[Audit Indicators and Suggested Ranges for Data Validation of Chemical Components in Ambient PM_2.5: A Case Study of...

Diagnostic indicators for the validation of PM_2.5 compositional data were calculated, based on the monitoring results of approximately 2100 ambient samples collected in the Yangzi River Delta from 2014 to 2017. According to the results of a correlation analysis, we propose that the audit indexes of the monitoring data of PM_2.5 components in ambient air should include:equivalent ratios of anion-cation charge balance(A/C), the consistency between sum of all measured components(∑species) and weighed PM_2.5, the consistency between mass reconstructed PM_2.5(PM_{2.5, reconstructed}) and weighed PM_2.5, the chemical consistency between elemental S and water-soluble SO₄^2-, elemental K and water-soluble K⁺, and the chemical consistency of theoretical and tested NH₄⁺. The double-sided 95% reference ranges of anion-cation equivalent balance (A/C), ∑species/PM_2.5, PM_{2.5, reconstructed}/PM_2.5, S/SO₄^2-, and K/K⁺ ratios were determined in terms of P_2.5 and P_97.5 as follows:(0.82, 1.35), (0.63, 0.94), (0.62, 1.00), (0.28, 0.50), and (0.66, 2.31). These diagnostic indicators were helpful for judging the errors of chemical component analysis and retain seasonal variation stability. In most cases, NH₄⁺ existed in the form of NH₄NO₃ and (NH₄)₂SO₄ in spring and summer. With the approach of autumn and winter, it transformed to NH₄NO₃, (NH₄)₂SO₄, and NH₄Cl. The results of literature verification showed the pass rate of A/C was 87.1% and the rate of other indexes was 100%, indicating that the above audit indexes we propose could be applied to not only the Yangzi River Delta but the entire country. Furthermore, there were certain conditions in applying the diagnostic indicators. The S/SO₄^2- ratio worked well with PM_2.5 ≥ 40 μg·m^-3 in summer and with 60 μg·m^-3 ≤ PM_2.5 ≤ 140 μg·m^-3 in spring, autumn, and winter. Other audit indexes were available universally in all weathers under the condition of PM_2.5 ≥ 60 μg·m^-3.