Factors influencing the large-scale distribution of Hg° in the Mexico City area and over the North Pacific

[摘要] Gas-phase elemental mercury (Hg°) was measured aboard the NASA DC-8aircraft during the Intercontinental Chemical Transport Experiment Phase B(INTEX-B) campaign in spring 2006. Flights were conducted around Mexico Cityand on two subsequent deployments over the North Pacific based out ofHonolulu, Hawaii and Anchorage, Alaska. Data obtained from 0.15–12 kmaltitude showed that Hg° exhibited a relatively constant verticalprofile centered around 100 ppqv. Highly concentrated pollution plumesemanating from the Mexico City urban agglomeration revealed that mixingratios of Hg° as large as 500 ppqv were related to combustion tracerssuch as CO, but not SO₂ which is presumably released locally from coalburning, refineries, and volcanoes. Our analysis of Mexico City plumesindicated that widespread multi-source urban/industrial emissions may have amore important influence on Hg° than specific point sources. Over thePacific, correlations with CO, CO₂, CH₄, and C₂Cl₄ werediffuse overall, but recognizable on flights out of Anchorage and Honolulu.In distinct plumes originating from the Asian continent the Hg°- COrelationship yielded an average value of ~0.56 ppqv/ppbv, in goodagreement with previous findings. A prominent feature of the INTEX-B datasetwas frequent total depletion of Hg° in the upper troposphere whenstratospherically influenced air was encountered. Ozone data obtained withthe differential absorption lidar (DIAL) showed that the stratosphericimpact on the tropospheric column was a common and pervasive feature on allflights out of Honolulu and Anchorage. We propose that this is likely amajor factor driving large-scale seasonality in Hg° mixing ratios,especially at mid-latitudes, and an important process that should beincorporated into global chemical transport models.

[学科分类] 大气科学