Explaining global surface aerosol number concentrations in terms of primary emissions and particle formation

Abstract

Abstract. We synthesised observations of total particle number (CN) concentration from 36 sites around the world. We found that annual mean CN concentrations are typically 300-2000 cm −3 in the marine boundary layer and free troposphere (FT) and 1000-10 000 cm −3 in the continental boundary layer (BL). Many sites exhibit pronounced seasonality with summer time concentrations a factor of 2-10 greater than wintertime concentrations. We used these CN obserCorrespondence to: D. V. Spracklen (dominick@env.leeds.ac.uk) vations to evaluate primary and secondary sources of particle number in a global aerosol microphysics model. We found that emissions of primary particles can reasonably reproduce the spatial pattern of observed CN concentration (R 2 =0.46) but fail to explain the observed seasonal cycle (R 2 =0.1). The modeled CN concentration in the FT was biased low (normalised mean bias, NMB=−88%) unless a secondary source of particles was included, for example from binary homogeneous nucleation of sulfuric acid and water (NMB=−25%). Simulated CN concentrations in the continental BL were also biased low (NMB=−74%) unless the number emission of anthropogenic primary particles was increased or a Published by Copernicus Publications on behalf of the European Geosciences Union. 4776 D. V. Spracklen et al.: Explaining global aerosol number concentrations mechanism that results in particle formation in the BL was included. We ran a number of simulations where we included an empirical BL nucleation mechanism either using the activation-type mechanism (nucleation rate, J , proportional to gas-phase sulfuric acid concentration to the power one) or kinetic-type mechanism (J proportional to sulfuric acid to the power two) with a range of nucleation coefficients. We found that the seasonal CN cycle observed at continental BL sites was better simulated by BL particle formation (R 2 =0.3) than by increasing the number emission from primary anthropogenic sources (R 2 =0.18). The nucleation constants that resulted in best overall match between model and observed CN concentrations were consistent with values derived in previous studies from detailed case studies at individual sites. In our model, kinetic and activation-type nucleation parameterizations gave similar agreement with observed monthly mean CN concentrations.

Boundary layer nucleation as a source of new CCN in savannah environment

“…To estimate the contribution of sulphuric acid on particle growth we calculated the so-called "sulphuric acid proxy", [H 2 SO 4 ] pro , based on [SO 2 ], solar radiation intensity and CS (Petäjä et al, 2009, proxy method 1;Vakkari et al, 2011). Since ambient OH radical concentrations are expected to be proportional to the radiation intensity (Rohrer and Berresheim, 2006), [H 2 SO 4 ] pro is expected to vary as the real gaseous sulphuric acid concentration.…”

“…During the observation period, clear new particle formation was observed on a total of 254 (or 69 %) of the days (Vakkari et al, 2011). For this study, we chose 187 new particle formation days with a nucleation mode growth up to sizes above 20 nm diameter.…”

“…The comprehensive measurement description is given by Laakso et al (2008) and Vakkari et al (2011) and here we list only briefly the measurements used in this study.…”

“…One such source, not considered by Ross et al (2003), is the atmospheric newparticle formation (Laakso et al, 2008;Vakkari et al, 2011), which has been observed globally in different environments (e.g. Kulmala et al, 2004).…”

Improving the simulation of global aerosol with size-segregated anthropogenic number emissions

“…Previous studies have shown that the implementation of an activation-type nucleation improves particle number concentration estimations in modeling (Spracklen et al, 2010;Makkonen et al, 2012). In our experiment, we coupled a binary sulphuric acid-water nucleation scheme (Vehkamäki et al, 2002) with an activationnucleation scheme described by Paasonen et al, (2010, Eq.…”