AS3.20

The pandemic of SARS-CoV-2 has led to a substantial reduction in anthropogenic activities globally. This is particularly true for traffic, which was reduced by 40-80 % in Eastern and Northern China. The imposed lockdown provides a unique opportunity to investigate the direct and indirect effects of anthropogenic activities (particularly traffic) on atmospheric new particle formation, atmospheric chemical cocktail and haze formation in polluted urban environments in the case when the emissions were substantially lower. Here, we utilize comprehensive, long term ground-based and satellite observations to investigate changes in the atmospheric composition and connect them with a continental scale gas-to-particle conversion producing both fresh particles and new aerosol mass.  We show  that despite the reductions in emissions, both new particle formation (NPF) and haze events still occur. The observational evidence confirms that the main NPF mechanism remains similar because of non-linear response of NPF and growth to local and regional vehicle emission reductions. Furthermore we are able follow the growth from NPF to haze and show, in the case study, that regional NPF makes a dominating contribution to the haze.

How to cite: Kulmala, M., Yan, C., Dada, L., Bianchi, F., Kokkonen, T., and Jiang, J. and the Aerosol and Haze Laboratory Team: The effect of COVID-19 restrictions of human activities on atmospheric chemical cocktail, new particle formation  and air quality in in Eastern and Northern China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12589, https://doi.org/10.5194/egusphere-egu21-12589, 2021.

Aerosol acidity is a key parameter in atmospheric aqueous chemistry and strongly influence the interactions of air pollutants and ecosystem. The recently proposed multiphase buffer theory provides a framework to reconstruct long-term trends and spatial variations of aerosol pH based on the effective acid dissociation constant of ammonia (K_a,NH3^*). However, non-ideality in aerosol droplets is a major challenge limiting its broad applications. Here, we introduced a non-ideality correction factor (c_ni) and investigated its governing factors. We found that besides relative humidity (RH) and temperature, c_ni is mainly determined by the molar fraction of NO₃^- in aqueous-phase anions, due to different NH₄⁺ activity coefficients between (NH₄)₂SO₄- and NH₄NO₃-dominated aerosols. A parameterization method is thus proposed to estimate c_ni at given RH, temperature and NO₃^- fraction, and is validated against long-term observations and global simulations. In the ammonia-buffered regime, with c_ni correction the buffer theory can well reproduce the K_a,NH3^* predicted by comprehensive thermodynamic models, with root-mean-square deviation ~0.1 and correlation coefficient ~1. Note that, while c_ni is needed to predict K_a,NH3^* levels, it is usually not the dominant contributor to its variations, as ~90% of the temporal or spatial variations in K_a,NH3^* is due to variations in aerosol water and temperature.

How to cite: Cheng, Y., Zheng, G., Su, H., Wang, S., and Pozzer, A.: The impact of non-ideality on reconstructing spatial and temporal variations of aerosol acidity with multiphase buffer theory, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9292, https://doi.org/10.5194/egusphere-egu21-9292, 2021.

