Seasonal and diel patterns of total gaseous mercury concentration in the atmosphere of the Central Valley of Costa Rica

Abstract

Abstract Monitoring of Hg in the atmosphere near volcanoes is limited with no previous data for Costa Rica. Seasonal and daily patterns of total gaseous mercury (TGM) were observed at the main sampling location at the Universidad Nacional, Heredia, Costa Rica. The area (lat. 10.000230 long. −84.109499) is located in the Central Valley of Costa Rica and is 27 km SE of the Poas volcano (lat. 10.199486 long. −84.231388). Measurements were made from May 2008 to May 2009 at this location with some additional values obtained at other sites near the Poas volcano including San Luis and Grecia as well as near, Turrialba and Irazu volcanoes. Total gaseous Hg was determined in samples collected at a height of 2 m using the Tekran 2537A (Tekran Inc.) gas-phase Hg vapor analyzer. Meteorological data (temperature, relative humidity, wind speed, wind direction, radiation and precipitation) were obtained from the airport weather station located at Alajuela. Monthly precipitation is typically 85 mm during the dry season (December to April) with winds from the west. The wet season begins in late April and continues to December with monthly rainfall of 328 mm and winds from the NE. The annual mean temperature is 20 °C. With the onset of the wet season TGM increased from typical values near 10 to 905 ng m −3 . Measurements made within 5 km of the Poas volcano were higher than at Heredia at that time. Diel values measured at the university site increased until midday along with temperature and radiation. Relative humidity showed a reciprocal pattern. It was found that high values of TGM were not related to wind velocity or direction. The strong diel pattern increased with sunrise, peaked at midday and was lowest during the night time. It would seem that Hg 0 from the volcano is oxidized and is deposited to the soils during the dry season when winds are blowing from the volcano. With the onset of heavy rains in April, Hg in the soil is reduced and re-volatilized resulting in the high levels in the atmosphere. Values at other volcano sites were provided. The role of atmospheric pollutants such as H 2 O 2 and O 3 should be included in future studies as they may result in oxidation of reduced Hg. The instability in the air masses may also be a factor and local pollution sources may result in high levels that is emitted from the volcanoes of Hg being circulated to ground level as radiation intensity increases.