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This study introduces an innovative outdoor nephelometer system designed to monitor the dynamic hygroscopic behavior of aerosols in ambient air. Field measurements conducted in the Pearl River Delta region of China unveil significant roles of relative humidity (RH) swings on aerosol phase states. Highlighting the occurrence of aerosol crystallization in the afternoon followed by gradual deliquescence as RH increases, particularly when minimum afternoon RH drops below 35%, with no such behavior observed when it exceeds 40%. Emphasizing that aerosol phase states are shaped not only by RH and chemical composition but also by their RH history, illustrating that RH levels of 70% in the morning or evening may correspond to fully dissolved metastable or partially dissolved metastable states of ambient aerosols. These findings underscore the need to account for RH history when predicting aerosol phase states and have broader implications for comprehending aerosol behavior and its atmospheric impacts. Plain Language Summary: This research introduces a new device to study how tiny particles called atmospheric aerosols in the air behave when they encounter moisture. We tested it in a region of China and discovered that relative humidity fluctuations from morning to evening impact significantly on phase state of atmospheric aerosols. In the afternoon, some of the particles might become solid, like tiny crystals, and later turn into liquid gradually as humidity increases. This phenomenon occurs when the afternoon minimum relative humidity is low, below 35%, but does not take place when it exceeds 40%. Suggesting that relative humidity history matters for aerosol phase. For example, relative humidity of 70% in the morning or evening, particles may be in a liquid or semi‐liquid state. This study helps us understand phase changes of atmospheric particles and their effects on our environment. Key Points: Innovative outdoor nephelometer system was developed to monitor aerosol hygroscopic behavior in ambient airAerosol crystallization in the afternoon followed by gradual deliquescence observedRelative humidity history plays significant roles in determining aerosol phase state [ABSTRACT FROM AUTHOR]

Submicron aerosol (PM1) species measured by aerosol mass spectrometers have been widely used to validate chemical transport models; however, the uncertainties due to chemical differences between PM1 and PM2.5 are poorly constrained. Here we characterized such differences in a highly polluted environment in north China in winter. Our results showed that the changes in PM1/PM2.5 ratios as a function of relative humidity (RH) were largely different for primary and secondary species. Secondary organic and inorganic aerosol (SOA and SIA) presented clear decreases in PM1/PM2.5 ratios at RH > 60% during periods with high SIA contributions (>50%), likely driven by the changes in aerosol hygroscopicity and phase states, while the traffic and coal combustion OA had limited dependence on RH. Thermodynamic modeling showed negligible impacts of PM differences on predictions of particle acidity, yet these impacts can cause a difference in aerosol water content by up to 50–70%. Plain Language Summary: Current air pollution studies rely largely upon aerosol mass spectrometers that provide real‐time measurements of submicron aerosol (PM1) species, and in many studies, PM1 aerosol species are used to validate those of PM2.5 in chemical transport models and estimate particle acidity and aerosol water content which are key parameters in studying heterogeneous reactions. However, the uncertainties in air pollution studies due to the chemical differences of PM1 and PM2.5 are poorly constrained, particularly in highly polluted environment, for example, China and India. We found that the changes in PM1/PM2.5 ratios as a function of relative humidity were largely different for primary and secondary aerosol species in highly polluted environment, which was likely driven by the changes in aerosol hygroscopicity and phase states. The chemical differences of PM1/PM2.5 ratios were also found to have negligible impacts on predictions of particle acidity, yet these impacts can cause a difference in aerosol water content by up to 50–70%. Considering the rapid increases in the deployments of aerosol mass spectrometers worldwide, the results in this study are of great importance for a better understanding of the uncertainties in both modelling and observations. Key Points: We observed large chemical differences between PM1 and PM2.5 under high relative humidity periods in highly polluted environmentThe RH dependence of PM1/PM2.5 ratios of primary and secondary species was largely different due to different hygroscopicityThe chemical differences between PM1 and PM2.5 have negligible influences on particle acidity prediction, yet have a large impact on AWC [ABSTRACT FROM AUTHOR]

