In situ measurements of speciated atmospheric mercury and the identification of source regions in the Mexico City Metropolitan Area

Abstract

Abstract. In order to expand the currently limited understanding of atmospheric mercury source-receptor relationships in the Mexico City Metropolitan Area, real time measurements of atmospheric mercury were made at a downtown urban site, and a rural site on the outskirts of Mexico City, during March 2006. Numerous short-lived increases in particulate mercury (PHg) and reactive gaseous mercury (RGM) concentrations were observed at the urban site during the 17 day study, and less frequent increases in gaseous elemental mercury (GEM) concentrations were measured at both the urban and rural sites. The episodic increases observed were attributed to plume impacts from industrial point source emissions in and around Mexico City. Average concentrations and standard deviations measured during the study were as follows: i) urban site; PHg=187±300 pg m−3, RGM=62±64 pg m−3, GEM=7.2±4.8 ng m−3, and; ii) rural site; GEM=5.0±2.8 ng m−3. Several source regions of atmospheric mercury to the urban and rural sites were determined using Concentration Field Analysis, in which atmospheric mercury measurements were combined with back trajectory data to determine source regions. Only some source regions correlated to mercury emission sources listed in the Federal Pollutant Release and Transfer Register, leaving the rest unaccounted for. Contributions of anthropogenic mercury point sources in and around Mexico City to concentration averages measured at the urban site during the study were estimated to be: 93±3% of reactive mercury (PHg and RGM), and; 81±0.4% of GEM. Point source contributions to GEM measured at the rural site were 72±1%. GEM and reactive mercury (PHg+RGM) were not found to correlate with biomass burning at either of the measurement sites.

Overview of receptor-based source apportionment studies for speciated atmospheric mercury

“…Similarly, higher CWT values for GEM in the Atlantic Ocean all year round suggested that the evasion of GEM from the ocean was a potential source of GEM in Nova Scotia (Cheng et al, 2013b). CFA results for the Mexico City sites indicated that the sites were impacted by known Hg point sources, such as cement and chemical production and paper and cardboard manufacturing, and by potential unregistered sources and volcanic emissions (Rutter et al, 2009).…”

“…CFA, RTWC, and CWT have been used to identify potential sources of TGM and speciated atmospheric Hg contributing to multiple sites in New York (Han et al, 2007), Mexico City (Rutter et al, 2009), Milwaukee, Wisconsin (de Foy et al, 2012; and Dartmouth, Nova Scotia (Cheng et al, 2013b). The New York and Milwaukee sites in the Great Lakes region and the Nova Scotia site identified industrial areas in Ohio and the eastern US as potential Hg sources (Han et al, 2007;de Foy et al, 2012;Cheng et al, 2013b).…”

“…CFA, RTWC, and CWT are also common back trajectory receptor models that have been used to identify potential source areas contributing to speciated atmospheric Hg measurements (Han et al, 2007;Rutter et al, 2009;de Foy et al, 2012;Cheng et al, 2013b). The most apparent difference between CFA/RTWC/CWT and previously described back trajectory receptor models is that the trajectory residence time in the grid cells have been weighted by the observed atmospheric Hg concentrations corresponding to the arrival of each trajectory.…”

“…In CFA studies for speciated atmospheric Hg, FLEXPART-WRF simulated the path of 100-1000 particles released from the receptor location (Rutter et al, 2009;de Foy et al, 2012). The particles were tracked for 48 h in Rutter et al (2009), since CFA was applied to speciated atmospheric Hg data. Six-day trajectories were determined by de Foy et al (2012) to simulate the transport of GEM.…”

Concentration-weighted trajectory approach to identifying potential sources of speciated atmospheric mercury at an urban coastal site in Nova Scotia, Canada

“…When the results are plotted on a map, these models illustrate potential source areas affecting a receptor site. There are several variations of TSMs that have been applied to speciated atmospheric Hg studies: Potential Source Contribution Function (PSCF) (Han et al, 2005(Han et al, , 2007Abbott et al, 2008;Choi et al, 2008;Xu and Akhtar, 2010;Fu et al, 2011), Gridded Frequency Distributions (GFD) (Weiss-Penzias et al, 2011;Gustin et al, 2012), Concentration Fields Analysis (Rutter et al, 2009) or Concentration-Weighted Trajectory (CWT), and Residence Time Weighted Concentration (RTWC) (Han et al, 2007). They all involve counting the frequency of back trajectory segment endpoints in grid cells that make up the geographical domain of interest for the receptor site.…”