CAPS-Based Nitrogen Dioxide and Particle Extinction Monitors
Aerodyne offers several state-of-the-art optical monitors based on the cavity attenuated phase shift (CAPS) technique. Similar in nature to cavity ringdown (CRD) detection, the method relies on producing very long optical paths (up to 2 km) using very high reflectivity mirrors in a sampling cell that is less than 30 cm in length. However, unlike most CRD-based instruments, these monitors utilize a light emitting diode (LED) as an optical source instead of a laser and measure the resultant signal in the frequency domain using a phase shift measurement. As a result, these monitors can be offered at a very favorable price compared to CRD-based instruments without any loss of performance.
The CAPS NO2 monitor provides a direct absorption measurement of nitrogen dioxide at 450 nm in the blue region of the electromagnetic spectrum. Unlike standard chemiluminescence-based monitors, these instruments require no conversion of NO2 to another species and thus are not sensitive to other nitro-containing species. Levels of detection (3σ noise levels) of less than 100 parts per trillion with a 10 second sampling period can readily be attained; a fast response version which offers 1 second time response at reduced levels of detection (< 2ppb) is also available. These instruments offer completely linear response at concentrations up to several parts-per-million of NO2; this limitation at high concentrations is purely one of optical opacity. This monitor can be rack mounted and requires only a source of NO2-free air for periodic (minutes to hours) baseline measurements. The standard gas flow is 0.85 lpm but lower flow rates with reduced time response can be chosen without loss of sensitivity. Twelve months of on-board data storage (at a 10 second sampling rate) is standard.
The CAPS PMex monitor provides a measurement of the optical extinction (the sum of scattering and absorption) of an ambient sample of particles. Currently, there is a choice of 3 different wavelengths - red (630 nm), green (530 nm) and blue (450 nm) - which match the spectral bands of most other particle optical properties measurement equipment. These instruments have a 1 second time response and provide a level of detection less than 2 Mm-1 with 1 second integration time. An ultraviolet version (365 nm, albeit with as yet undetermined but probably reduced sensitivity, ) will be available shortly. These monitors are entirely self-contained, requiring no consumables such as zero air and can be operated autonomously for 6 months using on-board data storage if one second averaging is chosen.
CAPS PMssa Monitor (under development)
The CAPS PMssa monitor provides simultaneous measurement of both particle-based total optical extinction and scattering, producing a direct measurement of the particle single scattering albedo (SSA), the ratio of scattering to total extinction. Based on the CAPS PMex particle extinction monitor, this instrument incorporates an integrating sphere within the optical path, allowing for an accurate and precise measurement of particle scattering with minimal truncation effects compared to commercial nephelometers. Specifications for this instrument are identical to that of the CAPS PMex monitors.