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

Abstract

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 (delta N-15) 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 delta N-15 of TN, remained N (mostly NH4+), and removed N (mostly NO3-). The highest mean delta N-15 values of TN (+16.9 parts per thousand +/- 4.5 parts per thousand) and remained N (+20.2 parts per thousand +/- 5.2 parts per thousand) are detected in summer (June-August) whereas the lowest mean delta N-15 values (+12.9 parts per thousand +/- 3.4 parts per thousand and +11.3 parts per thousand +/- 5.1 parts per thousand, respectively) are in winter (December-February). These trends can partly be explained by an enhanced contribution of N-15-enriched emissions from agricultural straw burning in China in a harvest season (summer and autumn). The mean delta N-15 of removed N showed an opposite trend: the lowest (+8.9 parts per thousand +/- 3.7 parts per thousand) in warm season (March-August) and the highest (+14.1 parts per thousand +/- 3.7 parts per thousand) 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- + NO3-) are associated with lower delta N-15 values of removed N as a result of less isotopic enrichment (epsilon(product-reactant)) during the reaction between HNO3 and dust particles. This study proposes that N-15/N-14 ratio can be regarded as process tracer of nitrogenous species in the atmosphere.