Water-soluble Inorganic Nitrogen Research Articles

Nitrogen aerosols in New Delhi, India: Speciation, formation, and sources

Delhi, the capital city of India, experiences severe air pollution and suffers from its adverse effects on human health and ecosystems. This pollution is characterized by high levels of pollutants, including atmospheric nitrogen in both the gaseous and particulate phases. However, there is a lack of simultaneous measurement of chemical composition, tracers and 15N data in aerosols to understand the influence of different sources on N aerosols over Delhi. Here, we measured total nitrogen (TN), water-soluble total nitrogen (WSTN), water-soluble inorganic nitrogen (WSIN), and N stable isotope compositions (δ15N) in PM2.5 samples covering the post-monsoon, winter, and summer periods of the year 2018–19. NH4+-N was the major N species, accounting for an average 58% of TN and 68% of WSIN. The temporal variations of TN, WSTN, NH4+-N, NO3−-N, and WSON showed peaks in the post-monsoon and winter seasons, exhibiting seasonality similar to PM2.5 and levoglucosan (a biomass-burning tracer) indicating their co-genetic sources. Based on the correlation analysis between δ15N and N-species, we identified two distinct secondary chemical processes: i) in an NH4+-poor atmosphere, the gas-to-particle (NH3 → NH4+) conversion and subsequent formation of NH4HSO4 was the main process controlling the 15N and nitrogen enrichments in PM2.5; whereas ii) under NH4+-rich conditions, the formation and dissociation of NH4NO3 dominated. The coupled HYSPLIT and PSCF analyses highlighted the transport and contributions of open biomass burning emissions under a northwesterly atmospheric flow during post-monsoon as well as from local biomass combustion (from cooking and heating) during winter in the city and its vicinity. Our results suggested that i) both NH4+-N and NO3−-N were mainly impacted by biomass combustion during post-monsoon and winter seasons, and ii) NO3−-N resulted of dust transport from the Thar Desert in the summer season, but not NH4+-N. Finally, we recommend that future research focuses on the study of the seasonality of atmospheric nitrogen composition using 15N data from their different sources to design tailor-made measures and policies regarding the different potential sources, combining them within a comprehensive framework to ultimately improve air quality and the living environment in Delhi.

Deposition Characteristics of Water-soluble Inorganic Nitrogen and Organic Nitrogen in Atmospheric Precipitation in the Northern Suburbs of Nanjing

Based on atmospheric precipitation collected in the northern suburbs of Nanjing from 2019 to 2020, the pH, conductivity, and chemical components of precipitation were analyzed. The seasonal variation in pH and conductivity of atmospheric precipitation in the northern suburbs of Nanjing were studied. The pollution levels and deposition characteristics of water-soluble inorganic nitrogen (WSIN) and organic nitrogen (WSON) in precipitation were also analyzed. The frequency of acid rain (pH<5.6) in atmospheric precipitation in the northern suburbs of Nanjing reached 37.18% during the observation period. Precipitation acidification was more serious in autumn and winter, and the pH value showed a variation trend of spring>summer>autumn>winter. The average conductivity of precipitation was 29.49 μS·cm-1; high pH and conductivity in spring were related to the high dust content in the atmosphere. The seasonal difference between WSIN and WSON in precipitation was significant. The highest and lowest concentrations of NO3--N and NH4+-N appeared in spring and summer, respectively. The concentration of WSON was the highest in autumn (2.63 mg·L-1). The average concentration ratio of WSON to water-soluble total nitrogen (WSTN) in precipitation was approximately 0.47, indicating that WSON played an important role in the study of total nitrogen. The average wet deposition fluxes of WSIN and WSON were 12.10 kg·(hm2·a)-1 and 11.13 kg·(hm2·a)-1, respectively, in which the inorganic nitrogen deposition was mainly NH4+-N.

