A three-dimensional modeling study of the correlations of Pb-210 with HNO3and peroxyacetylnitrate (PAN) at remote oceanic sites

We have used Pb-210, a tracer originating from the radioactive decay of Rn- 222 emitted from soils, to show that tracers not affected by complex chemic al reactions can provide a strong indication of the origin of air masses in the remote oceanic atmosphere and can help us understand the behavior of s pecies which are affected by chemistry. Using our three-dimensional off-lin e chemical transport model (CTM), TOMCAT, we show that Pb-210 tends to cova ry with altitude with HNO3 and peroxyacetylnitrate (PAN) at remote oceanic areas, suggesting the importance of transport for insoluble species (in our case Rn-222 and NOx) that are able to escape deep convective scavenging an d travel long distances before being transformed to soluble species (here P b-210 and HNO3). Near the continental coast, especially in the lower part o f the troposphere, model HNO3 levels are large, evident of the continental influence. PAN and Pb-210 levels are small. This indicates that Rn-222 has not yet decayed to Pb-210 and also PAN formation is not favored due to high er temperatures near the surface. Therefore, in these young air masses, onl y PAN and Pb-210 and correlate, both having low values. Higher up, in the m iddle to upper troposphere, our modeling study shows that HNO3 and Pb-210 t end to correlate, since-in these relatively older air masses there was suff icient time for Pb-210 formation.