Mass concentration and ion composition of coarse and fine particles in an urban area in Beirut: effect of calcium carbonate on the absorption of nitric and sulfuric acids and the depletion of chloride

[摘要] Levels of coarse (PM10-2.5) and fine (PM2.5) particles were determinedbetween February 2004 and January 2005 in the city of Beirut, Lebanon.While low PM mass concentrations were measured in the rainy season, elevatedlevels were detected during sand storms originating from Arabian desertand/or Africa. Using ATR-FTIR and IC, it was shown that nitrate, sulfate,carbonate and chloride were the main anionic constituents of the coarseparticles, whereas sulfate was mostly predominant in the fine particles inthe form of (NH₄)₂SO₄. Ammonium nitrate was not expected tobe important because the medium was defined as ammonium poor. In parallel,the cations Ca²⁺ and Na⁺ dominated in the coarse, andNH₄⁺, Ca²⁺ and Na⁺ in the fine particles. Coarse nitrateand sulfate ions resulted from the respective reactions of nitric andsulfuric acid with a relatively high amount of calcium carbonate. BothCaCO₃ and Ca(NO₃)₂ crystals identified by ATR-FTIR in thecoarse particles were found to be resistant to soaking in water for 24 hbut became water soluble when they were formed in the fine particlessuggesting, thereby, different growth and adsorption phenomena. The seasonalvariational study showed that nitrate and sulfate ion concentrationsincreased in the summer due to the enhancement of photochemical reactionswhich facilitated the conversion of NO₂ and SO₂ gases intoNO₃^- and SO₄^2-, respectively. While nitrate was mainlydue to local heavy traffic, sulfates were due to local and long-rangetransport phenomena. Using the air mass trajectory HYSPLIT model, it wasfound that the increase in the sulfate concentration correlated with windvectors coming from Eastern and Central Europe. Chloride levels, on theother hand, were high when wind originated from the sea and low during sandstorms. In addition to sea salt, elevated levels of chloride were alsoattributed to waste mass burning in proximity to the site. In comparison toother neighboring Mediterranean countries, relatively higher concentrationsof calcium in Beirut were good indication of calcitic crustal abundance.Considering the importance of the health and climate impacts of aerosolslocally and regionally, this study constitutes a point of reference foreastern Mediterranean transport modeling studies and local regulatory andpolicy makers.

[学科分类] 大气科学