Nitric Acid and Organic Acids Suppress the Role of Methanesulfonic Acid in Atmospheric New Particle Formation

Multicomponent atmospheric molecular clusters, typically comprising a combination of acids and bases, play a pivotal role in our climate system and contribute to the perplexing uncertainties embedded in modern climate models. Our understanding of cluster formation is limited by the lack of studies on complex mixed-acid-mixed-base systems. Here, we investigate multicomponent clusters consisting of mixtures of several acid and base molecules: sulfuric acid (SA), methanesulfonic acid (MSA), nitric acid (NA), formic acid (FA), along with methylamine (MA), dimethylamine (DMA), and trimethylamine (TMA). We calculated the binding free energies of a comprehensive set of 252 mixed-acid-mixed-base clusters at the DLPNO-CCSD(T0)/aug-cc-pVTZ//ωB97X-D/6-31++G(d,p) level of theory. Combined with the existing datasets, we simulated the new particle formation (NPF) rates using the Atmospheric Cluster Dynamics Code (ACDC). We find that the presence of NA and FA had a substantial impact, increasing the NPF rate by 60% at realistic conditions. Intriguingly, we find that NA and FA suppress the role of MSA in NPF. These findings suggest that even high concentration of MSA has a limited impact on NPF in polluted regions with high FA and NA. We outline a method for generating a lookup table that could potentially be used in climate models that sufficiently incorporates all the required chemistry. By unraveling the molecular mechanisms of mixed-acid-mixed-base clusters, we get one step closer to comprehending their implications for our global climate system.

A Monte Carlo approach to study the effect of ions on the nucleation of sulfuric acid-water clusters

The nucleation of sulfuric acid-water clusters is a significant contribution to the formation of aerosols as precursors of cloud condensation nuclei (CCN). Depending on the temperature, there is an interplay between the clustering of particles and their evaporation controlling the efficiency of cluster growth. For typical temperatures in the atmosphere, the evaporation of H 2 SO 4  H 2 O clusters is more efficient than the clustering of the first, small clusters, and thus their growth is dampened at its early stages. Since the evaporation rates of small clusters containing an HSO 4 - $$ {\mathrm{HSO}}_4^{-} $$ ion are much smaller than for purely neutral sulfuric acid clusters, they can serve as a central body for the further attachment of H 2 SO 4  H 2 O molecules. We here present an innovative Monte Carlo model to study the growth of aqueous sulfuric acid clusters around central ions. Unlike classical thermodynamic nucleation theory or kinetic models, this model allows to trace individual particles and thus to determine properties for each individual particle. As a benchmarking case, we have performed simulations at T = 300 K $$ T=300\kern0.5em \mathrm{K} $$ a relative humidity of 50% with dipole and ion concentrations of c dipole = 5 × 10 8 - 10 9 cm - 3 $$ {c}_{dipole}=5\kern0.5em \times \kern0.5em {10}^8-{10}^9\kern0.5em {\mathrm{cm}}^{-3} $$ and c ion = 0 - 10 7 cm - 3 $$ {c}_{ion}=0-{10}^7\kern0.5em {\mathrm{cm}}^{-3} $$ . We discuss the runtime of our simulations and present the velocity distribution of ionic clusters, the size distribution of the clusters as well as the formation rate of clusters with radii R ≥ 0.85 nm $$ R\ge 0.85\kern0.5em \mathrm{nm} $$ . Simulations give reasonable velocity and size distributions and there is a good agreement of the formation rates with previous results, including the relevance of ions for the initial growth of sulfuric acid-water clusters. Conclusively, we present a computational method which allows studying detailed particle properties during the growth of aerosols as a precursor of CCN.

