Successes and challenges of measuring and modeling atmospheric mercury at the part per quadrillion level: a critical review

Abstract

Measurements of atmospheric mercury (Hg) are being increasingly incorporated into monitoring networks worldwide. These data are expected to support and inform regulatory decision making aimed at protecting human and wildlife health. Here we critically review current efforts to measure Hg concentrations in the atmosphere and interpret these data with Hg models. There are three operationally defined forms of atmospheric Hg: Gaseous Elemental (GEM), Gaseous Oxidized (GOM), and Particulate Bound (PBM). While there is relative confidence in GEM measurements, GOM and PBM are less well understood. Field and laboratory investigations suggest the methods to measure GOM and PBM are impacted by analytical interferences that vary with environmental setting (e.g., ozone, relative humidity) and GOM concentrations can be biased low by a factor of 1.6–12 times depending on the chemical compound. Importantly, efforts to understand the fundamental limitations of atmospheric Hg measurement methods have provided clear evidence that the composition of GOM (e.g., HgBr 2, HgCl 2, HgBrOH) varies across space and time. This has significant implications for refining existing measurement methods and developing new ones, model/measurement comparisons, model development, and assessing trends. In addition, unclear features of previously published data may now be re-examined and possibly explained, which we present as a case study. Lastly, we outline recommendations for needed research directions as the Hg field moves forward. Priorities include GOM and PBM calibration systems, identification of GOM compounds in ambient air, and identification of redox mechanisms and associated rate coefficients. Determination of a quantitative correction factor for biased GOM and PBM data is also needed to facilitate model-measurement comparisons.