Improved chloride quantification in quadrupole aerosol chemical speciation monitors (Q-ACSMs)

Particulate chloride is an important component of fine particulate matter in marine air masses. Recent field studies also report elevated concentrations of gas-phase reactive chlorine species and particulate chloride related to anthropogenic activities. This work focuses on particulate chloride detection and quantification issues observed for some quadrupole aerosol chemical speciation monitors (Q-ACSM), which are designed for long-term measurement of ambient aerosol composition. The ACSM reports particle concentrations based on the difference between measurements of ambient air (sample mode) and particle-free ambient air (filter mode). For our long-term campaign in Krakow, Poland, the Q-ACSM reports apparent negative total chloride concentration for most of the campaign when analyzed with the default fragmentation table. This is the result of the difference signal from <em>m/z</em> 35 (³⁵Cl⁺) being negative which dominates over the positive difference signal from <em>m/z</em> 36 (H³⁵Cl⁺). Highly time-resolved experiments with NH₄Cl, NaCl and KCl particles show that the signal response of <em>m/z</em> 35 is non-ideal, where the signal builds up and decreases slowly for all three salts, leading to a negative difference measurement. In contrast, the <em>m/z</em> 36 signal exhibits a near step-change response for NH₄Cl during sampling and filter period, resulting in a positive difference signal. The response of <em>m/z</em> 36 for NaCl and KCl is not as prompt as for NH₄Cl but still fast enough to have a positive difference signal. Furthermore, it is shown that this behavior is mostly temperature-independent. Based on these observations, this work presents an approach to correct the chloride concentration time series by adapting the standard fragmentation table coupled with a calibration of NH₄Cl to obtain a relative ionization efficiency (RIE) based on the signal at <em>m/z</em> 36 (H³⁵Cl⁺). This correction can be applied for measurements in environments where chloride is dominated by NH₄Cl. Caution should be exercised when other chloride salts dominate the ambient aerosol. This repository contains the data corresponding to each figure presented in the published main text.