Separating refractory and non-refractory particulate chloride and estimating chloride depletion by aerosol mass spectrometry in a marine environment

Abstract. Aerosol composition and concentration measurements along the coast of California were obtained using an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-AMS) onboard the research vessel Atlantis during the CalNex study in 2010. This paper focuses on the measurement of aerosol chloride using the HR-AMS that can be ambiguous in regions with significant quantities of sea salt aerosols. This ambiguity arises due to large differences in the sensitivity of the HR-AMS to refractory chloride species (i.e., NaCl) and non refractory chloride species (i.e., NH₄Cl, HCl, etc.). Using the HR-AMS, the aerosol chloride signal is typically quantified using ion signals for ³⁵Cl⁺, H³⁵Cl⁺, ³⁷Cl⁺ and H³⁷Cl⁺ (H_xCl⁺). During this study, the highest aerosol chloride signal was observed during sea sweep experiments when the source of the aerosol chloride was NaCl present in artificially generated sea salt aerosols even though the HR-AMS has significantly lower sensitivity to such refractory species. Other prominent ion signals that arise from NaCl salt were also observed at m/z 22.99 for Na⁺ and m/z 57.96 for Na³⁵Cl⁺ during both sea sweep experiments and during periods of ambient measurements. Thus, refractory NaCl contributes significantly to the H_xCl⁺ signal, interfering with attempts to quantify non sea salt chloride (nssCl). It was found that during ambient aerosol measurements, the interference in the H_xCl⁺ signal from sea salt chloride (ssCl) was as high as 89%, but with a study wide average of 10%. The Na³⁵Cl⁺ ion signal was found to be a good tracer for NaCl. We outline a method to establish nssCl in the ambient aerosols by subtracting the sea salt chloride (ssCl) signal from the H_xCl⁺ signal. The ssCl signal is derived from the Na³⁵Cl⁺ ion tracer signal and the H_xCl⁺ to Na³⁵Cl⁺ ratio established from the sea sweep experiments. Ambient submicron concentrations of ssCl were also established using the Na³⁵Cl⁺ ion tracer signal and a scaling factor determined through simultaneous measurements of submicron aerosol chloride on filters. This scaling factor accounts for the low vaporization response of the AMS heater to ssCl, although regular calibration of this response is recommended in future applications. It follows that true total particulate chloride (pCl) is the sum of nssCl and ssCl. In this study, the median levels observed for the concentrations of pCl, nssCl and ssCl were 0.052, 0.017 and 0.024 μg m⁻³ respectively. The average contributions of nssCl and ssCl to pCl were 48 and 52% respectively, with nssCl dominating in periods of continental outflow and ssCl dominating during other periods. Finally, we propose a method to measure percentage chloride depletion of NaCl in ambient submicron sea salt aerosols, strictly using the AMS measurements of Na⁺ and Na³⁵Cl⁺ ion signals. The median chloride depletion in submicron aerosols in this study was found to be 78%.