Abstract
Abstract. A fog event was monitored with state-of-the art real-time aerosol mass spectrometers in an urban background location in London (England) during the REPARTEE-I experiment. Specific particle types rich in hydroxymethanesulphonate (HMS) were found only during the fog event. Formation of inorganic and organic secondary aerosol was observed as soon as fog was detected and two different mechanisms are suggested to be responsible for the production of two different types of aerosol. Nitrate aerosol is produced in the liquid phase within the droplet. Contrary to previous studies, the formation of HULIS was observed on interstitial particles rather than evaporated fog droplets, suggesting heterogeneous formation mechanisms depending on parameters other than the water content and not fully understood. Not only are secondary aerosol constituents produced during the fog event, but the primary aerosol is observed to be processed by the fog event, dramatically changing its chemical properties.
Highlights
Fogs constitute an aqueous reaction medium in which aerosol mass formation occurs through gas scavenging and chemical reaction in the droplets
The lower temperature values detected at the park in comparison to those taken at the BT tower support the presence of an inversion cap over London
Our results suggest that the unique HMOC particle types (HMOC1and HMOC2) are formed only in the interstitial air, not in evaporated fog droplets, suggesting heterogeneous formation mechanisms depending on parameters other than the water content and not fully understood
Summary
Fogs constitute an aqueous reaction medium in which aerosol mass formation occurs through gas scavenging and chemical reaction in the droplets. The two competing effects depend on several factors: the initial oxidative steps leading to (nonvolatile) particulate organic matter formation are likely to be more important at the very beginning of the fog formation when the reactant concentrations are at their maximum whilst deposition rates may increase over time with the growth of fog droplets (Lillis et al, 1999). Studies of the chemical composition of clouds/fogs have been largely focused on the processing of inorganic compounds. Reaction pathways for cloud-phase formation of low-molecular weight organic compounds have been studied in detail (Herrmann et al 2000; Herrmann 2003). The poor understanding of the formation of secondary organic particulate matter represents a major source of uncertainty in predictions of aerosol concentrations and properties affecting health, visibility and climate
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