Measurement of NO₂ by the photolysis conversion technique during the Transport and Chemical Evolution Over the Pacific (TRACE-P) campaign

An airborne instrument for in situ measurements of tropospheric nitrogen dioxide (NO₂) was developed using the photolytic conversion technique followed by chemiluminescence detection of NO. This instrument was used for the measurements of NO₂ on board the NASA P-3B aircraft during the Transport and Chemical Evolution Over the Pacific (TRACE-P) campaign. Comparison in the laboratory indicated less than 10% difference between our NO₂ instrument and two independent laser-induced fluorescence instruments in the NO₂ range of 30 parts per trillion by volume (pptv) to 50 ppbv. The magnitudes of potential errors in airborne tropospheric NO₂ measurements were further assessed using the TRACE-P data set. The systematic errors estimated for the median NO₂ mixing ratios, 70 pptv at 0-2 km (30 pptv at 2 8 km), were 19% (39%). The random errors for a 10 s integration time were estimated to be 5-10%, depending on altitude. The observed NO₂ mixing ratios were compared to those calculated by a photochemical box model. Overall, the calculated NO₂ values correlated very well with those observed (r² = 0.97), although the calculations were systematically higher than the observations by about 30%, except for the highest flight levels. The calculated/ observed NO₂ ratio remained nearly constant, having values close to 1.3 at 0-4 km, and decreased with altitude. The difference between the observed and model-calculated values, however, was within the combined uncertainty in the measurement and model calculation. The underlying causes for this difference are to be determined in future studies.