Understanding the mechanism of haze formation is crucial for the development of deliberate pollution control strategies. Multiphase chemical reactions in aerosol water have been suggested as an important source of particulate sulfate during severe haze (Cheng et al., 2016;Wang et al., 2016). While the key role of aerosol water has been commonly accepted, the relative importance of different oxidation pathways in the aqueous phase is still under debate, mainly due to questions about aerosol pH. To investigate the spatio-temporal variability of aerosol pH and sulfate formation during winter in the North China Plain (NCP), we have developed a new aerosol water chemistry module (AWAC) for the WRF-Chem model (Weather Research and Forecasting model coupled with Chemistry). Using the WRF-Chem-AWAC model, we performed a comprehensive survey of the atmospheric conditions characteristic for wintertime in the NCP, focusing on January 2013. We find that aerosol pH exhibited a strong vertical gradient and distinct diurnal cycle, which was closely associated with the spatio-temporal variation in the relative abundance of acidic and alkaline fine particle components and their gaseous counterparts. Over Beijing, the average aerosol pH at the surface layer was ~5.4 and remained nearly constant around ~5 up to ~2 km above ground level; further aloft, the acidity rapidly increased to pH ~0 at ~3 km. The pattern of aerosol acidity increase with altitude persisted over the NCP, while the specific levels and gradients of pH varied between different regions. In the region north of ~41°N, the mean pH values at surface level were typically >6 and the main pathway of sulfate formation in aerosol water was S(IV) oxidation by ozone. South of ~41°N, the mean pH values at surface level were typically in the range of 4.4 to 5.7, and different chemical regimes and reaction pathways of sulfate formation prevailed in four different regions, depending on reactant concentrations and atmospheric conditions. The NO₂ reaction pathway prevailed in the megacity region of Beijing and the large area of Hebei Province to the south and west of Beijing, as well as part of Shandong Province. The transition metal ion (TMI) pathway dominated in the inland region to the west and the coastal regions to the east of Beijing, and the H₂O₂ pathway dominated in the region extending further south (Shandong and Henan Provinces). In all of these regions, the O₃ and TMI pathways in aerosol water as well as the gas-particle partitioning of H₂SO₄ vapor became more important with increasing altitude. Although pH is sensitive to the abundance of NH₃ and crustal particles, we show that the rapid production of sulfate in the NCP can be maintained over a wide range of aerosol acidity (e.g., pH = 4.2-5.7) with transitions from TMI pathway dominated to NO₂/O₃ pathway dominated regimes.

How to cite: Tao, W., Su, H., Zheng, G., Wang, J., Wei, C., Liu, L., Ma, N., Li, M., Zhang, Q., Pöschl, U., and Cheng, Y.: Aerosol pH and chemical regimes of sulfate formation in aerosol water during winter haze in the North China Plain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7189, https://doi.org/10.5194/egusphere-egu21-7189, 2021.

Reactive oxygen species (ROS), such as hydroxyl radical (OH•), hydroperoxy radicals (HO2•/O2-), and hydrogen peroxide (H2O2), are produced in cloud droplets and aqueous aerosol. Multiphase model studies suggest that the Fenton reaction, i.e. the oxidation of Fe(II) by H2O2 represents one of the main sources of the OH radical in the aqueous phase.

Current cloud and aerosol multiphase chemistry models are usually initialized with equal iron concentrations in all droplets or particles as derived from bulk samples of cloud water or aerosol composition. However, analysis of single aerosol particles has revealed that only a small number fraction of particles and, thus, of cloud droplets contain iron.

The aim of our study is to identify the impacts of the iron distribution in cloud droplets or aqueous aerosol particles on the total (gas + aqueous) budgets of OH, HO2, H2O2 and O3 in the multiphase system.

By means of model studies, we compare predicted oxidant budgets based on the assumptions of iron distributed among all droplets or particles versus the same iron mass concentrated in a few droplets (or particles) in the total population only. Our results suggest that the traditional approach based on bulk iron concentrations may significantly underestimate total OH budgets, whereas the predicted levels of H2O2, HO2/O2- and ozone are less affected. The reasons for the different findings between (i) the various oxidants and (ii) cloud droplets vs aerosol particles will be discussed. In summary, our model studies suggest that oxidant levels and oxidation potentials of particulate matter in the atmosphere can only be accurately assessed if particle- and size-resolved aerosol composition is accounted for.

How to cite: Khaled, A., Zhang, M., and Ervens, B.: The impact of iron distribution among cloud droplets or aqueous aerosol particles on multiphase oxidant levels, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2102, https://doi.org/10.5194/egusphere-egu21-2102, 2021.

Hydrolysis of nitrogen dioxide (NO₂) has long been recognized as a major formation path of atmospheric nitrous acid (HONO), which is regarded as a dominant hydroxyl radical (OH) source, particularly in a polluted environment. Since HONO is moderately water soluble and its solubility can be highly dependent on the acidity of the water solution, the HONO formation rate and its ensuring fate may also be affected by the acidity of the water surfaces. In this work, we investigated the hydrolysis of NO₂ on dilute sulfuric acid water solutions with a pH value ranging from ~3 to ~6. Both the gaseous HONO and dissolved nitrous acid solution were quantified by a wet-chemistry based HONO analyzer and ion chromatography analyses, respectively. The results showed that significant amount of HONO can participate into the aqueous phase at low acidity and as the acidity increased gas-phase HONO also increased. These results indicated that liquid water on various surfaces may both provide a reaction site for HONO formation and serve as a reservoir of HONO that can be released when the liquid water was evaporated.

