Chemical Compound Classification by Elemental Signatures in Castle Dust Using SEM Automated X-ray Particle Analysis
Diana Ortiz-Montalvo, Edward P. Vicenzi, Nicholas W. Ritchie, Carol A. Grissom, Richard A. Livingston, Zoe Weldon-Yochim, Joseph M. Conny, Scott A. Wight
Discoloration on the Smithsonian Institution Building and Enid A. Haupt Garden gateposts was recently revealed to be related to a Mn enriched rock varnish. Mn does not appear to be derived locally from the building stone; therefore, its source is likely related to atmospheric dust transport. Minor oxygen isotopic ratios of sulfate in desert varnish demonstrate that atmospheric deposition of dust is an important component of the varnish formation process. A 2017 study of architectural rock varnish determined that vehicle emissions are a likely source of Mn. In this study, we evaluate airborne dust as a potential Mn source at a location where rock varnish is actively forming. Urban dust samples were collected on polycarbonate filters using a portable sampler with a size selective inlet (10 um diameter cut-off). The collection times were 24 hours, and the volumetric flow rate was 30 L/min. Samples were collected near the Haupt Garden gateposts and heavily trafficked Independence Ave. A TESCAN MIRA3 and a 20 keV/1 nA electron beam and 4 PulseTor silicon drift detectors (SDD) were used to analyze particles. Automated analysis was performed using the SEMantics extension to NIST DTSA-II. The sum of the 4 SDD spectra (400 ms dwell time) were used for quantification using NIST Graf. A novel algorithm was then used to cluster the data obtained for 39,491 particles. Data were then reprocessed using a manually developed rule set. The top four major particle classes of the particle population were silicate, Fe oxide, vehicle-related and CaMg carbonate. Only 52 particles had elevated levels of Mn (> 10 wt %). Overall, our results show low but detectable levels of Mn in the atmosphere in the Castle area. Efforts are underway to estimate the mass of Mn transported by atmospheric dust deposition for particles under 10 um in size. To our knowledge, this is the first study to examine the linkage between individual particle analysis of dust and active rock varnish formation.