Field measurements of the surface – atmosphere exchange of SO₂ and NH₃ have been made using two very different micrometeorological methods, a long term averaging method based on flux-gradient principles and an eddy covariance method, using a tunable diode laser fast response gas analysis system. The field data are then applied in surface-atmosphere models to calculate deposition of pollutant gases at the regional scale. The Time Averaged Gradient system (TAG) has been developed following a prototype implemented during a pilot study in 1998. The main goal of the TAG is to provide direct long-term average (1 to 4 weeks) flux-gradient measurements for a range of trace gas species, between atmosphere and terrestrial surfaces. Over daily periods, atmospheric conditions can range from high stability, where the vertical gradients of ambient concentration are enhanced due to very small diffusivity, to highly unstable conditions, in which concentration gradients are small due to the intense turbulent activity of the surface layer. By sampling continuously over a long-term period, the large vertical gradients generated by high stability would lead to an over-estimate of the actual flux; therefore it is necessary to avoid the bias due to these processes. To overcome this problem, the TAG system operates conditionally, sampling the micrometeorological variables within a carefully defined range of stability. A data series of five years, from 1999 to 2003, for SO₂ and a data series of two years, from 2001 to 2003 for NH₃ has been obtained at Auchencorth Moss, a field site in Southern Scotland. The measurements provided a characterisation of the chemical climate, meteorology, turbulent characteristics, as well as deposition-emission rates at the field site. A comparison with a continuous flux-gradient system running in parallel on the same field site allowed the reliability of the TAG system to be quantified: the correlation coefficients for u_* and H show a very good agreement (above 90% in both cases) between TAG protocol and continuous system suggesting the removal of stable conditions from the sampling period doesn’t modify the evaluation of the turbulent fluxes, although it introduces a bias. The SO₂ fluxes calculated omitting stable conditions under-estimate the fluxes of SO₂ measured by the continuous system by providing fluxes that are the 77% of the values estimated with the continuous system. A second TAG system has been implemented to improve estimates of gradients and reduce uncertainty on the fluxes and to increase the data coverage. It has been tested on the Easter Bush field site (Southern Scotland) measuring NH₃ fluxes form autumn 2001 to spring 2003. Field measurements of NH₃ fluxes using an eddy covariance technique were made for a total of 60 days between July and October 2002 at intensively managed grassland in Southern Scotland. The collected data demonstrate the suitability of a Tuneable Diode Laser Absorption Spectroscopy (TDLAS) system coupled with a sonic anemometer for eddy covariance measurements.