Processes controlling the seasonal variations in &amp;lt;sup&amp;gt;210&amp;lt;/sup&amp;gt;Pb and &amp;lt;sup&amp;gt;7&amp;lt;/sup&amp;gt;Be at the Mt. Cimone WMO-GAW global station, Italy: a model analysis

Erika Brattich,Hongyu Liu,Laura Tositti,James H Crawford,David B Considine

doi:10.5194/acp-17-1061-2017

Abstract

Abstract. We apply the Global Modeling Initiative (GMI) chemistry and transport model driven by NASA's MERRA assimilated meteorological data to simulate the seasonal variations in two radionuclide aerosol tracers (terrigenous 210Pb and cosmogenic 7Be) at the WMO-GAW station of Mt. Cimone (44°12′ N, 10°42′ E; 2165 m a.s.l.; Italy), which is representative of free-tropospheric conditions most of the year, during 2005 with an aim to understand the roles of transport and precipitation scavenging processes in controlling their seasonality. The total precipitation field in the MERRA data set is evaluated with the Global Precipitation Climatology Project (GPCP) observations, and generally good agreement is found. The model reproduces reasonably the observed seasonal pattern of 210Pb concentrations, characterized by a wintertime minimum due to lower 222Rn emissions and weaker uplift from the boundary layer and summertime maxima resulting from strong convection over the continent. The observed seasonal behavior of 7Be concentrations shows a winter minimum, a summer maximum, and a secondary spring maximum. The model captures the observed 7Be pattern in winter–spring, which is linked to the larger stratospheric influence during spring. However, the model tends to underestimate the observed 7Be concentrations in summer, partially due to the sensitivity to spatial sampling in the model. Model sensitivity experiments indicate a dominant role of precipitation scavenging (vs. dry deposition and convection) in controlling the seasonality of 210Pb and 7Be concentrations at Mt. Cimone.

Highlights

The use of atmospheric radionuclides to understand atmospheric dynamics, pollution transport and removal processes has a long history (e.g., Junge, 1963; Reiter et al, 1971; Gäggeler, 1995; Arimoto et al, 1999; Turekian and Graustein, 2003; World Meteorological Organization (WMO)-Global Atmosphere Watch (GAW), 2004; Dibb, 2007; Rastogi and Sarin, 2008; Froehlich and Masarik, 2010; Lozano et al, 2012)
It has been recognized that natural radionuclides are useful in a global monitoring network for atmospheric composition to support global climate change and air quality research, and they are measured at many of the regional, global, and contributing-partner stations in the Global Atmosphere Watch (GAW) network of the World Meteorological Organization (WMO) (WMO-GAW, 2004)
The Global Modeling Initiative (GMI, http://gmi.gsfc.nasa. gov) is a NASA-funded project aiming at improving assessments of anthropogenic perturbations to the Earth system; in this framework, a chemistry and transport model (CTM) appropriate for stratospheric assessments was developed (Rotman et al, 2001)

Summary

Introduction

The use of atmospheric radionuclides to understand atmospheric dynamics, pollution transport and removal processes has a long history (e.g., Junge, 1963; Reiter et al, 1971; Gäggeler, 1995; Arimoto et al, 1999; Turekian and Graustein, 2003; WMO-GAW, 2004; Dibb, 2007; Rastogi and Sarin, 2008; Froehlich and Masarik, 2010; Lozano et al, 2012). At Mauna Loa (19.47◦ N, 155.6◦ W; 3400 m a.s.l.; Hawaii) 210Pb seasonality was characterized by high concentrations in spring and summer and lower ones in winter, as opposed to the seasonal pattern found at higher latitudes, where the 210Pb maximum concentrations in winter are attributed to the advective transport of 210Pb aerosols from midlatitudes This behavior is due to the elevation of the site, representative of the conditions of the free troposphere rather than those of the PBL.

Data and methods

GMI model

Findings

Summary and conclusions