Four pot experiments are reported in which Norway spruce (Picea abies (L.) Karst) seedlings, of different nutrient status, were treated with acid mist for one growing season in open-top chambers (OTCs). Combinations of H+, SO42−, NH4+ and NO3− were applied at different frequencies of application and supplying different doses of S and N kg ha−1. Plant growth, visible injury, frost hardiness and nutrient status were observed. These experiments were undertaken to improve our understanding of the interaction of environmental factors such as nutrition and mist-exposure frequency on seedling response to N and S deposition.

Both acidity ([les ]pH 2·7) and SO42− ions were necessary to induce visible injury. Mist containing SO42−, H+ and to a lesser extent NH4+ significantly reduced winter frost hardiness. Increasing the misting frequency, and to a lesser extent the overall dose, increased the likelihood of acid mist causing visible injury and reducing frost hardiness. Post-planting stress, low N status and needle juvenility increased the likelihood of acid mist causing visible injury. Increased plant vitality, adequate N status and growth rate reduced the likelihood of acid-mist-induced reductions in frost hardiness.

Principles underlying the responses of spruce seedlings treated in controlled conditions to acid mist are discussed.

From an examination of the outline requirements given by Mr. Davies, it is apparent that pressure measuring techniques can be used in all stages of a flight although it must be acknowledged that some of the tolerances he has called for cannot be met. The advantage of the pressure technique is that it uses the Earth's atmosphere which is experienced by all airborne vehicles and so provides a common datum for height or altitude measurement. It should be noted that wrong impressions can be got from the present pattern of accepting altitude or height in feet as a fixed physical measurement. A pressure sensor measures the pressure and, as the millibar is a widely accepted unit of pressure in aeronautics, it would really be more sensible to express aircraft altitude and separation values in millibars.