How to cite: Zheng, J. and Ma, Y.: Heterogeneous formation and partitioning of nitrous acid on water surfaces: dependency on the acidity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2114, https://doi.org/10.5194/egusphere-egu21-2114, 2021.

Nitrous acid (HONO) is an important component of the nitrogen cycle. HONO can also be rapidly photolyzed by actinic radiation to form hydroxyl radicals (OH) and exerts a primary influence on the oxidative capacity of the atmosphere. The sources and sinks of HONO, however, are not fully understood. Soil nitrite, produced via nitrification or denitrification, is an important source for the atmospheric HONO production. [HONO]*, the equilibrium gas phase HONO concentration over the soil, has been suggested as key to understanding the environmental effects of soil fluxes of HONO (Su et al., 2011). But if and how [HONO]* may exist and vary remains an open question. In this project, a measurement method using a dynamic chamber has been developed to derive [HONO]* and the atmospheric soil fluxes of HONO can accordingly be quantified. We demonstrate the existence of [HONO]* and determine its variation in the course of soil drying processes. We show that when [HONO]* is higher than the atmospheric HONO concentration, HONO will be released from soil; otherwise, HONO will be deposited on soil. This work advances the understanding of soil HONO emissions, and the evaluation of its impact on the atmospheric oxidizing capacity and the nitrogen cycling.

How to cite: Bao, F., Su, H., Kuhn, U., and Cheng, Y.: Key Role of Equilibrium HONO Concentration over the Soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4166, https://doi.org/10.5194/egusphere-egu21-4166, 2021.

Measuring pH in individual aerosol droplet is essential for understanding and estimating physicochemical processes within aerosol microenvironments. Recently, aerosol optical tweezers coupling with Raman spectroscopy have been applied to measure the pH of single trapped microdroplets by utilizing conjugate acid-base equilibrium to infer pH shifts. However, such measurements are easily affected by many factors such as variations in detecting volumes and laser intensities, making it hard to directly determine these acid and base concentrations through their respective peak areas. To overcome these problems and accurately measure the concentrations of SO₄^2- and HSO₄⁻ within individual NaHSO₄ microdroplets, in this study a ratio-metric spectroscopic method is developed based on the peak area ratio of ν(SO₄²⁻)/ν(OH) and ν(HSO₄⁻)/ν(OH). Combined with the ion balance and ion activity coefficients, droplet pH is determined unambiguously. These experiment results were further used to evaluate the performance of activity models and thermodynamic models associated with aerosol pH, ion concentration and activity coefficient predictions. Pitzer, Simonson, and Clegg (PSC) model provides the best predictions of ion activity coefficients Extended Aerosol Inorganics Model vision IV (E-AIM IV) works well over a wide NaHSO₄ concentration range (0.4-8.8 mol/kg), while ACCENT Pitzer model predictions have extremely good agreement with the experiment results in low NaHSO₄ concentration condition (≤2.0 mol/kg). By contrast, ISORROPIA II shows relatively poor performance as compared with E-AIM IV.

How to cite: Li, M., Su, H., Zheng, G., Kuhn, U., Li, G., Ma, N., Pöschl, U., and Cheng, Y.: Individual Aerosol Droplet pH Measurement via a Ratio-metric Raman Method Using Aerosol Optical Tweezers: Evaluation of Thermodynamic and Activity Models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8057, https://doi.org/10.5194/egusphere-egu21-8057, 2021.

