Nitrogen dry deposition to Lake Superior and Lake Michigan

Nitrate (NO3-) levels in Lake Superior have increased from historic levels of about 5 mu M to its current concentration of about 25 mu M. The atmosphere makes a substantial contribution to the nitrogen budgets for Lake Superior and Lake Michigan. This study provides a more well-defined estimate of nitrogen dry deposition rates derived from the measurement of over-water concentrations, and in situ meteorological measurements, which were input into the Resistance Model. We obtained a nitrogen dry deposition rate of [(3.41 +/- 2.26) x 10(7) kg N/yr; (5.90 +/- 3.91) kg N/ha/yr] over Lake Michigan, and [(1.54 +/- 1.06) x 10(7) kg N/yr; (1.87 +/- 1.27) kg N/ha/yr] over Lake Superior. Nitric acid (HNO3), which originates from the combustion of fossil fuels, contributes 84% of the total nitrogen dry deposition to Lake Michigan; and 66% to Lake Superior. Ammonia (NH3), which originates from agricultural activities and gasoline combustion, is the second highest contributor of nitrogen dry deposition to both lakes: contributing 13% to Lake Michigan and 32% to Lake Superior. The nitrogen dry deposition is approximately 68% of the nitrogen wet deposition over lake Superior, and approximately 80% of wet deposition over Lake Michigan. The over-water dry deposition velocity of HNO3 and NH3 were also evaluated. We obtained morning deposition velocities of 0.099 cm/s for NH3 and 0.095 cm/s for HNO3; and afternoon values of 0.137 cm/s for NH3 and 0.132 cm/s for HNO3. Another key finding is that the atmospheric concentrations of nitrogen compounds near Lake Michigan and Lake Superior have decreased since 2003. (C) 2018 Published by Elsevier B.V. on behalf of International Association for Great Lakes Research.