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Measurements of total peroxy radicals (HO2 + RO2) and nitrate radical (NO3) were made on the NOAA research vessel R/V Brown along the U.S. Gulf Coast during the TexAQS 2006 field campaign. The measurements were modelled using a constrained box-model based upon the Master Chemical Mechanism (MCM). The agreement between modelled and measured HO2 + RO2 was typically within ∼40% and, in the unpolluted regions, within 30%. The analysis of the model results suggests that the MCM might underestimate the concentrations of some acyl peroxy radicals and other small peroxy radicals. The model underestimated the measurements of NO3 by 60–70%, possibly because of rapid heterogeneous uptake of N2O5. The MCM model results were used to estimate the composition of the peroxy radical pool and to quantify the role of DMS, isoprene and alkenes in the formation of RO2 in the different regions. The measurements of HO2 + RO2 and NO3 were also used to calculate the gas-phase budget of NO3 and quantify the importance of organic peroxy radicals as NO3 sinks. RO2 accounted, on average, for 12–28% of the total gas-phase NO3 losses in the unpolluted regions and for 1–2% of the total gas-phase NO3 losses in the polluted regions.

Commercially available ultraviolet light-emitting diodes (UV-LEDs) are evaluated in the laboratory as a light source for photolysis of atmospheric nitrogen dioxide (NO2) followed by chemiluminescence detection (P-CL) of the resulting nitric oxide (NO). Sensitivity, selectivity, and engineering simplicity of three UV-LED sources are compared. The most powerful source uses two 9-W Nichia LED modules and provides an NO2 photolysis frequency (j) of 8.4 s−1 corresponding to a nine-fold improvement over a 100-W Hg arc lamp; this source provides the most sensitivity, but requires water cooling. A pair of Hamamatsu 0.25-W LED modules provides intermediate sensitivity with photolysis efficiency comparable to a 100-W Hg arc lamp. The Hamamatsu LEDs require no additional cooling and are a simple and inexpensive package, providing sensitivity and stability appropriate for long-term, unattended measurements at remote surface sites. A Droplet Measurement Technologies Blue-Light Converter (BLC) represents the most complete off-the-shelf system available for atmospheric measurements of NO2. The BLC provides the least sensitivity of the LED-based converters, but is less subject to interference from HONO. Implementing the Nichia LEDs into the NOAA WP-3D aircraft P-CL instrument, in conjunction with improved sample gas handling, improves instrument time response by a factor of eight and permits 1-Hz retrieval of ambient NO and NO2 using a single detector simply by modulating the LEDs on and off. The performance and stability of the Nichia LEDs are further assessed during the CalNex (California Research at the Nexus of Air Quality and Climate Change) field study in May and June of 2010.