Effective density is one of the most important physical properties of atmospheric aerosol particles, and is linked to particle formation and aging process. Combined characterization of aerosol density, chemical composition, emission and aging processes may provide crucial information for better understanding their interactions and effects on environment and climate. In autumn of 2019, the effective density of sub-micrometer aerosol particles was measured in-situ at a heavily polluted rural site in the North China Plain (NCP). A tandem technique coupling a Centrifugal Particle Mass Analyzer (CPMA) with a differential mobility analyzer (DMA) and a Condensation Particle Counter (CPC) were used to determine the effective density of ambient aerosol particles with diameters of 50, 100, 150, 220 and 300 nm. The probability distribution of effective density exhibits double peak modes in majority cases, with a higher density mode (main-density) and a lower density mode (sub-density). The existence of sub-density particles normally ascribed to freshly emitted or partial aged black carbon (BC) with non-spherical morphology. The number fraction of sub-density mode varies from 4% to 67%, with mean of 22-27% at five particle sizes. Due to the higher aging degree of larger particles, the main-density exhibits an evident ascending trend with particle size. However, the sub-density decreases as mobility size increases, from 0.89 g/cm³ at 50 nm to 0.62 g/cm³ at 300 nm, since larger fresh soot particles usually present a more agglomerated morphology than small particles. A comparison was carried out between the mean effective density at 300 nm and ACSM-derived density using different approximations of BC density. The best agreement is achieved when assuming a BC density of 0.6 g/cm³, indicating that BC typically exists as non-spherical particles with fractal-like or porous morphology in the NCP in cold season.

How to cite: Zhou, Y., Ma, N., Wang, Z., Tao, J., Hong, J., Peng, L., Xie, L., Zhu, S., Chen, C., Zhang, Q., Su, H., and Cheng, Y.: Size-resolved effective density of submicron aerosol particles in the North China Plain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16424, https://doi.org/10.5194/egusphere-egu21-16424, 2021.

Simultaneous measurements of aerosol hygroscopicity and chemical composition were performed at a suburban site in the North China Plain in winter 2018 using a self-assembled hygroscopic tandem differential mobility analyzer (H-TDMA) and a capture-vaporizer time-of-flight aerosol chemical speciation monitor (CV-ToF-ACSM), respectively. During the experimental period, aerosol particles usually show an external mixture in terms of hygroscopicity, with a less hygroscopic particles mode (LH) and a more hygroscopic mode (MH). The average ensemble mean hygroscopicity parameter (κ_mean) are 0.16, 0.18, 0.16, and 0.15 for 60, 100, 150, and 200 nm particles, respectively. Two episodes with different RH/T conditions and secondary aerosol formations are distinguished. Higher aerosol hygroscopicity is observed for all measured sizes in the high RH episode (HRH) than in the low RH episode (LRH). In LRH, κ decreases as the particle size increases, which may be explained by the large contribution of non- or less-hygroscopic primary compounds in large particles due to the enhanced domestic heating emissions at low temperature. The number fraction of LH mode at 200 nm even exceeds 50%. Closure analysis is carried out between the HTDMA-measured κ and the ACSM-derived hygroscopicity using different approximations for the hygroscopic parameters of organic compounds (κ_org). The results indicate that κ_org is less sensitive towards the variation of its oxidation level under HRH conditions but has a stronger O: C-dependency under LRH conditions. The difference in the chemical composition and their corresponding physical properties under different RH/T conditions reflects potentially different formation mechanisms of secondary organic aerosols at those two distinct episodes.

How to cite: Shi, J., Hong, J., Ma, N., Luo, Q., Xu, H., Tan, H., He, Y., wang, Q., Tao, J., Zhou, Y., Peng, L., Cheng, Y., Su, H., and Sun, Y.: On the difference of aerosol hygroscopicity between high and low RH environment in the North China Plain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13951, https://doi.org/10.5194/egusphere-egu21-13951, 2021.

