Spatial and Temporal Characterization of PM_(2.5) Mass Concentrations in California, 1980-2007

摘要

Systematic measurement of fine particulate matter (aerodynamic diameter less than 2.5 μm [PM_(2.5)]) mass concentrations began nationally with implementation of the Federal Reference Method (FRM) network in 1998 and 1999. In California, additional monitoring of fine particulate matter (PM) occurred via a dichotomous sampler network and several special studies carried out between 1982 and 2002. The authors evaluate the comparability of FRM and non-FRM measurements of PM_(2.5) mass concentrations and establish conversion factors to standardize fine mass measurements from different methods to FRM-equivalent concentrations. The authors also identify measurements of PM_(2.5) mass concentrations that do not agree with FRM or other independent PM_(2.5) mass measurements. The authors show that PM_(2.5) mass can be reconstructed to a high degree of accuracy (r~2 > 0.9; mean absolute error ~2 μg m~(-3)) from PM with an aerodynamic diameter ≤10 μm (PM_(10)) mass and species concentrations when site-specific and season-specific conversion factors are used and a statewide record of fine PM mass concentrations by combining the FRM PM_(2.5) measurements, non-FRM PM_(2.5) measurements, and reconstructions of PM_(2.5) mass concentrations. Trends and spatial variations are evaluated using the integrated data. The rates of change of annual fine PM mass were negative (downward trends) at all 22 urban and 6 nonurban (Interagency Monitoring of Protected Visual Environments [IMPROVE]) monitoring locations having at least 15 yr of data during the period 1980-2007. The trends at the IMPROVE sites ranged from -0.05 to -0.25 μg m~(-3) yr~(-1) (median -0.11 μg m~(-3) yr~(-1)), whereas urban-site trends ranged from -0.13 to -1.29 μg m~(-3) yr~(-1) (median -0.59 μg m~(-3) yr~(-1)). The urban concentrations declined by a factor of 2 over the period of record, and these decreases were qualitatively consistent with changes in emissions of primary PM_(2.5) and gas-phase precursors of secondary PM. Mean PM_(2.5) mass concentrations ranged from 3.3 to 7.4 μg m~(-3) at IMPROVE sites and from 9.3 to 37.1 μg m~(-3) at urban sites.