Airborne measurements of the photolysis frequency of NO₂

Abstract

A set of photoelectric detectors for airborne measurements of the photolysis frequency of NO2, i.e., JNO2, was developed and integrated aboard the research aircraft Hercules C‐130 operated by the U.K. Meteorological Office. The instrument consists of two separate sensors, each of which provides an isotropic response over a solid angle of 2π steradian (sr). The sensors are mounted on top and below the aircraft, respectively, to obtain a field of view of 4π sr, and permit the discrimination of the upwelling and downwelling components of the actinic flux. From experimental tests and model calculations it is demonstrated that small differences between the spectral sensitivity of the sensors and the spectral response of JNO2 can lead to significant errors in the determination of JNO2, especially under cloudy conditions. We present correction factors for clear sky conditions and suggest the use of a new filter combination in the sensors which requires only small corrections and provides acceptable accuracy, even under cloudy conditions. A climatology of JNO2 values is presented from a series of flights made in 1993 at latitudes of 36°–59°N. For clear sky conditions and solar zenith angles of 33°–35°, JNO2 was 8.3 × 10−3 s−1 at sea level and increased with altitude to values of 13 × 10−3 s−1 at 7.5 km altitude. Above clouds, JNO2 reached maximum values of 24 × 10−3 s−1, and peak values of 29 × 10−3 s−1 were observed for very short periods in the uppermost layers of clouds. Enhancement of the actinic flux due to light scattered from clouds was also observed at altitudes below 0.5 km. Comparison of the clear sky data with predictions from different radiative transfer models reveals the best agreement for models of higher angular resolution. The Delta Eddington method underpredicts the measurements significantly, whereas the JNO2 values predicted by the discrete ordinate method and multidirectional model are only about 5% smaller than our measurements, a difference that is within the experimental uncertainties.

Upgrading photolysis in the p-TOMCAT CTM: model evaluation and assessment of the role of clouds

“…An issue which should also be considered is the quality of the measurements. According to Volz-Thomas et al (1996), the estimated accuracy of the J (NO 2 ) measurements is 8%, including cases of cloudy conditions, low sun, or when the aircraft is inclined during turns. The authors do not assess J (O 1 D) measurements, which are known to involve larger uncertainties (Bohn et al, 2008).…”

Ozone photochemistry in boreal biomass burning plumes

“…The statistical uncertainty in BC mass concentration was 5 % for an average time of 15 s, and accuracy was within 20 % due to possible differences between the instrumental response to biomass burning BC and the calibrant (Laborde et al, 2012). Measurement of NO 2 photolysis rates were made using upper and lower fixed bandwidth radiometers at wavelengths predominantly between 208 and 500 nm, at a frequency of 1 Hz and with accuracy of 10 % (Volz- Thomas et al, 1996). Measurements of VOCs used in this study were made using a dual channel gas chromatograph system with flame ionisation detection (GC-FID) (Hopkins et al, 2003(Hopkins et al, , 2011 to analyse whole air canister samples taken during each flight.…”

Photolysis frequency measurement techniques: results of a comparison within the ACCENT project

“…The instrumental setup and properties of j (NO 2 )-FR and j (O 1 D)-FR were described in detail by Volz-Thomas et al (1996) and Junkermann et al (1989), respectively. Briefly 30 mm diameter quartz receivers with 140 mm diameter horizontal shadow rings were used for 2 π sr radiation collection.…”

“…Consequently, filter radiometers were designed to specifically measure the photolysis frequencies j (NO 2 ) (Reaction 3) or j (O 1 D) (Reaction 5) (Junkermann et al, 1989;Volz-Thomas et al, 1996). The main advantage of filter radiometers is that the instruments are light-weight and easy to handle making them ideal for routine measurements with high time resolution (1 s).…”