The study of atmospheric nitrous acid (HONO), which is the primary source of OH radicals, is crucial with respect to understanding atmospheric photochemistry and heterogeneous chemical processes. Heterogeneous NO2 chemistry under haze conditions has been identified as one of the missing sources of HONO on the North China Plain, and also produces sulfate and nitrate. However, controversy exists regarding the various proposed HONO production mechanisms, mainly regarding whether SO2 directly takes part in the HONO production process and what roles NH3 and the pH value play. In this paper, never before seen explosive HONO production was reported and evidence was found – for the first time in field measurements during fog (usually with 4< pH <6) and haze episodes under high relative humidity (pH ≈4) – that NH3 was the key factor that promoted the hydrolysis of NO2 , leading to the explosive growth of HONO and nitrate under both high and relatively lower pH conditions. The results also suggest that SO2 plays a minor or insignificant role in HONO formation during fog and haze events, but was indirectly oxidized upon the photolysis of HONO via subsequent radical mechanisms. Aerosol hygroscopicity significantly increased with rapid inorganic secondary aerosol formation, further promoting HONO production as a positive feedback. For future photochemical and aerosol pollution abatement, it is crucial to introduce effective NH3 emission control measures, as NH3 -promoted NO2 hydrolysis is a large daytime HONO source, releasing large amounts of OH radicals upon photolysis, which will contribute largely to both atmospheric photochemistry and secondary aerosol formation. [ABSTRACT FROM AUTHOR]

The aerosol asymmetry factor (g) is one of the most important factors for assessing direct aerosol radiative forcing. However, little attention has been paid to the measurement and parameterization of g. In this study, the characteristics of g are studied based on field measurements over the North China Plain (NCP) using the Mie scattering theory. The results show that calculated g values for dry aerosol can vary over a wide range (between 0.54 and 0.67). Furthermore, when ambient relative humidity (RH) reaches 90 %, g is significantly enhanced by a factor of 1.2 due to aerosol hygroscopic growth. For the first time, a novel method of calculating g based on measurements from the humidified nephelometer system is proposed. This method can constrain the uncertainty of g to within 2.56% for dry aerosol populations and 4.02% for ambient aerosols, providing that aerosol hygroscopic growth is taken into account. Sensitivity studies show that aerosol hygroscopicity plays a vital role in the accuracy of predicting g. [ABSTRACT FROM AUTHOR]

Water condensed on ambient aerosol particles plays significant roles in atmospheric environment, atmospheric chemistry and climate. Before now, no instruments were available for real-time monitoring of ambient aerosol liquid water contents (ALWCs). In this paper, a novel method is proposed to calculate ambient ALWC based on measurements of a three-wavelength humidified nephelometer system, which measures aerosol light scattering coefficients and backscattering coefficients at three wavelengths under dry state and different relative humidity (RH) conditions, providing measurements of light scattering enhancement factor f (RH). The proposed ALWC calculation method includes two steps: the first step is the estimation of the dry state total volume concentration of ambient aerosol particles, Va(dry), with a machine learning method called random forest model based on measurements of the "dry" nephelometer. The estimated Va(dry) agrees well with the measured one. The second step is the estimation of the volume growth factor Vg(RH) of ambient aerosol particles due to water uptake, using f (RH) and the Ångström exponent. The ALWC is calculated from the estimated Va(dry) and Vg(RH). To validate the new method, the ambient ALWC calculated from measurements of the humidified nephelometer system during the Gucheng campaign was compared with ambient ALWC calculated from ISORROPIA thermodynamic model using aerosol chemistry data. A good agreement was achieved, with a slope and intercept of 1.14 and -8.6 μm3 cm-3 (r2 =0.92), respectively. The advantage of this new method is that the ambient ALWC can be obtained solely based on measurements of a three-wavelength humidified nephelometer system, facilitating the real-time monitoring of the ambient ALWC and promoting the study of aerosol liquid water and its role in atmospheric chemistry, secondary aerosol formation and climate change. [ABSTRACT FROM AUTHOR]

