Influence of transport over a mountain ridge on the chemical composition of marine aerosols during the ACE-2 HILLCLOUD experiment.

Allen, A. G. and Davison, B. M. and James, J. D. and Robinson, L. and Harrison, R. M. and Hewitt, CN (2002) Influence of transport over a mountain ridge on the chemical composition of marine aerosols during the ACE-2 HILLCLOUD experiment. Journal of Atmospheric Chemistry, 41 (1). pp. 83-107. ISSN 0167-7764

Abstract

Aerosol chemical composition and trace gas measurements were made at twolocations on the northeastern peninsula of Tenerife during the ACE-2HILLCLOUD experiment, between 28 June and 23 July 1997. Measurementswere made of coarse (#gt;2.5 m aerodynamic diameter) and fine (#lt; 2.5m) aerosol Cl–, NO3 –,SO4 2–, non-sea saltSO4 2– (NSSS),CH3SO3 – (MSA) andNH4 +, and gas phase dimethylsulphide (DMS), HCl,HNO3, SO2, CH3COOH, HCOOH andNH3. Size distributions were measured using a cascadeimpactor. Results show that in marine air masses NSSS and MSA wereformed via DMS oxidation, with additional NSSS present in air massescontaining a continental component. Using a Eulerian box model approachfor aerosols transported between upwind and downwind sites, a mean NSSSproduction rate of 4.36 × 10–4 gm–3 s–1 was calculated for daytimeclear sky periods (highest insolation), with values for cloudy periodsduring daytime and nighttime of 3.55 × 10–4 and2.40 × 10–4 g m–3s–1, respectively. The corresponding rates for MSA were6.23 × 10–6, 8.49 × 10–6and 6.95 × 10–6 g m–3s–1, respectively. Molar concentration ratios forMSA/NSSS were 8.7% (1.8–18.2%) and 1.9%(1.3–3.5%) in clean and polluted air masses, respectively.Reactions occurring within clouds appeared to have a greater influenceon rates of MSA production, than of NSSS, while conversely daytime gasphase reactions were more important for NSSS. For MSA, nighttimein-cloud oxidation rates exceeded rates of daytime gas phase productionvia OH oxidation of DMS. NSSS, MSA and ammonium had trimodal sizedistributions, with modes at 0.3, 4.0 and >10.0 m (NSSS andNH4 +), and 0.3, 1.5 and 4.0 m (MSA). Nosignificant production of other aerosol species was observed, with theexception of ammonium, which was formed at variable rates dependent onneutralisation of the aerosol with ammonia released from spatiallynon-uniform surface sources. Seasalt components were mainly present incoarse particles, although sub-micrometre chloride was also measured.Losses by deposition exceeded calculated expectations for all species,and were highest for the seasalt fraction and nitrate.