Development and first results of an aircraft-based, high time resolution technique for gaseous elemental and reactive...

We developed a high time resolution (2.5 min) aircraft instrument for gaseous elemental mercury (GEM) and reactive gaseous mercury (RGM). The system measures RGM with complementary denuder and difference techniques, which can be compared to check for consistency. In laboratory tests, the agreement of the two techniques was 15% (+/- 13%). In five test flights, RGM was generally below the detection limit of the difference technique (0.08-0.16 ng/m3) except for values of 200-500 pg/m3 in airmasses between 600-700 mb (Flight 1) and 850-550 mb (Right 4), which is consistent with previous observations of RGM at Mt Bachelor. There was a linear correlation between the denuder and difference techniques in each flight (range of slopes, 0.27-1.24) and across all flights (slope = 0.37, p < 1e-6). The correlation is evidence that the difference technique is able to measure RGM in real time, although RGM appears to not be fully captured or recovered by the denuder. The only factor common to all RGM enhancements was the low aerosol scattering coefficient (< 2 M/m). Particulate mercury was below the detection limit (27 pg/m3) for all samples. The mean GEM and total mercury (THg) profiles are in the middle of the range of other published profiles. The THg profile showed no gradient to 5.5 km (p = 0.12, r2 = 0.009).