Further development of a new flow-through directional passive air sampler for monitoring ambient nitrogen dioxide

Abstract

We have previously introduced a new type of rotatable flow-through directional passive air sampler (DPAS) for monitoring trace air pollutants in ambient air. In wind tunnel tests, the sampler turns into the prevailing wind direction and retains NO₂ (used as a test pollutant) on an internal sampling medium ring of triethanolamine (TEA)-coated meshes to indicate the source of pollution. However, these meshes can become saturated, after exposure times of tens of hours or a few days, due to the relatively small masses of TEA which can be coated onto them. This paper outlines the saturation problem and presents a possible redesign of the DPAS sampling approach, to allow longer-term (weeks–months) deployments, where air passes over larger volumes of TEA retained in a compartmentalised carousel. Investigations varying the volume, depth and mixing of TEA in sampling compartments suggested that the limiting step of NO₂ uptake was its rate of supply from the atmosphere. In wind tunnel trials, NO₂ uptake into TEA continued linearly in response to a stable air concentration over periods of tens of days, showing no signs of saturation. Uptake was wind velocity dependent across the range of 0.50, 2.00, 5.00 and 8.00 m s⁻¹. Results indicate that the total sampling time and storage capacity of the TEA for NO₂ can be varied to meet deployment time requirements, indicating that long-term, cheap, directional passive air sampling is achievable.