Pollution Characteristics and Sources of Water-soluble Organic Nitrogen in PM2.5 in Jiangbei New Area, Nanjing

The seasonal variation characteristics of water-soluble organic nitrogen (WSON) in PM2.5 were analyzed in the Nanjing Jiangbei new area. The relationship between WSON and water-soluble inorganic nitrogen (WSIN) was also discussed. The results showed that the variation range of WSON in PM2.5 was 0.446 to 4.200 μg·m-3, with an average value of 2.04 μg·m-3. This value was slightly higher than the observed results in Beijing, Shanghai, and Changzhou. The average value of WSON was the highest in autumn[(2.967±0.643) μg·m-3], which was approximately 1.7 times that of the other three seasons. The average contribution ratio of WSON to water-soluble total nitrogen (WSTN) in fine particles of Nanjing accounted for 25%, with high ratios in summer-autumn and low ratios in winter-spring. For example, the proportion of WSON/WSTN in winter was only approximately 50% of that in summer and autumn. WSON had the highest correlation with NO2--N and the lowest correlation with NO3--N in WSIN, which may be related to the volatilization of NO3--N caused by high temperatures in summer. Principal component analysis (PCA) showed that WSON of PM2.5 in Nanjing was mainly derived from secondary transformation, sea salt, dust, and biomass combustion.

Gridded distribution of total suspended particulate matter (TSP) and their chemical characterization over Delhi during winter.

In the present study, total suspended particulate matter (TSP) samples were collected at 47 different sites (47 grids of 5 × 5 km2 area) of Delhi during winter (January-February 2019) in campaign mode. To understand the spatial variation of sources, TSP samples were analyzed for chemical compositions including carbonaceous species [organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC)], water-soluble total nitrogen (WSTN), water-soluble inorganic nitrogen (WSIN), polycyclic aromatic hydrocarbons (16 PAHs), water-soluble inorganic species (WSIS) (F-, Cl-, SO42-, NO2-, NO3-, PO43-, NH4+, Ca2+, Mg2+, Na+, and K+), and major and minor trace elements (B, Na, Mg, Al, P, S, Cl, K, Ca, Ti, Fe, Zn, Cr, Mn, Cu, As, Pd, F, and Ag). During the campaign, the maximum concentration of several components of TSP (996 μg/m3) was recorded at the Rana Pratap Bagh area, representing a pollution hotspot of Delhi. The maximum concentrations of PAHs were recorded at Udhyog Nagar, a region close to heavily loaded diesel vehicles, small rubber factories, and waste burning areas. Higher content of Cl- and Cl-/Na+ ratio (>1.7) suggests the presence of nonmarine anthropogenic sources of Cl- over Delhi. Minimum concentrations of OC, EC, WSOC, PAHs, and WSIS in TSP were observed at Kalkaji, representing the least polluted area in Delhi. Enrichment factor <5.0 at several locations and a significant correlation of Al with Mg, Fe, Ti, and Ca and C/N ratio indicated the abundance of mineral/crustal dust in TSP over Delhi. Principal component analysis (PCA) was also performed for the source apportionment of TSP, and extracted soil dust was found to be the major contributor to TSP, followed by biomass burning, open waste burning, secondary aerosol, and vehicular emissions.

Size distributions and dry deposition fluxes of water-soluble inorganic nitrogen in atmospheric aerosols in Xiamen Bay, China.

Size-segregated aerosol particles were collected using a high volume MOUDI sampler at a coastal urban site in Xiamen Bay, China, from March 2018 to June 2020 to examine the seasonal characteristics of aerosol and water-soluble inorganic ions (WSIIs) and the dry deposition of nitrogen species. During the study period, the annual average concentrations of PM1, PM2.5, PM10, and TSP were 14.8 ± 5.6, 21.1 ± 9.0, 35.4 ± 14.2 μg m−3, and 45.2 ± 21.3 μg m−3, respectively. The seasonal variations of aerosol concentrations were impacted by the monsoon with the lowest value in summer and the higher values in other seasons. For WSIIs, the annual average concentrations were 6.3 ± 3.3, 2.1 ± 1.2, 3.3 ± 1.5, and 1.6 ± 0.8 μg m−3 in PM1, PM1-2.5, PM2.5–10, and PM>10, respectively. In addition, pronounced seasonal variations of WSIIs in PM1 and PM1-2.5 were observed, with the highest concentration in spring-winter and the lowest in summer. The size distribution showed that SO42−, NH4+ and K+ were consistently present in the submicron particles while Ca2+, Mg2+, Na+ and Cl− mainly accumulated in the size range of 2.5–10 μm, reflecting their different dominant sources. In spring, fall and winter, a bimodal distribution of NO3− was observed with one peak at 2.5–10 μm and another peak at 0.44–1 μm. In summer, however, the fine mode peak disappeared, likely due to the unfavorable conditions for the formation of NH4NO3. For NH4+ and SO42−, their dominant peak at 0.25–0.44 μm in summer and fall shifted to 0.44–1 μm in spring and winter. Although the concentration of NO3–N was lower than NH4–N, the dry deposition flux of NO3–N (35.77 ± 24.49 μmol N m−2 d−1) was much higher than that of NH4–N (10.95 ± 11.89 μmol N m−2 d−1), mainly due to the larger deposition velocities of NO3–N. The contribution of sea-salt particles to the total particulate inorganic N deposition was estimated to be 23.9—52.8%. Dry deposition of particulate inorganic N accounted for 0.95% of other terrestrial N influxes. The annual total N deposition can create a new productivity of 3.55 mgC m−2 d−1, accounting for 1.3–4.7% of the primary productivity in Xiamen Bay. In light of these results, atmospheric N deposition could have a significant influence on biogeochemistry cycle of nutrients with respect to projected increase of anthropogenic emissions from mobile sources in coastal region.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10874-021-09427-8.

