Technical note: An improved approach to determining background aerosol concentrations with PILS sampling on aircraft

Abstract

h i g h l i g h t sBackground distributions of PILS þ offline IC measurements do not necessarily follow parametric statistics. Histograms of background distributions can be fit to lognormal distributions for accurate background determination. This procedure leads to substantially lower calculated limits of detection for ammonium and other inorganic ions. a r t i c l e i n f o Particle-into-Liquid Samplers (PILS) have become a standard aerosol collection technique, and are widely used in both ground and aircraft measurements in conjunction with off-line ion chromatography (IC) measurements. Accurate and precise background samples are essential to account for gas-phase components not efficiently removed and any interference in the instrument lines, collection vials or off-line analysis procedures. For aircraft sampling with PILS, backgrounds are typically taken with in-line filters to remove particles prior to sample collection once or twice per flight with more numerous backgrounds taken on the ground. Here, we use data collected during the Front Range Air Pollution and Photochemistry Experiment (FRAPP E) to demonstrate that not only are multiple background filter samples are essential to attain a representative background, but that the chemical background signals do not follow the Gaussian statistics typically assumed. Instead, the background signals for all chemical components analyzed from 137 background samples (taken from~78 total sampling hours over 18 flights) follow a log-normal distribution, meaning that the typical approaches of averaging background samples and/or assuming a Gaussian distribution cause an over-estimation of background samples e and thus an underestimation of sample concentrations. Our approach of deriving backgrounds from the peak of the lognormal distribution results in detection limits of 0. 25, 0.32, 3.9, 0.17, 0.75 and 0.57 and calcium (Ca 2þ ), respectively. The difference in backgrounds calculated from assuming a Gaussian distribution versus a log-normal distribution were most extreme for NH 4 þ , resulting in a background that was 1.58Âthat determined from fitting a log-normal distribution.

Aerosol Electroanalysis by PILSNER: Particle-into-Liquid Sampling for Nanoliter Electrochemical Reactions

“…However, even given a hypothetical situation where 99% of the water in the aerosol has evaporated, the estimate of the PILSNER Pb LOD would still be around 100 ng/m 3 , which is still appropriately sensitive for sampling natural aerosols. These LODs compare favorably with typical LODs and background species detection limits for traditional bulk PILS, which vary with the target aerosol but have been reported to be in the roughly 50–200 ng/m 3 range. , We hypothesize this LOD may still be improved by aforementioned optimization steps. For example, the LOD of PILSNER is dependent in part on the LOD of the anodic stripping voltammetry being applied, which itself is based in large part on the preconcentration time applied to the collector.…”

“…These LODs compare favorably with typical LODs and background species detection limits for traditional bulk PILS, which vary with the target aerosol but have been reported to be in the roughly 50−200 ng/m 3 range. 27,28 We hypothesize this LOD may still be improved by aforementioned optimization steps. For example, the LOD of PILSNER is dependent in part on the LOD of the anodic stripping voltammetry being applied, which itself is based in large part on the preconcentration time applied to the collector.…”

Air Quality in the Northern Colorado Front Range Metro Area: The Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ)

“…In addition to the GeoTaso instrument the campaigns also served as a testbed for the advancement of in situ aircraft instrumentation. This included a fast, infrared absorption spectrometer for the measurement of ethane with a 1-s time resolution (Richter et al, 2015), a novel approach of measuring background water soluble aerosol composition (Fukami et al, 2016), and a Chemical Ionization Mass Sectrometer (CIMS) technique for the fast measurement of atmospheric peroxides (Treadaway et al, 2018).…”