US EPA applications of the Monitor for AeRosols and GAses in ambient air (MARGA) to measure ambient gaseous and particulate pollutants and dry deposition fluxes

摘要

The US EPA has been deploying Monitor for AeRosols and GAses in ambient air (MARGA) analyzers as part of numerous research-orientated intensive sampling campaigns since 2012 which have included cooperation with universities and other government agencies. The MARGA is a semi-continuous, multipollutant analyzer that samples soluble gases (HNO_3, HONO, SO_2, and NH_3) and particles (NO_3~-, SO_4~(-2), NH_4~+, Ca~(+2), Mg~(+2), K~+, Na~+) via an absorption medium and quantifies their concentrations via on-line ion chromatography on an hourly basis. The analyzer can be deployed in a conventional setup with a single sampling 'box' (containing a wet, rotating, denuder for gases and a steam jet aerosol collection device for particles) or in a 'gradient mode' setup in which two sampling boxes are installed at different heights allowing for measurement of an hourly averaged concentration gradient. EPA research efforts using the conventional MARGA setup have focused on the assessment of integrated measurement techniques such as the CASTNET filter pack for effects due to shorter time-scale phenomena (i.e. meteorology, diurnal variation) to provide more information on pollutant transport, potential reservoirs, and source apportionment. MARGA measurements are also compared with co-located measurements of other pollutants (e.g. O_3, NO_y). The gradient mode setup allows the measurement of hourly concentration gradients to be coupled with micro-meteorological measurements to provide direct measurements of deposition fluxes and velocities. The system performance was characterized in terms of analytical accuracy and chemical gradient detection limits to estimate the uncertainties of the flux measurement and these were found to be sufficiently precise for flux gradient applications (Rumsey and Walker, 2016). Results from this study were taken over a short grass surface, the MARGA systems are currently being deployed on a tall tower over a forest canopy at the Coweeta Hydrologic Laboratory in western NC and over a coastal marshland site in Charleston, SC to expand the research to different types of ecosystems. This improved information on pollutant speciation and deposition will help to develop more complete N and S deposition budgets and will help to develop algorithms in regulatory models such as the Community Multiscale Air Quality (CMAQ) model for use in assessing ecosystem exposure and vulnerability to excess deposition.