Water-soluble organic and inorganic nitrogen in ambient aerosols over the Himalayan middle hills: Seasonality, sources, and transport pathways

Atmospheric nitrogen is ubiquitous in the environment and hence plays an essential role in the nutrient balance over the whole ecosystem. However, its abundance and characteristics, particularly in the Himalayas, are not well understood. Therefore, to understand the abundance, sources, and seasonality of soluble nitrogenous species in the middle hills of the central Himalayas, aerosol samples were collected at Dhulikhel in Nepal from January to December 2018. The results of this study revealed that water-soluble inorganic nitrogen (WSIN) contributed the most to water-soluble total nitrogen with an abundance of ammonium nitrogen (NH4+–N). Moreover, water-soluble organic nitrogen (WSON) contributed approximately 18% to aerosol total water-soluble nitrogen. The aerosol mass and WSIN species exhibited strong seasonality with considerably higher concentrations during dry periods and lower concentrations during the wet period. Furthermore, for dry periods, the HYSPLIT model revealed that nitrogen aerosols mainly originated from the Indo-Gangetic Plain region and were transported and deposited in the Himalayas through long-range atmospheric transport. The strong correlations of WSON with nssK+ (biomass burning) and nssCa2+ (crustal sources) and lack of a significant correlation with SO42− indicated that primary sources are responsible for generating WSON rather than secondary processes in the Himalayas. The estimated dry deposition fluxes for NO3−–N, NH4+–N, and WSON were 1.56, 8.35, and 4.06 kg ha−1 y−1, respectively. This study also shows that increasing air contaminants and emissions over South Asia can enter the Himalayas and affect the human health and ecology of this fragile area.

Chemical characterization and stable nitrogen isotope composition of nitrogenous component of ambient aerosols from Kanpur in the Indo-Gangetic Plains

Measurements of water-soluble total nitrogen (WSTN), water-soluble inorganic nitrogen (WSIN), water-soluble organic nitrogen (WSON) and ẟ15NTN (total N) was carried out on PM2.5 aerosol samples during wintertime to understand the major sources of ambient nitrogenous species at a heavily polluted location of Kanpur in north India. During the nighttime sampling campaign, WSON and NH4+_N contributed dominantly to the WSTN. Ammonium-rich condition persisted during sampling (NH4+/SO42− average equivalent mass ratio = 3.1 ± 0.7), suggesting complete neutralization of SO42− and formation of NH4NO3, which is stable in winter due to low temperature and high relative humidity (RH). Stagnant atmospheric conditions during wintertime enhanced concentrations of ionic species (SO42−, NH4+, and NO3−) at this location. Good correlations between NO3−_N, NH4+_N and biomass burning tracer K+BB (and also between NO3−_N, NH4+_N and SO42−) suggests a strong impact of biomass burning activities. Multi-linear regression (MLR) analysis shows a strong dependence of ẟ15N on NO3−_N, SO42− and WSON in night-1 (10:00 pm to 2:00 am) and on NO3−_N and SO42− in night-2 (2:00 am to 6:00 am) depicting different formation and removal mechanism of aerosols during both the time-periods. ẟ15NTN in PM2.5 varied from +8.8 to +15.5‰ (10.8 ± 1.3), similar to the variability observed for many urban locations in India and elsewhere. NH4+_N and WSON control the final ẟ15N value of nitrogenous aerosols. High relative humidity during nighttime enhanced the secondary organic aerosols formation due to aqueous-phase formation and gas to particle-phase partitioning. Isotopic fractionations associated with multi-phase reactions during gas to particle conversion of NH3 would result in an increase in ẟ15N by ~48‰ to 51‰ (at T of 5.4 °C to 15.4 °C) than that of the emission source(s), which indicates the most likely N-emission sources at Kanpur to be from agriculture activities and waste generation.

