Transport mechanisms for pesticides in the atmosphere

It is fairly certain that large-scale transport of pesticides such as DDT occurs primarily via the atmosphere. Atmospheric transport is therefore of basic importance for the regional and global distribution of these trace substances. Global variability of constituents within the troposphere is strongly correlated to tropospheric residence time. If this is Ionger than about half a year the constituent shows a fairly uniform distribution, so that the details of the atmospheric transport are unimportant. The parameters which determine the residence time of pesticides and PCBs are discussed, and it is shown that at the present time we can not be certain about its magnitude even for DDT. If the residence time is similar to that of aerosols, i.e. about a week, atmospheric transport and removal processes become very important. Some of them are briefly discussed. INTRODUCI'ION At this Congress, which is devoted to the role of pesticides in our environment, we are concerned, among other things, with the possibilities for nature to cope with the variety of substances introduced by man into bis 'Lebensraum' and with the resulting large-scale-scale and long-term distribution and its variation of these substances in soil, water and atmosphere. Because of the toxicity of some of these substances as well as their chemical stability, it has become apparent that some of the pesticides have in certain areas reached dangeraus Ievels for the biosphere. This serious threat to life has led to intense interdisciplinary research to Iook into the details of chemical and biological degradation of these substances in nature and their fate on a locaL regional and global basis. With respect to the topic of my paper, two important facts have so far emerged from these studies with a high degree of certainty. First, some ofthese substances have reached the biosphere ranging from pole to pole and indicating the degree of global dissemination, as for instance demonstrated by Figure 1. Interpreting this figure we have to keep two things in mind: the occurrence of DDT and other substances in certain tissues of animals (and particularly man) is no direct and accurate measure of the generat occurrence in the environment because of strong local influences in ·areas of application on both hemispheres and because of a large variety of biological amplification factors due to the peculiarities of the food chain, etc. The best 95 C.E.JUNGE indicators for large-scale distribution are likely to be the birds, and considering only these data in Figure 1 we see that even in Antarctica DDT and PCB are abundant (see also ref. 2). The same is true for the Arctic regions of the North Atlantic3, where considerable amounts of PCB and DDT were found in birds more than 250 miles north of the mainland and in species which never migrate far south in areas where the sea water is affected by Europe. I would have preferred for our discussion instead of Figure 1 a similar one for concentrations of these substances in air or rainwater, but unfortunately such data are still very scanty or unreliable at the present time. The second important fact for our discussion isthat at least for DDT most of the material is transported through the atmosphere (and not via rivers) into the oceans, which most probably act as the huge and final sink area Apart 100.0 1000.0 <> [ <>s a. 10.0 ~ 1oo.o'E $" ~ -" u -o :I ~ ~ .... "' c 3 0 -o ~ ~ .0 1- 0 0 :I 0 I 1.0 s~ 10.0 =g - 0 cn I E :I () ~ Q1 1.0 e ~ ih = ~ %5 I - Strontium-90(mCi/km2) o :EDDT in dietary meat,fish,poultry,eggs OIDDT in fish,birds(incl.skuo),eggs c. :EDDTin dietary milk products (fai,li~~er, can::ass) (fai basis) ~:EDDTin ather wildlife OI:DDTin human milk(fat basis) c::o PCBin fish,birds,eggs 11 I:DDT in crude fish oil - PCB in other wildli~ II I: D DT in crude vegetable oil o I:DDTin soil,grass,mud (lake,river) ~I:DDT in human adipose tissue A I:DDTin rivers(aU<Q001p.p.m.) •:EDDTin seVIIOge eftlueni • :EDDT in air (ng/m3 ) ~Muliiple poinis Residue values of 0.001 p.p.mplolted at Q001p.p.m.,as are non-deteded values Figure 1. Global distribution of chlorinated hydrocarbons and other trace substances in the biosphere, according to Appleby1 96 E 01 c üc 0 u L. ·c; 1- 0 0 • TRANSPORT MECHANISMS FüR PESTICIDES IN THE ATMOSPHERE from the fact that direct injection into the air by spray application is very common, any DDT in soil or deposited on plants is shown to disappear primarily by vaporization4 • 5. Typical half-life times in soil seem to be 5 years, varying ~omewhat with soil type and other conditions, such as total content and degree of depletion. These two facts indicate that transpoft processes by and through the atmosphere are of basic importance for our understanding of regional and global distribution problems. If we consider the