Size-dependent solubility is prevalent in atmospheric nanoparticles, but a molecular level understanding is still insufficient, especially for organic compounds. Here, we performed molecular dynamics simulations to investigate the size dependence of succinic acid solvation on the scale of ~1-4 nm with the potential of mean forces method. Our analyses reveal that the surface preference of succinic acid is stronger for a droplet than the slab of the same size, and the surface propensity is enhanced due to the curvature effect as the droplet becomes smaller. Energetic analyses show that such surface preference is primarily an enthalpic effect in both systems, while the entropic effect further enhances the surface propensity in droplets. On the other hand, with decreasing droplet size, the solubility of succinic acid in the internal bulk volume may decrease, imposing an opposite effect on the size dependence of solubility as compared with the enhanced surface propensity. Meanwhile, structural analyses, however, show that the surface to internal bulk volume ratio increases drastically, especially when considering the surface in respect to succinic acid, e.g., for droplet with radius of 1 nm, the internal bulk volume would be already close to zero for the succinic acid molecule.

How to cite: Chen, C., Wang, X., Binder, K., Ghahremanpour, M. M., van der Spoel, D., Pöschl, U., Su, H., and Cheng, Y.: Energetic analysis of succinic acid in water droplets: insight into the size-dependent solubility of atmospheric nanoparticles, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6415, https://doi.org/10.5194/egusphere-egu21-6415, 2021.

This study aims at comparing the gas-to-particle conversion mechanisms adopted by regional chemical transport models (CTMs). We use the results from twelve regional CTMs from the third phase of the Model Inter-Comparison Study for Asia (MICS-Asia III). The simulations are conducted over East Asia for the whole year of 2010. The models used are WRF-CMAQ (version 4.7.1 and v5.0.2), WRF-Chem (v3.6.1 and v3.7.1), GEOS-Chem, NHM-Chem, NAQPMS and NU-WRF. Measurements from 54 EANET sites, 86 sites of the Air Pollution Indices (API) and 35 local sites, remote sensing products from AERONET and satellite data from MODIS are used to evaluate model performance on PM₁₀, PM_2.5 and its components and aerosol optical depth (AOD). To investigate the inter-model differences in secondary aerosol formation, we compare the Sulfur Oxidation Ratio (SOR) and Nitrogen Oxidation Ratio (NOR) values by different models with observations at the EANET sites. The preliminary results show that the inter-model differences in the oxidation ratio (50%) are almost of the same magnitude as those in simulating the concentrations of particles. The results suggest large uncertainties in the gas-particle conversion process in modelling secondary aerosol formation.

How to cite: Tan, J., Fu, J., Carmichael, G., Su, H., and Cheng, Y.: Results on an Inter-model Comparison on Secondary Aerosol Formation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11154, https://doi.org/10.5194/egusphere-egu21-11154, 2021.

Ammonium salts (NH₄⁺) is the important component of PM_2.5 and has a significant impact on air quality, climate, human health, and natural ecosystems. The contribution of NH₄⁺ to PM_2.5 is increasing at urban sites. Ammonia (NH₃) with global emissions estimated at greater than 33 Tg(N) Yr^-1 is the only precursor of particulate NH₄⁺ in the atmosphere. Thus, it is important to understand the conversion kinetics from NH₃ to NH₄⁺ in the atmosphere. However, the uptake coefficient of NH₃ (γ_NH3) on aerosol particles are scarce at the present time. In this work, we reported the γ_NH3 on ambient PM_2.5 in Beijing and Shijiazhuang in China. The γ_NH3 values on ambient PM_2.5 are (1.13±12.4)×10^-4 and (6.88±40.7)×10^-4 in Shijiazhuang and Beijing, respectively. They are significantly lower than those on sulfuric acid droplet (0.1-1), aqueous surface (~5×10^-3-0.1) and acidified secondary organic aerosol (~10^-3-~10^-2), while are comparable with that on ice surface (5.3±2.2 ×10^-4) and on sulfuric acid in the presence of organic gases (2×10^-4-4×10^-3). An annual increase of γ_NH3 in the statistic sense is observed and the possible reason related to the aerosol acidity has also been discussed.

How to cite: Liu, Y., Feng, Z., Zhan, J., and Bao, X.: Heterogeneous uptake of NH3 on ambient PM2.5 in Beijing and Shijiazhuang: Possible influence of aerosol acidity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8105, https://doi.org/10.5194/egusphere-egu21-8105, 2021.

