Investigating missing sources of sulfur at Fairbanks, Alaska.

We investigated disparities in elemental sulfur and inorganic sulfate concentrations in ambient fine particulate matter (PM2.5) data from 2005 to 2012 at a monitoring station in Fairbanks, AK. In approximately 28% of the observations from 2005 to 2012, elemental sulfur by X-ray fluorescence (XRF) spectroscopy significantly exceeded the inorganic sulfur by ion chromatography (IC), suggesting the presence of a significant quantity of unmeasured sulfur compounds. The mean ratio of sulfur by XRF to that by IC for only these cases was 1.22 ± 0.11. The largest discrepancies between elemental sulfur and sulfate were most frequently observed in the summer, although discrepancies were observed year round. Assuming the additional sulfur (other than inorganic sulfate) as the upper limit estimate, this work shows that organosulfur species (or the additional sulfur) account for 1.29% of organic carbon (OC) and 0.75% of PM2.5 in Fairbanks. An analysis of all available air quality system (AQS) data suggests that these recurring phenomena are linked to seasons, total carbon, inorganic nitrate, and elemental sources during cold periods and ozone during warm periods.