Ion balances of size-resolved tropospheric aerosol samples: implications for the acidity and atmospheric processing of aerosols

A large set of size-resolved aerosol samples was inspected with regard to t heir ion balance to shed light on how the aerosol acidity changes with part icle size in the lower troposphere and what implications this might have fo r the atmospheric processing of aerosols. Quite different behaviour between the remote and more polluted environments could be observed. At the remote sites, practically the whole accumulation mode had cation-to-anion ratios clearly below unity, indicating that these particles were quite acidic. The supermicron size range was considerably less acidic and may in some cases have been close to neutral or even alkaline. An interesting feature common to the remote sites was a clear jump in the cation-to-anion ratio when goin g from the accumulation to the Aitken mode. The most likely reason for this was cloud processing which, via in-cloud sulphate production, makes the sm allest accumulation-mode particles more acidic than the non-activated Aitke n-mode particles. A direct consequence of the less acidic nature of the Ait ken mode is that it can take up semi-volatile, water-soluble gases much eas ier than the accumulation mode. This feature may have significant implicati ons for atmospheric cloud condensation nuclei production in remote environm ents, In rural and urban locations, the cation-to-anion ratio was close to unity over most of the accumulation mode, but increased significantly when going to either larger or smaller particle sizes. The high cation-to-anion ratios in the supermicron size range were ascribed to carbonate associated with mineral dust. The ubiquitous presence of carbonate in these particles indicates that they were neutral or alkaline, making them good sites for he terogeneous reactions involving acidic trace gases. The high cation-to-anio n ratios in the Aitken mode suggest that these particles contained some wat er-soluble anions not detected by our chemical analysis. This is worth keep ing in mind when investigating the hygroscopic properties or potential heal th effects of ultrafine particles in polluted environments. (C) 2001 Elsevi er Science Ltd. All rights reserved.