Size-segregated water-soluble N-bearing species in the land-sea boundary zone of East China

Size-segregated aerosol samples were collected in the land-sea boundary zone of East China, the offshore area of East China Sea (ECS), to determine the concentrations of water-soluble inorganic nitrogen (WSIN), water-soluble organic nitrogen (WSON), urea, and free amino acids (FAAs), to quantify their sources, and to estimate their dry deposition fluxes. The results indicated that the mean concentrations of WSIN, WSON, urea, and FAAs in total suspended particle (TSP) were 547.14 nmol/m3, 258.64 nmol/m3, 12.69 nmol/m3, and 2589.03 pmol/m3, respectively. The WSIN, WSON, and urea concentrations in TSP showed remarkably spatial variation with the higher one during the shipping line (SL) 1 and SL4, while they exhibited the lower ones during SL3 and SL5. The biomass burning and fertilizer application in the continent provided important precursors for WSIN, WSON, and urea in the offshore areas. Besides, O3-related photochemical process also promoted the secondary formation of these species. In contrast, these anthropogenic sources played the minor roles on the relatively remote marine region (SL5). It was interesting to note that the total FAAs during SL5 did not show the lowest concentration, which might be contributed by the bubble bursting on the sea surface and the release of bacteria. The Positive Matrix Factorization (PMF) method identified fertilizer application and secondary formation were important sources for WSIN (35.54% and 27.53%) and WSON (43.07% and 40.46%), respectively. However, fertilizer application and combustion sources played the crucial important roles on urea (65.93% and 19.13%) and FAAs (51.35% and 16.35%). The mean dry deposition fluxes of WSIN, WSON, urea, and FAAs in the ECS offshore area were 206763.57 nmol m−2 d−1, 103763.07 nmol m−2 d−1, 4746.20 nmol m−2 d−1, and 1047.08 nmol m−2 d−1, respectively. The present study revealed that the ambient N-bearing particles in the land-sea boundary zone suffered from the combined effects of continental transport and the release of marine organisms.

Characteristics and Sources of Water-soluble Organic Carbon/Nitrogen in PM2.5 During Spring in Changzhou

To understand the characteristics and sources of water-soluble organic carbon (WSOC) and organic nitrogen (WSON) in atmospheric aerosols during spring in Changzhou, 84 fine particle (PM2.5) samples were collected from March 1 to May 30, 2017, in Changzhou. The water-soluble components, including water-soluble organic carbon, water-soluble total nitrogen (WSTN), water-soluble ions, and carbonaceous components (OC and EC), were analyzed. The levels of WSOC and WSON and their source characteristics were discussed. The results show that the average concentrations of PM2.5, WSOC, and WSON are 101.97, 7.63, and 1.50 μg·m-3, respectively, during the sampling period. The WSON accounts for 12.9% of the WSTN and the water-soluble inorganic nitrogen mainly exists in two forms, that is, NH4+ and NO3-, accounting for 86.15% of the WSTN. The WSOC is weakly correlated with WSON (r=0.58), indicating that WSOC and WSON do not have the same sources. The WSOC is related to SOC, K+, and secondary ions (SO42-, NH4+, and NO3-), indicating that it is mainly derived from biomass burning and secondary conversion; WSON is strongly correlated to secondary ions, indicating that it is mainly derived from secondary conversion. The wind speed is the main factor affecting the WSOC and WSON concentration levels. Furthermore, the WSON is positively correlated with the air pressure and negatively correlated with the temperature. The results of the principal component analysis show that PM2.5 mainly originates from four sources:secondary formation, dust, coal combustion, biomass burning, and the ocean. The backward trajectory analysis indicates that the total concentrations of PM2.5, WSOC, and WSON in air masses from long-distance transformation are higher than that from short-distance transmission, whereas there is no significant difference in the WSON/WSTN ratio from different transmission paths.

