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

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 μm aerodynamic diameter) and fine (#lt; 2.5μm) aerosol Cl−, NO3−,SO42−, non-sea saltSO42− (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 μgm−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 μg m−3s−1, respectively. The corresponding rates for MSA were6.23 × 10−6, 8.49 × 10−6and 6.95 × 10−6 μg 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 μm (NSSS andNH4+), and 0.3, 1.5 and 4.0 μm (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.