Weak global sensitivity of cloud condensation nuclei and the aerosol indirect effect to Criegee + SO<sub>2</sub> chemistry

Abstract

Abstract. H2SO4 vapor is important for the nucleation of atmospheric aerosols and the growth of ultrafine particles to cloud condensation nuclei (CCN) sizes with important roles in the global aerosol budget and hence planetary radiative forcing. Recent studies have found that reactions of stabilized Criegee intermediates (CIs, formed from the ozonolysis of alkenes) with SO2 may be an important source of H2SO4 that has been missing from atmospheric aerosol models. For the first time in a global model, we investigate the impact of this new source of H2SO4 in the atmosphere. We use the chemical transport model, GEOS-Chem, with the online aerosol microphysics module, TOMAS, to estimate the possible impact of CIs on present-day H2SO4, CCN, and the cloud-albedo aerosol indirect effect (AIE). We extend the standard GEOS-Chem chemistry with CI-forming reactions (ozonolysis of isoprene, methyl vinyl ketone, methacrolein, propene, and monoterpenes) from the Master Chemical Mechanism. Using a fast rate constant for CI+SO2, we find that the addition of this chemistry increases the global production of H2SO4 by 4%. H2SO4 concentrations increase by over 100% in forested tropical boundary layers and by over 10–25% in forested NH boundary layers (up to 100% in July) due to CI+SO2 chemistry, but the change is generally negligible elsewhere. The predicted changes in CCN were strongly dampened to the CI+SO2 changes in H2SO4 in some regions: less than 15% in tropical forests and less than 2% in most mid-latitude locations. The global-mean CCN change was less than 1% both in the boundary layer and the free troposphere. The associated cloud-albedo AIE change was less than 0.03 W m−2. The model global sensitivity of CCN and the AIE to CI+SO2 chemistry is significantly (approximately one order-of-magnitude) smaller than the sensitivity of CCN and AIE to other uncertain model inputs, such as nucleation mechanisms, primary emissions, SOA (secondary organic aerosol) and deposition. Similarly, comparisons to size-distribution measurements show that uncertainties in other model parameters dominate model biases in the model-predicted size distributions. We conclude that improvement in the modeled CI+SO2 chemistry would not likely lead to significant improvements in present-day CCN and AIE predictions.

Aerosol loading in the Southeastern United States: reconciling surface and satellite observations

“…Cloudy conditions as well as enhanced oxidant concentrations in summertime could also augment sulfate production, which we show is moderately underestimated at the surface in summertime in our simulation. It has also been suggested that including reactions of stabilized Criegee intermediates with sulfur dioxide in models can produce a 10-25 % increase in sulfuric acid (H 2 SO 4 ) annually and a 100 % increase in July (Pierce et al, 2013).…”

Semi-empirical parameterization of size-dependent atmospheric nanoparticle growth in continental environments

“…This precursor could be similar to the extra SOA treated by Spracklen et al (2011) or, e.g., a fraction of some other condensable material treated in the model, as discussed above. We used the GEOS-Chem-TOMAS model (as described in Pierce et al, 2013) to study the seasonal behavior of this extra SOA and to determine if it can be used to describe our "background". As suggested by Spracklen et al (2011), 100 Tg SOA yr −1 correlated with anthropogenic CO emissions was added to the model beyond the standard biogenic SOA-precursor emissions.…”

“…Schematic of processes leading to nanoparticle growth from the perspective of the parameterization: formation of SORG MT , SORG bg and sulfuric acid from biogenic volatile organic compounds (BVOCs) and anthropogenic pollution via atmospheric oxidation and condensation of these vapors onto nanoparticles. Model simulations with GEOS-Chem-TOMAS (Pierce et al, 2013) show that SORG bg can be linked to the products of oxidation of biogenic organics in the presence of anthropogenic pollutants (Hoyle et al, 2011;Spracklen et al, 2011). particle growth. The parameterization is aimed to be simple and thus easily applicable in large-scale atmospheric models.…”

Reactivity of stabilized Criegee intermediates (sCIs) from isoprene and monoterpene ozonolysis toward SO<sub>2</sub> and organic acids

“…Their results supported the experimental observations by Mauldin III et al (2012), showing that a signif-icant fraction (several tens of percents) of ground-level gas phase sulfuric acid originates probably from sCI-initiated oxidation of SO 2 . Pierce et al (2013) took a step further and studied the role of the sCI + SO 2 reaction to global aerosol and cloud condensation nuclei (CCN) concentrations by the global climate model. They found, in accordance with Boy et al (2013), that sCIs can contribute significantly to gas phase H 2 SO 4 in the lower troposphere above forested areas.…”

“…However, due to further aerosol dynamical processes during particle growth to CCN sizes, the influence of sCI on sulfuric acid concentration was only feebly projected to CCN concentrations, and thus to radiative forcing. However, Pierce et al (2013) used reaction rate coefficients, including the upper limit for the sCI loss rate (decomposition and reaction with water vapour), obtained by Welz et al (2012) for CH 2 OO. Furthermore, the sCI reaction rate coefficients, including sCI loss in a reaction with water, may be strongly dependent on the sCI structure.…”