Nitrogen dry deposition to Lake Superior and Lake Michigan

Nitrate (NO ₃ ⁻ ) levels in Lake Superior have increased from historic levels of about 5 μM to its current concentration of about 25 μM. 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) × 10 ⁷ kg N/yr; (5.90 ± 3.91) kg N/ha/yr] over Lake Michigan, and [(1.54 ± 1.06) × 10 ⁷ kg N/yr; (1.87 ± 1.27) kg N/ha/yr] over Lake Superior. Nitric acid (HNO ₃ ), 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 (NH ₃ ), 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 HNO ₃ and NH ₃ were also evaluated. We obtained morning deposition velocities of 0.099 cm/s for NH ₃ and 0.095 cm/s for HNO ₃ ; and afternoon values of 0.137 cm/s for NH ₃ and 0.132 cm/s for HNO ₃ . Another key finding is that the atmospheric concentrations of nitrogen compounds near Lake Michigan and Lake Superior have decreased since 2003.