High-volume PM2.5 samples were collected hourly from 4 December to 13 December 2012 at an urban site in Xi'an and analyzed for organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), water-soluble total nitrogen (WSTN), water-soluble organic nitrogen (WSON) and inorganic ions to investigate the sources and formation mechanism of WSON. The results showed that during the sampling period the averaged hourly concentration of WSON was (12 +/- 9.4) microg x m(-3) and maximized at 31 microg x m(-3), accounting for 47% +/- 9.8% of WSTN with NH4(+) -N and NO3(-) -N being 29% +/- 8.5% and 23% +/- 8.1%, respectively. WSON: WSOC (N: C) mass ratios ranged from 0.04 to 0.65 with an average of 0.31 +/- 0.13 during the observation period. WSON was (1.6 +/- 0.9) microg x m(-3), (6.5 +/- 3.9) microg x m(-3) and (23 +/- 4.7) microg x m(-3) in non-haze days (visibility > 10 km), light haze days (5 km < visibility < 10 km) and heavy haze days (visibility < 5 km), respectively. WSOC/OC mass ratio throughout the observation period showed no significant change, but WSON/WSOC(N: C) mass ratio increased significantly from a lower value of 0.2 +/- 0.1 in non-haze days to 0.3 +/- 0.1 on light haze days and 0.4 +/- 0.1 on heavy haze days, in consistence with the enhanced acidity of the fine particles. In addition, during the whole sampling period, WSON was strongly correlated with NH4(+), SO4(2-) and NO3(-) (R2 > 0.80), and negatively correlated with cation-anion equivalent ratio (R2 = 0.53). These phenomena can be mainly ascribed to a gas-particle conversion of gaseous water-soluble nitrogen-containing organic compounds like amines via acid-base reactions, which was sharply increased under the favorable meteorological conditions (e.g., low temperature and high humidity) during the heavy haze days.