Data on concentrations of base cations in air and precipitation are important in calculating the net deposited acidity to ecosystems. Measurements of many trace metals in air have been made at three rural sites in the United Kingdom over the past 20 years. Measurements of the concentrations of the base cations, Ca2+, Mg2+, K+, and Na+ have also been made but not routinely reported nor considered. Some data were missing for Ca2+ for the period between 1985 and 1989, and a large number of less than values were present. Despite these uncertainties, these data represent the only long-term record of their kind in the United Kingdom. An examination of the data revealed that concentrations of non-sea-salt Ca2+ in air at these three sites have declined over the past 20 years by between approximately 40 and 60%. Concentrations of Mg2+ and K+ have also declined, and Ca2+, Mg2+, and K+ all show greater variability in earlier years. Concentrations of Na+ in air showed no detectable long-term trend. These declines of base cations in air are similar to those observed in precipitation elsewhere in North America and mainland Europe, and these trends are similar to declines in emissions of sulphur dioxide. The estimated emissions of calcium from particles deriving from fossil fuel combustion was 21 kt in 1991 and 67 kt in 1970, a decline of 67%. Present-day rates of non-sea-salt Ca2+ deposition to the United Kingdom from wet and dry deposition are estimated to be 86 and 27 kt yr-1, respectively. It is clear that these calculated emissions of Ca from fossil fuel combustion alone cannot adequately explain the estimated deposition nor the decline of concentrations in air. The observed decline of scandium, a crustal element, provides good evidence that the decline in industrial particle emissions partly explains the decline in base cations in air. It is necessary to consider the possible implications of the trends revealed in this work as the declines in concentrations of base cations in air, and thus deposition may offset some of the benefits resulting from emissions control of acidic gases, if acid-sensitive receptors are to show stabilization or recovery. ¿ 1998 American Geophysical Union