Organic aerosols (OA) are major components of fine particulate matter, yet their formation mechanism remains unclear, especially in polluted environments. Laboratory studies have shown that the OA formation processes may be different under irradiated and dark conditions, but few studies have explored this aspect in ambient air. Here we investigate the diurnal chemical composition and formation processes of OA in carbonaceous particles during winter in Beijing using aerosol time-of-flight mass spectrometry. Our results show that 84.5% of carbonaceous particles undergo aging processes, characterized with larger size and more secondary species compared to fresh carbonaceous particles, and present different chemical compositions of OA in the daytime and nighttime. During the day, organosulfates and oligomers exist in the aged carbonaceous particles, which are mixed with a higher abundance of nitrate compared with sulfate. At night, distinct spectral signatures of hydroxymethanesulfonate and organic nitrogen compounds, and a minor abundance of other hydroxyalkylsulfonates and high molecular weight organic compounds are present in the aged carbonaceous particles, which are mixed with a higher abundance of sulfate compared with nitrate. Our results indicate that photochemistry dominates the formation of OA under high oxidant concentrations in the daytime, while aqueous chemistry plays an important role in the formation of OA under high relative humidity in the nighttime. The findings can help improve the performance of air quality and climate models on OA simulation.

How to cite: Ma, T., Furutani, H., Duan, F., Kimoto, T., Ma, Y., Zhu, L., Huang, T., Toyoda, M., and He, K.: Distinct diurnal formation processes of organic aerosols in winter in Beijing, China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15866, https://doi.org/10.5194/egusphere-egu21-15866, 2021.

Organic aerosol (OA) has been considered as one of the most important uncertainties in climate modeling due to the complexity in presenting its chemical production and depletion mechanisms. To better understand the capability of climate models and probe into the associated uncertainties in simulating OA, we evaluate the Community Earth System Model version 2.1 (CESM2.1) configured with the Community Atmosphere Model version 6 (CAM6) with comprehensive tropospheric and stratospheric chemistry representation (CAM6-Chem), through a long-term simulation (1988–2019) with observations collected from multiple datasets in the United States. We find that CESM generally reproduces the inter-annual variation and seasonal cycle of OA mass concentration at surface layer with correlation of 0.40 as compared to ground observations, and systematically overestimates (69 %) in summer and underestimates (-19 %) in winter. Through a series of sensitivity simulations, we reveal that modeling bias is primarily related to the dominant fraction of monoterpene-formed secondary organic aerosol (SOA), and a strong positive correlation of 0.67 is found between monoterpene emission and modeling bias in eastern US during summer. In terms of vertical profile, the model prominently underestimates OA and monoterpene concentrations by 37–99 % and 82–99 % respectively in the upper air (>500 m) as validated against aircraft observations. Our study suggests that the current Volatility Basis Set (VBS) scheme applied in CESM might be parameterized with too high monoterpene SOA yields which subsequently result in strong SOA production near emission source area. We also find that the model has difficulty in reproducing the decreasing trend of surface OA in southeast US, probably because of employing pure gas VBS to represent isoprene SOA which is in reality mainly formed through multiphase chemistry, thus the influence of aerosol acidity and sulfate particle change on isoprene SOA formation has not been fully considered in the model. This study reveals the urgent need to improve the SOA modeling in climate models.

How to cite: Liu, Y., Dong, X., Wang, M., Emmons, L., Liu, Y., Liang, Y., Li, X., and Shrivastava, M.: Analysis of Secondary Organic Aerosol Simulation Bias in the Community Earth System Model (CESM2.1), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1915, https://doi.org/10.5194/egusphere-egu21-1915, 2021.

