Deposition of nitrogen from the NOx emitted at the Metropolitan Area of Buenos Aires on waters of de la Plata River

PINEDA ROJAS, A.L.; VENEGAS, L.E.

Xiamen, China

Conferencia; SOLAS Open Science Conference 2007; 2007

Surface Ocean - Lower Atmosphere Study

The Metropolitan Area of Buenos Aires (MABA), composed by Buenos Aires City and the Greater Buenos Aires, has an extension of 3830km2 and is the third mega-city of Latin America. This work presents the first estimations of the deposition of nitrogen (N), originated from the NOx emitted at MABA, on the waters of de la Plata River. Results are analysed in relation with the behaviour of meteorological variables. The atmospheric dispersion models CALPUFF and DAUMOD-RD have been applied to point and area emission sources, respectively. These models include parameterizations of formation and deposition of gaseous NO2 and HNO3 and NO3- aerosol. Chemical transformation rates are hourly calculated as functions of environmental variables. Dry deposition is estimated through the resistance method and wet deposition is calculated using scavenging coefficients. Computations have been performed hourly over a 100x107 grid cells of 1km2 including MABA and 3539km2 of the river. Three years of meteorological data measured at a coastal site and a recently developed high resolution NOx emission inventory for MABA have been used in calculations. Mean total N deposition varies between 5643-9129kg-N/month. NO2 dry deposition is greater in May-August as a consequence of greater offshore wind frequencies and lower oxidation rates. Monthly averaged diurnal transformation rates for the NO2 loss and HNO3 formation vary between 8.3-31.2%/h and 2.8-9.4%/h, respectively; while NO3- formation rates are in the range 26.7-86.6%/h. Despite the low available HNO3, its high solubility results in deposition values comparable to those of NO2. Particularly high HNO3 dry deposition is obtained for December as a result of a greater number of favourable situations for deposition during diurnal hours (greater NOx emission + HNO3 formation) in relation to other months of the same season. Monthly NO3- dry deposition is greatly determined by temperature variation through nitrate formation. HNO3 and NO3- wet depositions show strong seasonal variations associated to monthly variation of transformation rates and simultaneous occurrence of precipitation with offshore winds. On the other hand, monthly wet and dry total deposition horizontal distributions vary significantly according to monthly wind direction distributions. Mean annual total N deposition is 84158kg. NO2, HNO3 and NO3- dry depositions constitute 42.7%, 22.7% and 19.8% of this value, respectively; while HNO3 and NO3- wet depositions represent 3.5% and 11.3%, respectively. Wet deposition contribution to total N deposition is relatively low because precipitation occurred during ~5% of the hours with offshore winds. Greater deposition values (up to 136.7kg-N/km2yr) occur near the shoreline. These values reduce to 50% at ~5km from the coast. Wet deposition contribution to total N deposition in each receptor varies between 8.7-23.4% and is greater far from the coast.