Effects of Atmospheric Dry Deposition on External Nitrogen Supply and New Production in the Northern South China Sea

The South China Sea (SCS) is one of the world’s largest oligotrophic marginal seas. Increases in biomass and primary production in the surface layer of the northern SCS are affected by anthropogenic aerosol use among north Asian peoples. The seasonal variation of dry deposition and its contribution to new production in the ocean are vital to determining the effect that such dry deposition has on the biogeochemical cycle of the SCS. This study collected 240 samples of total suspended particles at Dongsha Island in the northern SCS from April 2007 to March 2009; the major ions and water-soluble nitrogen species in the samples were analyzed. The analysis results indicated that the concentration distributions of major water-soluble ions and nitrogen species in total suspended particles exhibited significant seasonal (source) variation. The north-east monsoon seasons (autumn to spring) brought relatively high concentrations because most air masses during this period arrived from the northern continental region. We found that the concentration of nitrogen species shows a latitude distribution, gradually decreasing from north to south. In addition, this study also discovered that the ratio of organic nitrogen to total dissolved or water-soluble nitrogen also varies in a similar manner, resulting in a concentration of &lt;20% for locations north of 30° N and &gt;30% for those south of 30° N. Aerosols at Dongsha Island mainly comprised sea salt; however, significant chloride depletion was observed during the north-east monsoon season. The molar ratio of NH4+ to non–sea salt (NSS) sulfate (nss-SO42−) was 0.8, indicating that the amount of artificially produced NH4+ in the region was insufficient for reaction with nss-SO42−. Therefore, NH4+ was mainly present in the form of NH4HSO4. The fluxes of water-soluble inorganic nitrogen (WSIN) and water-soluble organic nitrogen (WSON) within the region were 23 ± 13 and 27 ± 15 mmol m−2 y−1, respectively. The new production converted from atmospheric water-soluble nitrogen species in the northern SCS was estimated to be 0.52–0.81 mmol C m−2 d−1. This flux made about 5.6–8.7% (the global average was about 3.5%) contribution to the primary production (9.24 mmol C m−2 d−1) of the SCS surface water. This result indicates that the ocean’s external nitrogen supply, provided by anthropogenic aerosols, is vital for the biogeochemical cycle in Asian marginal seas, particularly the northern SCS.

Water-soluble components in rainwater over Xi'an in northwest China: Source apportionment and pollution controls effectiveness evaluation

Rainwater samples were collected to investigate the water-soluble species levels and their sources at a semi-arid urban site of Xi'an, China, during 2015. The precipitation samples were analyzed for pH, electric conductivity (EC), ionic species (Na+, K+, NH4+, Ca2+, Mg2+, F−, Cl−, NO3−, and SO42−), and water-soluble organic carbon and nitrogen (WSOC and WSTN). The annual volume-weighted mean (VWM) of pH and EC values were 6.50 and 46.9 μS cm−1, respectively. The total ionic concentrations in rainwater varied widely, ranging from 121.9 to 2448.2 μeq L−1 with an annual VWM of 686.4 μeq L−1. SO42− was the largest contributor to anions while Ca2+ was the most important cation. SO42− and NO3− levels decreased in a large scale by comparing with prior study in six years ago. Moreover, the total wet depositions flux of water-soluble inorganic nitrogen (sum of NH4+-N and NO3−-N) and SO42--S were reduced by 54.5 and 81.1% in 2015 compared with 2009, respectively. The results indicated that the pollution controls during the past years were more effective over Xi'an. The average concentrations of water-soluble organic carbon and total nitrogen (WSOC and WSTN) were 2.9 and 3.9 mg L−1. The water-soluble organic nitrogen was the dominant component in WSTN. The neutralization factor showed that NH4+ and Ca2+ were the dominant neutralization substances in the rainwater, whereas the effects of Mg2+, K+ and Na+ were negligible. The enrichment factor analysis indicated that SO42− and NO3− were mainly originated from anthropogenic sources, while Ca2+ was associated with terrestrial sources. Source apportionment by a positive matrix factorization receptor model showed that the anthropogenic emissions were the largest contributor to SO42− (61.9%), NO3− (65.3%), and NH4+ (70.5%), followed by crustal sources (14.9, 21.3, and 18.4%) and biomass burning (13.8, 12.1, and 6.8%). For the major cations of Ca2+, crustal source was the dominant contributor (75.6%), followed by biomass burning (14.6%).