Secondary new particle formation is an important source of the number concentration of atmospheric aerosols. Despite relatively high coagulation sinks contributed by pre-existing aerosols, intensive new particle formation occurs frequently in polluted atmospheric environments such as in urban Beijing. Considering the measured concentrations of sulfuric acid and organic compounds, the contrast between the high coagulation sink and the frequent intensive NPF events in urban Beijing indicates an efficient nucleation mechanism. Based on long-term atmospheric measurements conducted at the campus of Beijing University of Chemical Technology, we show that sulfuric acid-amine nucleation is a governing mechanism to initiate new particle formation in urban Beijing. The molecular-level mechanism of sulfuric acid-amine nucleation, especially with low amine concentrations and high aerosol concentrations, are discussed. We present evidence for the existence of the missing amine molecules in the measured H₂SO₄-amine clusters. A neutral cluster needs to be ionized before it is detected by a mass spectrometer. Deprotonation or clustering with an additional reagent ion changes the stability of the original neutral cluster. Therefore, the amine molecules in neutral H₂SO₄-amine clusters may dissociate before detection. Combining measurements and cluster kinetic simulations, we show that although not directly detected, a considerable proportion of H₂SO₄ monomers exist in the form of (H₂SO₄)₁(amine)₁, where the amine is most likely to be dimethylamine or trimethylamine. The evaporation rate of (H₂SO₄)₁(amine)₁ is moderate and forming (H₂SO₄)₁(amine)₁ is a critical step for H₂SO₄-amine nucleation. According to nucleation theory, (H₂SO₄)₁(amine)₁ is the critical cluster at a low amine concentration, whereas H₂SO₄-amine nucleation may occur without a free energy barrier at a high amine concentration. The clustering between (H₂SO₄)₁(amine)₁ and (H₂SO₄)_n(amine)_n is a major reaction pathway for the initial growth of H₂SO₄-amine clusters. These findings are supported by the measured H₂SO₄ dimer concentration and its dependencies on amine concentrations and temperature in urban Beijing. Besides, the enhancement of cluster growth rate due to synergy between amines and ammonia are discussed.

How to cite: Cai, R., Yan, C., Zheng, J., Wang, L., Kulmala, M., and Jingkun, J.: Secondary new particle formation initiated by sulfuric acid-amine nucleation in Beijing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9729, https://doi.org/10.5194/egusphere-egu21-9729, 2021.

The formation of secondary aerosols (SA, including secondary organic and inorganic aerosols, SOA and SIA) were the dominant sources of aerosol particles in the North China Plain and can result in significant variations of particle size distribution (PNSD) and hygroscopicity. Earlier studies have shown that the mechanism of SA formation can be affected by relative humidity (RH), and thus has different influences on the aerosol hygroscopicity and PNSD under different RH conditions. Based on the measurements of size-resolved particle activation ratio (SPAR), hygroscopicity distribution (GF-PDF), PM_2.5 chemical composition, PNSD, meteorology and gaseous pollutants in a recent field campaign McFAN (Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain) conducted at Gucheng site from November 16^th to December 16^th in 2018, the influences of SA formation on CCN activity and CCN number concentration (N_CCN) calculation at super-saturation of 0.05% under different RH conditions were studied. Measurements showed that during daytime, SA formation could lead to a significant increase in N_CCN and a strong diurnal variation in CCN activity. During periods with daytime minimum RH exceeding 50% (high RH conditions), SA formation significantly contributed to the particle mass/size changes in wide particle size range of 150 nm to 1000 nm, and led to an increase of N_CCN in particle size range of 200 nm to 300 nm, while increases in particle mass concentration mainly occurred within particle sizes larger than 300nm. During periods with daytime minimum RH below 30% in (low RH conditions), SA formation mainly contributed to the particle mass/size and N_CCN changes in particle sizes smaller than 300 nm. As a result, under the same amount SA formation induced mass increase, the increase of N_CCN was weaker under high RH conditions, while stronger under low RH conditions. Moreover, the diurnal variations of aerosol mixing state (inferred from CCN measurements) due to SA formation was different under different RH conditions. If the variations of the aerosol mixing state were not considered, estimations of N_CCN would bear significant deviations. By applying aerosol mixing state estimated by number fraction of hygroscopic particles from measurements of particle hygroscopicity or mass fraction of SA from measurements of particle chemical compositions, N_CCN calculation can be largely improved with relative deviation within 30%. This study improves the understanding of the impact of SA formation on CCN activity and N_CCN calculation, which is of great significance for improving parameterization of SA formation in aerosol models and CCN calculation in climate models.