A density functional theory study of the molecular interactions between a series of amides and sulfuric acid

Amides, a class of nitrogen-containing organic pollutants in the atmosphere, may affect the formation of atmospheric aerosols by the interactions with sulfuric acid. Here, the molecular interactions of sulfuric acid with formamide, methylformamide, dimethylformamide, acetamide, methylacetamide and dimethylacetamide was investigated by density functional theory. Geometry optimization and Gibbs free energy calculation were carried out at M06-2X/6-311++G(3df,3pd) level. The results indicate that the addition of amides to H₂SO₄ might have a promoting effect on atmospheric new particle formation at 298.15 K and 1 atm. In the initial stage of new particle formation, the binding capacity of amides and sulfuric acid is stronger than ammonia, but weaker than methylamine. It is worth noting that the trans-methylacetamide could have similar capabilities of stabilizing sulfuric acid as dimethylamine. In the presence of water, amides are found to only have a weak enhancement capability on new particle formation. In addition, we can infer from evaporation rate that the small molecule clusters of formamide and sulfuric acid may be more energetically favorable than macromolecule clusters.

Simulation on different response characteristics of aerosol particle number concentration and mass concentration to emission changes over mainland China

In this study, Nested Air Quality Prediction Modeling System with Advance Particle Microphysics module (NAQPMS+APM) is applied to simulate the response characteristics of aerosol particle number concentration and mass concentration to emission changes over mainland China. It is the first attempt to investigate the response of both aerosol mass concentration and number concentration to emission changes using a chemical transport model with detailed aerosol microphysics over mainland China. Results indicate that the response characteristics are obviously different between aerosol particle number concentration and mass concentration. Generally, the response of number concentration shows a more heterogeneous spatial distribution than that of mass concentration. Furthermore, number concentration has a higher sensitivity not only to primary particles emission but also to precursor gases than that of mass concentration. Aerosol particle mass concentration exhibits a consistent trend with the emission change and yet aerosol number concentration does not. Due to the nonlinearity of aerosol microphysical processes, reduction of primary particles emission does not necessarily lead to an obvious decrease of aerosol number concentration and it even increases the aerosol number concentration. Over Central-Eastern China (CEC), the most polluted regions in China, reducing primary particles emission rather than precursor gas emissions is more effective in reducing particles number concentration. By contrast, the opposite is true over the northwestern China. The features of fine particles pollution revealed in this study are associated with the spatial differences in China's population, geography, climate and economy. Considering the more adverse effects of ultrafine particles on human health and the spatial distribution of population, making different measures in controlling particles number concentration from that controlling mass concentration in different regions over mainland China is indicated.

MAIN FINDINGS

FPN concentration responds more heterogeneously to emission than FPM. Spatial difference of response of FPN to emission is distinguished by a boundary line.

Improving organic aerosol treatments in CESM/CAM5: Development, application, and evaluation

New treatments for organic aerosol (OA) formation have been added to a modified version of the CESM/CAM5 model (CESM-NCSU). These treatments include a volatility basis set treatment for the simulation of primary and secondary organic aerosols (SOAs), a simplified treatment for organic aerosol (OA) formation from glyoxal, and a parameterization representing the impact of new particle formation (NPF) of organic gases and sulfuric acid. With the inclusion of these new treatments, the concentration of oxygenated organic aerosol increases by 0.33 µg m-3 and that of primary organic aerosol (POA) decreases by 0.22 µg m-3 on global average. The decrease in POA leads to a reduction in the OA direct effect, while the increased OOA increases the OA indirect effects. Simulations with the new OA treatments show considerable improvement in simulated SOA, oxygenated organic aerosol (OOA), organic carbon (OC), total carbon (TC), and total organic aerosol (TOA), but degradation in the performance of HOA. In simulations of the current climate period, despite some deviations from observations, CESM-NCSU with the new OA treatments significantly improves the magnitude, spatial pattern, seasonal pattern of OC and TC, as well as, the speciation of TOA between POA and OOA. Sensitivity analysis reveals that the inclusion of the organic NPF treatment impacts the OA indirect effects by enhancing cloud properties. The simulated OA level and its impact on the climate system are most sensitive to choices in the enthalpy of vaporization and wet deposition of SVOCs, indicating that accurate representations of these parameters are critical for accurate OA-climate simulations.