Water-soluble nitrogen and phosphorus in aerosols and dry deposition in Jiaozhou Bay, North China: Deposition velocities, origins and biogeochemical implications

To probe the deposition velocities (Vd), fluxes, origins and biogeochemical significance of atmospheric dry deposition (ADD) of nutrients in coastal waters, aerosol and dry deposition samples were collected in Jiaozhou Bay (JZB) from June 2015 to May 2016. We measured water-soluble inorganic nitrogen (WSIN; including ammonium (NH4N), nitrate (NO3N) and nitrite (NO2N)), organic nitrogen (WSON), inorganic phosphorus (WSIP; including reactive phosphate (PO4P)) and organic phosphorus (WSOP) for all samples. The concentrations of aerosol N species were comparatively higher than those in other aquatic ecosystems worldwide. Although WSIN and WSIP represented relatively large percentages of species, WSON (~16%) and WSOP (~35%) cannot be ignored in aerosols. The dry deposition fluxes for NH4N, NO3N, NO2N, WSON, PO4P and WSOP were 29.4, 29.9, 0.058, 15.4, 0.099 and 0.165 mmol m−2 yr−1, respectively, with apparent seasonal variations. The average annual Vd values of NH4N, NO3N, NO2N, WSON, PO4P and WSOP were indirectly calculated as 0.17 ± 0.11, 0.35 ± 0.20, 0.34 ± 0.25, 0.30 ± 0.22, 0.43 ± 0.28 and 0.91 ± 0.60 cm s−1, respectively. The considerable differences between the percentages of corresponding nutrient concentrations and ADD fluxes were mainly attributed to Vd variations. Anthropogenic emissions were the most common source of WSON and WSIN based on a correlation analysis. WSIP was highly correlated with WSOP, suggesting that they may share similar sources. Additionally, the long-range transport of natural and anthropogenic pollutants, especially the air-masses originating from the northwest direction, exerted greater effects than local emissions on aerosol N and P species in JZB based on an air mass backward trajectory analysis. The increasing ADD flux may have non-negligible biogeochemical effects on JZB by increasing primary productivity and altering the nutrient structure due to the high WSIN/WSIP ratio.

Water-Soluble Organic Nitrogen in High Mountain Snow Samples from Central Japan

ABSTRACTWe measured water-soluble organic nitrogen (WSON) in snow pit samples, which were collected at the Murodo-Daira snowfield near the summit of Mt. Tateyama, central Japan in 2008, 2009, and 2011. The concentrations of WSON ranged from 12.8 to 96.7 ng g–1, which were significantly lower than those reported in continental wet deposition samples from the Asian continent. WSON may be largely diluted in the snow samples during snowing processes over the high mountains. We found that WSON significantly correlated with nss-Ca2+ and water-soluble organic carbon (WSOC). Concentrations of WSON are likely controlled by the intensity of Asian dust events. Contributions of WSON to water soluble total nitrogen (WSTN) in snow pit sequence was found to be 15 ± 10%, which is lower than those (63–91%) of reference dust materials collected in China. Mass concentration ratio of WSOC/WSON was on average 23.7, which is significantly higher than the C/N weight ratio (5.6) calculated from the Redfield ratio. This result suggests that WSOC is largely produced by secondary photochemical oxidation of anthropogenic volatile organic compounds during the transport from East Asia to the high mountain areas in Japan. On the other hand, WSON may be contributed from Asian dusts from arid areas in China whereas water-soluble inorganic nitrogen may be derived from pollution sources.

Occurrence of water soluble organic nitrogen in aerosols at a coastal area

Aerosol samples (205) were collected from January 2005 to November 2006 at a coastal site at Keelung City (Taiwan), on the southern East China Sea. The atmospheric concentrations of major ions indicated that the area was dominated by different air masses at different times of the year: continental Asia from January to May and November to December; air masses from Indonesia and surrounding islands (south-eastern region) dominated during early-summer (July 2005 and June 2006), and air masses directly from the adjacent ocean dominated during late-summer (July to August 2006) and early-autumn (September to October 2005). The atmospheric concentrations of water soluble organic nitrogen (WSON) corresponded to the different sources and weather conditions. The results of a factor analysis of combined major ions, water soluble inorganic nitrogen (WSIN) and WSON indicated that a fossil fuel combustion source and biomass burning were the two major controlling factors during the sampling period. We discuss the correlations between WSON, WSIN and major anthropogenic ions in relation to the different formation mechanisms for the various sources. We propose a mechanism whereby biomass burning is the major input of aerosols derived from mainland China. Fossil fuel combustion was found to be important during periods when aerosols were derived from the south-eastern area. Our data indicate that WSON was predominantly associated with aerosols derived from biomass burning when the origin of the air mass was mainland China.