Calibration and tests of the filter-collection method for measuring clean-air, ambient levels of nitric acid

Abstract

A portable permeation tube based system has been developed for the field calibration of nitric acid vapor collection on Nylon filters during actual atmospheric sampling. Extensive testing has been undertaken to determine the effects of: transfer line materials, the uptake of nitric acid on the filter holder apparatus, the linearity of dilution of the nitric acid effusing from the permeation tube source achievable and the effects of the permeation of gases, other than nitric acid, on the permeation tube calibration. Data demonstrating significant uptake of HNO 3 on commonly used materials are given, together with specific recommendations on the use of materials to avoid such problems. Nitrogen dioxide is shown to be a significant contaminant of the HNO 3 permeation tubes used requiring corrections of the order of 15%. Nitric acid “spiking” during actual field sampling of clean tropospheric air at a remote site in the Rocky Mountains west of Boulder, Colorado demonstrated agreement between the measured Nylon filter nitrate loading and the calculated loading to + 0.1, −0.4 μg at the 60% confidence level. At the atmospheric sampling rate used of 120 std ℓ min −1, these limits correspond to +5, −20 pptv for a one hour sampling time, with proportionately smaller uncertainties on the mean HNO 3 mixing ratio for longer sampling times.