Interaction of mineral dust with gas phase nitric acid and sulfur dioxide during the MINATROC II field campaign: First estimate of the uptake coefficient HNO₃ from atmospheric data

Mineral dust, one of the most abundant aerosols by mass in the atmosphere, may have a lasting but to date almost unexplored effect on the trace gases nitric acid (HNO3) and sulfur dioxide (SO2). These gases have an important influence on, for example, the tropospheric ozone cycle, aerosol formation or acid rain. Within the second part of the MINATROC project (Mineral Dust and Tropospheric Chemistry) we investigated the interaction of mineral dust with gaseous HNO3 and SO2. The measurements were performed on a high mountain plateau (Izaña, Tenerife, 2367 m asl) using the highly sensitive CIMS (Chemical Ionization Mass Spectrometry) technique. During five periods of medium and one period of high atmospheric dust load, the HNO3 concentration decreased with increasing dust concentrations, and in all cases the HNO3 detection limit was reached. From the HNO3 decrease the uptake coefficient γHNO3 was calculated for the first time on the basis of in situ measurements. For the observed events, γHNO3 varied between 0.017 and 0.054. Moreover, during the dust events a significant decrease of ozone (O3) of the order of 30% was detected. The measurements and the analyses made in this paper show that the direct uptake of O3 on dust is a minor pathway for O3 depletion compared to the indirect effect, i.e., HNO3 depletion on dust which takes away a source of the O3 precursors nitrogen oxides. In contrast, a general interaction between SO2 and mineral dust was not observed. Positive as well as negative and no correlations between SO2 and mineral dust were detected