Notes: 1. The area considered is that from the Mississippi River east to the Atlantic Coast and Nova Scotia (60E), north of central Alabama (33N) and south of 50N (Winnipeg to the Gulf of St. Lawrence).

2. Canadian emissions for 1970 are given in Rennie and Halstead (1973): these were dominated by the emissions from the Sudbury smelters, which have probably been relatively constant during the period considered (Beamish and Harvey 1972, Balsillie, personal communication).

3. U.S. emissions were estimated by interpolating from the data for 1950, 1960, and 1970 in EPA (1973), assuming that 80 of power plant emissions and 50 of other emissions were in the area considered (cf. 13–10 and Figure 6–2).

4. Natural emissions were estimated by scaling the global estimate of ref. 1 in proportion to the area under consideration (about 2 percent of the earth’s land surface) (cf. Rodhe 1972).

5. An increase of 3 percent per year is assumed for the period 1965–1972: this is consistent with Table 7–1 and other data quoted in the text, and also with 6–9b.