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Abstract. The uptake of HNO 3 to deliquescent airborne seasalt particles (RH = 55%, P = 760 torr, T = 300 K) at concentrations from 2 to 575 ppbv is measured in an aerosol flow tube using 13 N as a tracer. Small particles (≈ 70 nm diameter) are used in order to minimize the effect of diffusion in the gas phase on the mass transfer. Below 100 ppbv, an uptake coefficient (γ upt ) of 0.50 ± 0.20 is derived. At higher concentrations, the uptake coefficient decreases along with the consumption of aerosol chloride. Data interpretation is further supported by using the North American Aerosol Inorganics Model (AIM), which predicts the aqueous phase activities of ions and the gas-phase partial pressures of H 2 O, HNO 3 , and HCl at equilibrium for the NaCl/HNO 3 /H 2 O system. These simulations show that the low concentration data are obtained far from equilibrium, which implies that the uptake coefficient derived is equal to the mass accommodation coefficient under these conditions. The observed uptake coefficient can serve as input to modeling studies of atmospheric sea-salt aerosol chemistry. The main sea-salt aerosol burden in the marine atmosphere is represented by coarse mode particles (> 1 µm diameter). This implies that diffusion in the gas-phase is the limiting step to HNO 3 uptake until the sea-salt has been completely processed.

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Aerosols from the Sarychev volcano eruption (Kuril Islands, northeast of Japan) were observed in the Arctic lower stratosphere a few days after the strongest SO₂ injection which occurred on 15 and 16 June 2009. From the observations provided by the Infrared Atmospheric Sounding Interferometer (IASI) an estimated 0.9 Tg of sulphur dioxide was injected into the upper troposphere and lower stratosphere (UTLS). The resultant stratospheric sulphate aerosols were detected from satellites by the Optical Spectrograph and Infrared Imaging System (OSIRIS) limb sounder and by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and from the surface by the Network for the Detection of Atmospheric Composition Changes (NDACC) lidar deployed at OHP (Observatoire de Haute-Provence, France). By the first week of July the aerosol plume had spread out over the entire Arctic region. The Sarychev-induced stratospheric aerosol over the Kiruna region (north of Sweden) was measured by the Stratospheric and Tropospheric Aerosol Counter (STAC) during eight balloon flights planned in August and September 2009. During this balloon campaign the Micro Radiomètre Ballon (MicroRADIBAL) and the Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et NOx (SALOMON) remote-sensing instruments also observed these aerosols. Aerosol concentrations returned to near-background levels by spring 2010. The effective radius, the surface area density (SAD), the aerosol extinction, and the total sulphur mass from STAC in situ measurements are enhanced with mean values in the range 0.15–0.21 μm, 5.5–14.7 μm² cm⁻³, 5.5–29.5 × 10⁻⁴ km⁻¹, and 4.9–12.6 × 10⁻¹⁰ kg[S] kg⁻¹[air], respectively, between 14 km and 18 km. The observed and modelled e-folding time of sulphate aerosols from the Sarychev eruption is around 70–80 days, a value much shorter than the 12–14 months calculated for aerosols from the 1991 eruption of Mt Pinatubo. The OSIRIS stratospheric aerosol optical depth (AOD) at 750 nm is enhanced by a factor of 6, with a value of 0.02 in late July compared to 0.0035 before the eruption. The HadGEM2 and MIMOSA model outputs indicate that aerosol layers in polar region up to 14–15 km are largely modulated by stratosphere–troposphere exchange processes. The spatial extent of the Sarychev plume is well represented in the HadGEM2 model with lower altitudes of the plume being controlled by upper tropospheric troughs which displace the plume downward and upper altitudes around 18–20 km, in agreement with lidar observations. Good consistency is found between the HadGEM2 sulphur mass density and the value inferred from the STAC observations, with a maximum located about 1 km above the tropopause ranging from 1 to 2 × 10⁻⁹ kg[S] kg⁻¹[air], which is one order of magnitude higher than the background level

Abstract. Surface active organic compounds have been observed in marine boundary layer aerosol. Here, we investigate the effect such surfactants have on the uptake of nitric acid (HNO 3 ), an important removal reaction of nitrogen oxides in the marine boundary layer. The uptake of gaseous HNO 3 on deliquesced NaCl aerosol was measured in a flow reactor using HNO 3 labelled with the short-lived radioactive isotope 13 N. The uptake coefficient γ on pure deliquesced NaCl aerosol was γ =0.5±0.2 at 60% relative humidity and 30 ppb HNO 3 (g). The uptake coefficient was reduced by a factor of 5-50 when the aerosol was coated with saturated linear fatty acids with carbon chain lengths of 18 and 15 atoms in monolayer quantities. In contrast, neither shorter saturated linear fatty acids with 12 and 9 carbon atoms, nor coatings with the unsaturated oleic acid (C18, cisdouble bond) had a detectable effect on the rate of HNO 3 uptake. It is concluded that it is the structure of the monolayers formed, which determines their resistance towards HNO 3 uptake. Fatty acids (C18 and C15), which form a highly ordered film in the so-called liquid condensed state, represent a significant barrier towards HNO 3 uptake, while monolayers of shorter-chain fatty acids (C9, C12) and of the unsaturated oleic acid form a less ordered film in the liquid expanded state and do not hinder the uptake. Similarly, high contents of humic acids in the aerosol, a structurally inhomogeneous, quite water soluble mixture of oxidised high molecular weight organic compounds did not affect HNO 3 uptake. As surfactant films on naturally occurring aerosol Correspondence to: M. Ammann (markus.ammann@psi.ch) are expected to be less structured due to their chemical inhomogeneity, it is likely that their inhibitory effect on HNO 3 uptake is smaller than that observed here for the C15 and C18 fatty acid monolayers.