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

Aerosol chemical composition and trace gas measurements were made at two locations on the northeastern peninsula of Tenerife during the ACE-2 HILLCLOUD experiment, between 28 June and 23 July 1997. Measurements were made of coarse (>2.5 mum aerodynamic diameter) and fine (<2.5 mu m) aerosol Cl-, NO3-, SO42-, non-sea salt SO42- (NSSS), CH3SO3- (MSA) and NH4+, and gas phase dimethylsulphide (DMS), HCl, HNO3, SO2, CH3COOH, HCOOH and NH3. Size distributions were measured using a cascade impactor. Results show that in marine air masses NSSS and MSA were formed via DMS oxidation, with additional NSSS present in air masses containing a continental component. Using a Eulerian box model approach for aerosols transported between upwind and downwind sites, a mean NSSS production rate of 4.36 x 10(-)4 mug m(-3) s(-1) was calculated for daytime clear sky periods (highest insolation), with values for cloudy periods during daytime and nighttime of 3.55 x 10(-4) and 2.40 x 10(-4) mug m(-3) s(-1), respectively. The corresponding rates for MSA were 6.23 x 10(-6), 8.49 x 10(-6) and 6.95 x 10(-6) mug m(-3) s(-1), respectively. Molar concentration ratios for MSA/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 influence on rates of MSA production, than of NSSS, while conversely daytime gas phase reactions were more important for NSSS. For MSA, nighttime in-cloud oxidation rates exceeded rates of daytime gas phase production via OH oxidation of DMS. NSSS, MSA and ammonium had trimodal size distributions, with modes at 0.3, 4.0 and >10.0 mum (NSSS and NH4+), and 0.3, 1.5 and 4.0 mum (MSA). No significant production of other aerosol species was observed, with the exception of ammonium, which was formed at variable rates dependent on neutralisation of the aerosol with ammonia released from spatially non-uniform surface sources. Seasalt components were mainly present in coarse 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.