Water-soluble Organic Nitrogen And Urea In Qingdao Aerosols: Influence Of Different Weather Conditions

Organic nitrogen (ON) is one of important components of reactive nitrogen in atmospheric aerosols. Atmospheric deposition of organic nitrogen in ocean could enhance primary productivity and affect the community structure of marine ecosystems. It was reported that those phytoplanktons which can assimilate organic nitrogen became dominate populates in laboratory experiments. However, the knowledge on the contribution of ON to the total nitrogen in ocean via atmospheric deposition is still limited. Accurate estimation of the contribution would help better understanding ON's effects to marine ecosystem.In this study, 67 total suspended particulate (TSP) samples were collected in Qingdao from January to December in 2008. The concentration of water-soluble organic nitrogen (WSON) and urea in these TSP samples were analyzed. The contribution of WSON to the total nitrogen and the contribution of the urea nitrogen to the total WSON were computed. The atmospheric deposition flux of WSON was discussed in terms of meteorological conditions and air mass sources. We draw some conclusions which were summarized as below:The average concentration of WSON in the TSP samples was 433.2±387.7 nmol·m^-3 (2489.9±2572.9μmol·g^-1) in 2008. The concentration of particulate organic nitrogen was obviously determined by the meteorological conditions. For example, the water-soluble organic nitrogen average concentration of WSON in the TSP samples collected after rain decreased by about 80% of that before rain. During haze episodes, the average concentrations of WSON in the TSP samples were 789.2±549.0 nmol·m^-3 while the average was 432.3±361.5 nmol·m^-3in fog events. The averages in haze and fog events were about 4 times and twice of that collected in clear days. However, the particle mass concentrations in haze and fog days were similar to that in clear days. The average mass concentration of the TSP during dust episodes was 6 times of that collected in clear days, but the average concentration of WSON incerased only by 30%. The water-soluble organic nitrogen average concentration of WSON was also associated with air mass sources in sampling. The highest average concentration of WSON was 621.6±430.5 nmol·m^-3 in the samples in which the air masses were passed over the developed areas, followed by the air mass passing over the undeveloped areas (133.4±40.9 nmol·m^-3), and the lowest from the clear marine atmsophere (74.9±39.6 nmol·m^-3).The urea concentration in the TSP samples ranged from 0.4 nmol·m^-3 to 53.9 nmol·m^-3(17.8±14.6nmol·m^-3, on average). The higher average concentration of urea was observed from March to May while the lowest occurred in August. The urea nitrogen accounted for 6.6±5.9% (on average) of the total WSON. The highest contribution of the urea nitrogen to the WSON occurred in spring while the contribution was the lowest in summer. The average concentration of urea in the TSP samples collected in clear days was considered as the background. The urea concentration increased slightly by 30% during haze and fog episodes, but it increased obviously by 130% in dust days. The air mass back trajectory analysis indicated that the average concentration of urea nitrogen in the TSP samples was related to air mass sources. The lowest average concentration of urea nitrogen occurred when the air mass came from clean marine atmosphere, while the higher concentration was associated with the air mass derived from the continent. However, the difference of the urea concentration was small between the samples where the air masses passing over the undeveloped and developed areas was separated from each otherIn Qingdao, the atmospheric dry deposition fluxes of organic nitrogen were between 7.7 and 1074.7μmol·m^-2·d^-1(224.6±201.0μmol·m^-2·d^-1, on average). The average deposition flux was lowest in summer (81.0μmol·m^-2·d^-1), but there was no obvious difference between the other three seasons (213.9²67μmol·m^-2·d^-1). In different meteorological conditions, the deposition fluxes of WSON varied a lot. The flux was highest in haze days with the deposition fluxes of 409.1μmol·m^-2·d^-1; the secondary highest occurred in fog days (224.1μmol·m^-2·d^-1); the third highest was occurred in dust days (132.6μmol·m^-2·d^-1), and it was lowest after rainfall with the deposition fluxes of 29.6μmol·m^-2·d^-1. The deposition flux of WSON was also affected by different air mass sources. The flux was highest when air masses passed over the developed areas (322.2μmol·m^-2·d^-1), followed by the air masses passing over the undeveloped areas (69.1μmol·m^-2·d^-1), and the lowest from the clear marine atmosphere (38.8μmol·m^-2·d^-1). The average deposition flux of atmospheric urea nitrogen was 17.7±14.6μmol·m^-2·d^-1, accounting for 12.6±11.4% of the total deposition flux of WSON. The urea nitrogen made the highest contribution to WSON in spring, which account for 55% of the total deposition fluxes of the whole year. But it was lowest in summer, which only account for 9%.