The effect of “moss bag” shape in the air monitoring of metal(oid)s in semi-arid sites: Influence of wind speed and moss porosity

Abstract

The moss bag technique has been widely used in Europe for the monitoring of metal(oid)s in the air. However, the effect of moss bag shape and moss porosity in the capture and retention of metal(oid)s linked to particulate matter (PM) has not been studied in detail. The objective of the study was to compare the capture of metal(oid)s in air between two shapes of moss bag, rounded bag (traditional shape) and semi-sphere bag. The factors and methods considered were i) wind speed and porosity characteristics of the medium (moss-matrix), evaluated by modeling and simulations in 3D with the COMSOL™ platform, and ii) net accumulations (NE) of Cu, Fe, Al, Zn, Sb, Ba and Pb with the moss bag technique. Exposure period was 30 days (August to November) in two districts of Santiago de Chile (Las Condes and Ñuñoa). Results revealed that i) semi-sphere bag had a greater accumulation by sedimentation of PM (>2.5 μm), mainly in its upper zone, with the most effective (average) porosity range for the capture and retention of PM being 0.45 to 0.7, and ii) the semi-sphere moss bag had higher NE for Al, Zn, Sb and Pb and lower coefficients of variation for Fe, Al, Pb, Ba and Cu. Our results show that in high wind speeds events, the shape of the moss bag can determine PM retention.