Measurements of nitrogen oxides, ozone, and carbon monoxide during the FIELDVOC'94 campaign in Tábua

Measurements of NO,NO_x,NO_y,CO,O₃,J_NO2 (the photolysis frequency of NO₂) and of meteorological parameters made during the FIELDVOC'94 campaign in Tábua, Portugal, are described. The situation at the site was dominated by a land–sea breeze system coupled with thermal up-slope on the mountain ridge, which caused northwesterly flow from the coastal area during daytime and southeasterly flow at night. O₃ up to 100 ppbv was observed in photochemically aged air masses that reached the site in the afternoon from the coastal areas. At night, O₃ mixing ratios at the surface dropped below 10 ppbv. NO_x mixing ratios varied between 200 pptv during daytime and 3 ppbv at night with peaks up to 6 ppbv. NO_y (the sum of NO_x and its oxidation products) varied between 1 and 6 ppbv and CO between 100 and 280 ppbv. Simultaneous measurements of NO₂ by photolytic conversion/NO-chemiluminescence and by tunable diode laser spectroscopy showed a tight correlation, in particular at night. Taking the identified difference in calibration between the two methods (8%) into account, the disagreement in the atmosphere, was ⩽2%, on average. Photochemical ozone formation in air transported from the coast was identified by a positive correlation between O₃ and the oxidation products of NO_x referenced as NO_z. The slope of 16 represents an upper limit to the efficiency of NO_x in catalyzing ozone formation in air. The relatively high ozone formation efficiency is supported by the large O₃/CO ratio of about 0.4. The correlation between CO and NO_y indicates an upper limit for the CO/NO_x emission ratio of 28.