Long-term relationships between SO2 and NOx emissions and SO42− and NO3− concentration in bulk deposition at the Hubbard Brook Experimental Forest, NH
A highly significant second-order polynomial relation between SO2 emissions and SO42− concentrations during 1970–2000 (r2 = 0.80, p = <0.001), and a linear relation between NOx and NO3− concentrations during 1991–2000 (r2 = 0.67, p = 0.004) in bulk precipitation were found for the Hubbard Brook Experimental Forest, NH based on emissions from a 24 h, back-trajectory determined source area. Earlier periods (1965–1980) for SO2 ∶ SO42− and longer periods (1965–2000) for NOx ∶ NO3− had poorer linear relations, r2 = 0.03, p = 0.51 and r2 = 0.22, p = 0.004, respectively. Methodology by the US Environmental Protection Agency for calculating emissions data during this period has changed significantly and frequently, making trend analysis difficult. Given the large potential for errors in estimating emissions and to a lesser extent, deposition, the robust relations between SO2 emissions and SO42− concentrations in bulk precipitation at the Hubbard Brook Experimental Forest show that careful, long-term measurements from a single monitoring site can provide sound and reasonable data on trends in air pollution.