2 Results and discussion for regulated organic contaminants
2.1 Long-term monitoring in air
2.1.3 Dichlorodiphenyltrichloroethane (DDTs)
DDTs are intentionally produced chemicals that have been used worldwide as a pesticide to protect humans and agricultural crops from vector-borne diseases. The production and use of DDTs were banned in Europe, the United States and Canada during 1970s to 2000 and is regulated by the Aarhus protocol on POPs (UN/ECE, 1998b) and the Stockholm Convention on POPs (Stockholm Convention, 2007). It is still in use in some parts of the world for disease vector control (primarily malaria). The Stockholm Convention allows the production of DDT for use in disease vector control and as an intermediate in the production of dicofol, although the latter use is anticipated to cease globally in the near future due to the inclusion of dicofol in the Stockholm Convention in May 2019. Furthermore, the World Health Organization (WHO) recommends indoor residual spraying with DDT as one of three
primary means of malaria control, the others being use of insecticide treated bednets and prompt treatment of confirmed cases with artemisinin-based combination therapies (WHO, 2006). The Conference of the Parties to the Stockholm Convention on POPs evaluates the continued need for DDT for disease vector control approximately every second year in consultation with WHO.
The six DDT congeners; o,p’- and p,p’- DDT, DDD, and DDE, have been monitored at Zeppelin since 1994, and at Birkenes and Andøya since 2010. In 2019, monitoring of DDTs at Zeppelin continued with weekly samples and the monitoring at Birkenes consisted of one sample per month, as the last years.
The DDTs in air are sampled on filter and polyurethane foam (PUF) plugs and thus the concentrations of DDTs in air in this monitoring programme represent the bulk phase (i.e. gas+particle phase). The detection frequencies in 2019 varied among the individual congeners and between the sites. For example, o,p’- and p,p’-DDT, and o,p’- and p,p’-DDE were detected in most samples at both sites. Low detection frequencies (i.e. more than 50% of the samples <LOD) were observed for p,p’-and o,p’-DDD at Zeppelin and o,p’-DDD at Birkenes. This suggests low concentrations of these DDT-congeners at the Zeppelin and Birkenes. Instead, p,p’-DDE was the most abundant congener followed by o,p’-DDT at the two sites. The weekly concentrations of sum DDTs at Zeppelin in 2019 ranged between 0.04-1.1 pg/m3. The monthly concentrations of sum DDTs at Birkenes ranged between 0.56-13 pg/m3 (including one extreme measurement in November and one high measurement in April). All DDT congeners were 5-10 times higher during the measurement in November 2019 than the other months. The reason for this is unknown. The analytical quality parameters for this sample do not suggest any analytical anomalities.
Figure 10: Annual mean concentrations of sum DDTs (pg/m3) in air. 2007 at Zeppelin is excluded as it is an unexplained high outlier. The annual mean includes all six congeners although some congeners are <LOD in most samples at some sites and for some years.
The annual mean concentrations of sum DDTs and the individual congeners in 2019 were as in previous years higher at Birkenes (3.0 pg/m3) than at Zeppelin (0.3 pg/m3) (Figure 10). The reason for higher concentrations at Birkenes compared to the more northern Norwegian sites may be explained by closer distances to possible emission sources (secondary repositories) and is also seen by the spatial distribution of DDTs in annual monitoring programmes and scientific case-studies within the EMEP region (Aas, 2019, Halse et al. 2011). Although the concentrations observed at Birkenes are higher than at Zeppelin, they are still one to two orders of magnitude lower than the concentrations found on the European continent (Pribylova et al., 2012, Aas et al., 2018). The annual mean concentrations of sum DDTs and all congeners at Zeppelin were the same as the last two years (2017-2018), which are also the lowest concentrations measured at Zeppelin (Figure 10). At Birkenes, instead, the annual mean concentration in 2019 was the highest ever observed. This is biased by the high monthly measurement
in November and April. This shows the limitation of having one sample per month as one individual high value (out of twelve) has a high influence on the annual concentration. Whether the high annual concentration is an outlier or not will be evaluated over time.The annual concentrations observed at Birkenes and Zeppelin during the last years suggest that the concentrations of DDTs in air are in slow decline or have reached a temporal remote state, where reduction rates are controlled by degradation in secondary repositories (Stroebe et al., 2004). The indicator ratio (p,p’-DDE+p,p’-DDD/p,p’-DDT) were high (3-12) at both sites in winter, spring and autumn indicating input only from aged DDT. The ratios were lower in summertime at Birkenes (2).
Figure 11: Seasonal variability of sum DDT and the four detected individual congeners at Birkenes and Zeppelin in 2019.
A strong seasonality of the DDT concentrations was observed at Zeppelin with five to ten times higher concentrations in wintertime (October-April) compared to warmer months (May-September) (Figure 11). This seasonality was seen for sum DDTs as well as o,p’- and p,p’-DDE and DDT, but not for DDD.
No such seasonality was observed at Birkenes where DDT only is measured in only one sample per month. The higher concentrations in winter at Zeppelin can be connected to the Arctic Haze season in the Arctic area during winter time in which the transportation of particles to the Arctic is higher and
removal rates of the DDTs are lower than in summer time (Hung et al., 2016). However, this may not be the full explanation as DDTs tend to be found to larger extent in gas-phase than in particle phase.
The lower temperature in winter may however shift the partitioning towards more particle bound DDTs which supports the explanation of the Arctic haze.