Non-sea-salt chloride in submicron particles at multiple locations in the Northern hemisphere

Description

Non-sea-salt chloride is often observed as a minor component of submicron particles, accounting for 1–2% of the mass worldwide (Zhang et al. 2007 GRL). Recent studies indicate that despite its low concentration, it may play disproportionally important roles affecting the atmospheric composition and the climate (Thornton et al. 2010 Nature). This is because chloride-containing particles react with gas phase N2O5 heterogeneously, forming nitryl chloride (ClNO2) with greater than 50% yield (Thornton et al. 2005 JGR). ClNO2 accumulates during the night but quickly photolyzes into chlorine radical (∙Cl) and NO2 after sunrise. Since both ∙Cl and NO2 are important precursors of major oxidants (e.g., ∙OH and O3) that regulate the concentrations and life cycles of a wide spectrum of atmospheric species including secondary aerosols, this chloride activation mechanism may have important implications for climate. The significance of this mechanism in the atmosphere relies on the availability of aerosol chloride and NOx—precursors of N2O5. Even in continental air masses, in which sea-salt aerosol loadings are expected to be low, significant concentrations of ClNO2 (>0.1 ppb) are frequently observed (Thornton et al. 2010 Nature). This is an indication that non-refractory chloride in the form of ammonium chloride may have played an important role and that more information is needed to understand the chemistry of this species in airborne particles. Here we report the results of a survey using the global Aerosol Mass Spectrometer (AMS) field measurement database and present an overview of the concentrations and temporal variation of non-refractory chloride in submicrometer aerosols. We will also present the correlations of chloride with other non-refractory species including nitrate, sulfate, ammonium, and organic components. Implications for atmospheric chemistry and formation and aging will be discussed.