A three‐dimensional modeling study of the correlations of 210Pb with HNO3 and peroxyacetylnitrate (PAN) at remote oceanic sites

Abstract

We have used 210Pb, a tracer originating from the radioactive decay of 222Rn emitted from soils, to show that tracers not affected by complex chemical 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 species which are affected by chemistry. Using our three‐dimensional off‐line chemical transport model (CTM), TOMCAT, we show that 210Pb tends to covary with altitude with HNO3 arid peroxyacetylnitrate (PAN) at remote oceanic areas, suggesting the importance of transport for insoluble species (in our case 222Rn and NOx) that are able to escape deep convective scavenging and travel long distances before being transformed to soluble species (here 210Pb and HNO3). Near the continental coast, especially in the lower part of the troposphere, model HNO3 levels are large, evident of the continental influence. PAN and 210Pb levels are small. This indicates that 222Rn has not yet decayed to 210Pb and also PAN formation is not favored due to higher temperatures near the surface. Therefore, in these young air masses, only PAN and 210Pb correlate, both having low values. Higher up, in the middle to upper troposphere, our modeling study shows that HNO3 and 210Pb tend to correlate, since in these relatively older air masses there was sufficient time for 210Pb formation.