The impact of dust on sulfate aerosol, CN and CCN during an East Asian dust storm

Abstract

Abstract.A global model of aerosol microphysics is used to simulate a large East Asian dust storm during the ACE-Asia experiment. We use the model together with size resolved measurements of aerosol number concentration and composition to examine how dust modified the production of sulfate aerosol and the particle size distribution in East Asian outflow. Simulated size distributions and mass concentrations of dust, sub-and super-micron sulfate agree well with observations from the C-130 aircraft. Modeled mass concentrations of fine sulfate (D p <1.3 µm) decrease by ∼10% due to uptake of sulfur species onto super-micron dust. We estimate that dust enhanced the mass concentration of coarse sulfate (D p >1.0 µm) by more than an order of magnitude, but total sulfate concentrations increase by less than 2% because decreases in fine sulfate have a compensating effect. Our analysis shows that the sulfate associated with dust can be explained largely by the uptake of H 2 SO 4 rather than reaction of SO 2 on the dust surface, which we assume is suppressed once the particles are coated in sulfate. We suggest that many previous model investigations significantly overestimated SO 2 oxidation on East Asian dust, possibly due to the neglect of surface saturation effects. We extend previous model experiments by examining how dust modified existing particle concentrations in Asian outflow. Total particle concentrations (condensation nuclei, CN) modeled in the dust-pollution plume are reduced by up to 20%, but we predict that dust led to less than 10% depletion in particles large enough to act as cloud condensation nuclei (CCN). Our analysis suggests that E. Asian dust storms have only a minor impact on sulfate particles present at climate-relevant sizes.

Impact of nucleation on global CCN

“…While dust can be an important source of ice nuclei and makes a significant proportion of aerosol mass in some regions, the number concentration of dust is always low compared to other aerosol types. Our resent modelling results including dust have shown that dust makes only a minor contribution to CN or CCN even during severe dust storms (Manktelow et al, 2009). Therefore, neglecting dust has no significant impact on our results.…”

A review of natural aerosol interactions and feedbacks within the Earth system

“…Bauer and Koch (2005) found that interaction between sulphate and dust would reduce the sulphate direct forcing from −0.25 to −0.18 W m −2 when heterogeneous oxidation of SO 2 is included, as a result of a reduced concentration of externally mixed sulphate. Dentener et al (1996) found that 50-70% of global sulphate formation is associated with dust, however a more recent study evaluated against observations suggests a 2% effect even in a dust storm because changes in fine and coarse dust essentially compensate (Manktelow et al, 2010). A significant fraction of nitric acid is also likely to be associated with dust (Dentener et al, 1996).…”

“…For instance, Lee et al (2008) and Manktelow et al (2010) show that in dusty regions, CCN concentrations can be reduced by 10-20% due to particles providing a condensation sink for sulphuric acid vapor and a coagulation sink for ultrafine particles. Also, increased dust deposition on snow can reduce surface albedo leading to enhanced snow-melt and feedbacks on regional climate (Krinner et al, 2006).…”

Global cloud condensation nuclei influenced by carbonaceous combustion aerosol

“…Our model does not include dust or nitrate. However, we have previously shown that dust has relatively little impact on CCN, reducing CCN concentrations by up to 10 % locally (Manktelow et al, 2010). The value of κ for organic material is uncertain, but is likely to lie within the range 0.009 to 0.4 (Petters and Kreidenweis, 2007).…”