large-scale conditions for transpoft and distribution of trace substances in and by the atmosphere, it soon becomes clear that they are determined in an important way by two things, namely by the chemical characteristics of the substance itself as weil as by the meteorological transport, mixing and other processes in the atmosphere. The chemical properties of trace substances introduced by man (not only pesticides) range from those of almost noble gases to those of reactive water-soluble substances with or without any vapour pressure which can only be present in the form of aerosols. In the subsequent sections we shall point out the importance of both these factors. THE ROLE OF RESIDENCE TIME FOR THE GLOBAL TRANSPORT AND DISTRIBUTION OF A TRACE CONSTITUENT For the cycle and behaviour of pollutants injected by man we can restriet our discussion tothat part ofthe atmosphere which is bounded by the tropopause in 8-16 km altitude and by the earth surface. It is known as the region of active weather and is called the troposphere. All trace substances present in the troposphere have certain average global residence or lifetime (we prefer the expression residence time) which is determined by the efficiency of their removal by rainout, washout, chemical destruction and such processes at the earth surface as absorption, impaction, sedimentation, etc. If this efficiency is low, the residence is long,-and vice versa. The magnitude of the residence time of a constituent is very important with respect to its global atmospheric transport and distribution. If we assume the injection rate of a trace constituent into the atmosphere to be balanced by its removal rate on a global basis, i.e. ifwe assume steady state conditions, the average variability throughout the troposphere given, for instance, by the standard deviation a is approximately inversely related to the residence time T, as demonstrated in Figure 2 (in which all relevant data on atmospheric trace constituents so far available are compiled). Based on these results it can be estimated that a i~ smaller than about 0.3 if T is Ionger than about half a year, which is approximately the case for carbon monoxide. This is an important realization. It means that a constituent does not on the a verage vary much more than by a factor of about 2 if its residence time is Ionger than about half a year. This does, of course, not exclude the fact that concentrations are much higher near sources, but we are here primarily concerned with the large-scale distributions. If, therefore, we know that a constituent has a T of more than about half a year, we need not be concerned about the atmospheric transport processes at all if we want to know whether it affects remote areas. It passes even the equatorial region sufficiently fast to be present in both hemispheres at about the same Ievel if the sources are preferentially located in one hemi97 PAC-42/1-2/E C.E.JUNGE 101 'i:H20 Rn , 100 ~ c 0 CH4 .Q .2 ii; 10-1 ~ R N20 "'0 10-2 Model/ 2' 'E ~ c 10-3 CO~ ' "'0 , ..... r· a = 0.14 c .E 1/l 10-4 ' He ..... ~ Cl ' ' I I .9 ~ ...... 02 I I 10-5 I 1\. ' '' 10-6 ' 10-J 10-l 10-1 ' 10u ld 10l 10J 104 105 lOb 10' 108 Residence time, years Figure 2. Log standard deviation from the average tropospheric mixing ratio of a trace gas as function of residence time, according to a compilation of all available data6• The size of the boxes gives the uncertainty ofthe data For helium the measuring method is not accurate enough to obtain the real variations; for oxygen the technique applied is on the Iimit. The dashed Iine indicates a representative line through the data. The solid line represents calculations for a simple atmospheric model. sphere. If, on the other band, a constituent has a T shorter than half a year, the tropospheric transport mechanisms become more and more important for our understanding of its distribution within the troposphere. In this case the average concentration will vary considerably with the geographical location and climatological condition and will decrease considerably with increasing distance from the source area. THE FACI'ORS THAT DETERMINE THE RESIDENCE TIME OF A TRACE CONSTITUENT The question which arises at this point is what determines T and what do we know about T for pesticides and related pollutants? We can generally say that T is to a large extent determined by the chemical properties of the constituent because they control the efficiency with which the various cleansing mechanisms of the atmosphere work. This efficiency is, for instance, very low in the case of a stable and inert gas which does not absorb at the land surface and at the aerosol particles in the air and is not or only slightly soluble in the ocean water. In this case it has no choice other than to stay or accumulate in the atmosphere. Noblegases ofthe atmosphere are the best e