How to cite: Tao, J., Kuang, Y., Ma, N., Hong, J., Sun, Y., Xu, W., Zhang, Y., He, Y., Luo, Q., Xie, L., Su, H., and Cheng, Y.: Secondary aerosol formation alters CCN activity in the North China Plain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11204, https://doi.org/10.5194/egusphere-egu21-11204, 2021.

Organic aerosols (OA) that make up a large fraction (up to 90%) of the fine aerosol (PM_2.5) mass have severe impact on the Earth’s climate system and can cause adverse risk to human health. Diacids and related compounds are ubiquitous in PM_2.5 in different environments and accounts for a substantial fraction in OA. Because of their high water-solubility, they can influence the hygroscopic properties and capacity of cloud condensation nuclei formation activity of aerosols and thus affect the indirect radiative forcing in the atmosphere. However, their origins, secondary formation and transformations and seasonality are not fully understood yet. To better understand the seasonal characteristics, origins and photochemical processing of OA in the Tianjin region, North China, we studied the molecular distributions and seasonal variations of water-soluble diacids, oxoacids and α-dicarbonyls in PM_2.5 collected at an urban and a suburban sites in Tianjin, an ideal location to study the aerosols, over a one-year period from July 2018 to June 2019. We found significant changes in concentrations and composition of diacids and related compounds from season to season at both the sites. Here, based on the results obtained together with the meteorology, oxidants (O₃ and NO₂) and SO₂, loading and the backward air mass trajectories, we discuss the possible origins and possible secondary formation pathways of diacids and related compounds in the Tianjin region.

How to cite: Li, P., Pavuluri, C. M., Dong, Z., Xu, Z., Fu, P., and Liu, C.-Q.: Seasonal Changes in Molecular Distributions of Diacids, Oxoacids and α-Dicarbonyls in PM2.5 in Tianjin, North China: Implications for Origins and Secondary Formation Pathways in Cold and Warm Periods, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13804, https://doi.org/10.5194/egusphere-egu21-13804, 2021.

Hygroscopic properties of 23 organic compounds with different physico-chemical properties including carboxylic acids, amino acids, sugars and sugar alcohols were measured using a Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA). We converted our experimental GF data of organics at 90% RH to κ to facilitate the comparison and we find that organic compounds with different molecular functionality present quite different hygroscopicity. Compounds with extra functional groups usually show higher hygroscopicity compared to their parental molecular compounds. Moreover, some compounds share the same molecular structure or functionality but vary differently in hygroscopicity. In general, the hygroscopicity of organics increase with functional groups in the following order: (-CH3/-NH2) < (-OH) < (-COOH/C=C/C=O). For highly soluble organics, the hygroscopicity decreases with molecular weight; while for slightly soluble organics which are not fully dissolved in aerosol droplets, their hygroscopicity can be divided into two categories. One is non-hygroscopic compounds, which may not fully deliquesce in the aerosol droplets. The other is moderate hygroscopic compounds, of which the hygroscopicity is mainly limited by their water solubility. Moreover, the hygroscopicity of organic compounds generally increased linearly with O:C ratios, although some of them have the same O:C ratio of but with different hygroscopicity. The experimental determined hygroscopicity are also compared with model predictions using the Extended Aerosol Inorganics Model (E-AIM) and the UManSysProp at 10-90% RH. Both models poorly represent the hygroscopic behavior of some organics, which may due to that the phase transition and intermolecular interactions are not considered in the simulations.

How to cite: han, S., Hong, J., Luo, Q., Xu, H., Tan, H., Wang, Q., Tao, J., Ma, N., Cheng, Y., and Su, H.: Hygroscopicity of organic compounds as a function of organic functionality, water solubility, molecular weight and organic oxidation level, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13985, https://doi.org/10.5194/egusphere-egu21-13985, 2021.