by changes in source regions and emission strengths of nitrogenous species, as well as difference in secondary aerosol nitrogen formation between warm and cold seasons. Higher ratios of Ca²⁺/ Na⁺ and the lowest ratios of Na⁺/(Cl^- + NO ₃^-) are associated with lower δ¹⁵N values of removed N as a result of less isotopic enrichment (ε _{product-reactant}) during the reaction between HNO₃ and dust particles. This study proposes that ¹⁵N/¹⁴N ratio can be regarded as process tracer of nitrogenous species in the atmosphere.