Environmental Statement volume 4 - Chiltern Evergreen3

J4CALCULATION OF ACID DEPOSITION AND NUTRIENT NITROGENDEPOSITIONContributions to acid deposition have been derived from rail concentrations ofNO X and SO 2 . Acid deposition can occur through dry and wet mechanisms.However, according to EA guidance (1) , for short-range effects, NO 2 and SO 2wet deposition is less significant compared with dry deposition. Therefore forNO 2 and SO 2 , only dry deposition has been assessed.Dry deposition rates were calculated using methods recommended by theEnvironment Agency (EA) (2) , as follows:Step 1: Calculate dry deposition flux.Dry deposition flux (µg m -2 s -1 ) = Rail concentration (µg m -3 ) × Depositionvelocity (m s -1 )The following deposition velocities (Table J4.1), as recommended by the EA,have been used.Table J4.1 Dry Deposition Velocities (m s -1 )Pollutants Grassland ForestSO 2 0.012 0.024NO 2 0.0015 0.03Step 2: Convert units from µg m -2 s -1 to units of kg ha -1 yr -1 by multiplyingthe dry deposition flux by standard conversion factors in Table J4.2.Table J4.2 Conversion Factors from µg m -2 s -1 to kg ha -1 yr -1Pollutants From µg m -2 s -1 to kg ha -1 yr -1SO 2 157.7NO 2 96Step 3: Convert to unit of equivalents (keq ha -1 yr -1 ) which is a measure ofhow acidifying the chemical species can be, by multiplying the drydeposition flux in units kg ha -1 yr -1 by standard conversion factors in TableJ4.3.(1) Spanton, A.M., Hall, D.J. and Powlesland, C.B. Calculation of Near-field Critical Load Exceedance from GeneratingStations, Environment Agency, May 2008.( 2 ) AQTAG06 – Technical Guidance on Detailed Modelling Approach for an Appropriate Assessment for Emissions to Air,Environment Agency, produced 06/02/04, Version 8.ENVIRONMENTAL RESOURCES MANAGEMENTJ12CHILTERN RAILWAYS