The number concentration of cloud condensation nuclei (CCN) plays a fundamental role in cloud physics. Instrumentations of direct measurements of CCN number concentration (NCCN) based on chamber technology are complex and costly; thus a simple way for measuring NCCN is needed. In this study, a new method for NCCN calculation based on measurements of a three-wavelength humidified nephelometer system is proposed. A three-wavelength humidified nephelometer system can measure the aerosol light-scattering coefficient (σsp) at three wavelengths and the light-scattering enhancement factor (fRH). The Ångström exponent (Å) inferred from σsp at three wavelengths provides information on mean predominate aerosol size, and hygroscopicity parameter (κ) can be calculated from the combination of fRH and Å. Given this, a lookup table that includes σsp, κ and Å is established to predict NCCN. Due to the precondition for the application, this new method is not suitable for externally mixed particles, large particles (e.g., dust and sea salt) or fresh aerosol particles. This method is validated with direct measurements of NCCN using a CCN counter on the North China Plain. Results show that relative deviations between calculated NCCN and measured NCCN are within 30 % and confirm the robustness of this method. This method enables simplerNCCN measurements because the humidified nephelometer system is easily operated and stable. Compared with the method using a CCN counter, another advantage of this newly proposed method is that it can obtain NCCN at lower supersaturations in the ambient atmosphere. [ABSTRACT FROM AUTHOR]

Aerosol surface area concentration ( S A ) is crucial for studying atmospheric chemical reactions happened in aerosol water or on aerosol surface. However, there is no commercial instrument that can provide direct measurements of ambient S A . In this paper, we propose a method to calculate ambient S A based only on measurements of a three-wavelength humidified nephelometer system, which measures aerosol optical properties at three wavelengths under dry state and different relative humidity (RH) conditions. Two critical steps are required in this method: (1) Calculating surface area concentration of ambient aerosols in dry state, S A (dry) , using a trained random forest machine learning model based only on optical properties measured by the "dry" nephelometer. The proposed machine learning method is evaluated with particle number size distributions (PNSD) datasets from eight field campaigns conducted on the North China Plain during different seasons. The square of correlation coefficients between predicted and calculated S A (dry) for PNSD is about 0.99, the average ratio between predicted and calculated S A (dry) is 1.01 and 70% of data points has a relative difference less than 10%. (2) Calculating the surface area growth factor f S (RH) of ambient aerosol particles due to water uptake using the proposed f S (RH) parameterization scheme f S (RH) = (1 + κ S R H 100 − R H) 2 3 , and the hygroscopicity parameter κ S can be calculated using measured aerosol light scattering enhancement factor and Å ngstr o ¨ m exponent. The ambient S A values during two field campaigns which are conducted on the North China Plain (Wangdu campaign and Gucheng campaign, in summer and winter respectively) are calculated using the proposed method. The ambient S A ranges from 42 to 1871 μ m 2 / c m 3 with an average of 319 μ m 2 / c m 3 during Wangdu campaign, and ranges from 19 to 4156 μ m 2 / c m 3 with an average of 788 μ m 2 / c m 3 during Gucheng campaign. Drastic daily variations of ambient S A are observed during these two campaigns. The results demonstrate that aerosol hygroscopic growth impacts significantly on variations in ambient S A especially under high RH conditions. The f S (RH) ranges from near 1 to 4 with an average of 1.4 during Wangdu campaign, and ranges from near 1 to 2.7 with an average of 1.3 during Gucheng campaign. The results indicate that the larger RH, the more sensitive f S (RH) becomes to variations in κ S which highlights that real-time measurements of aerosol hygroscopicity are required for accurate calculations of ambient S A . The advantage of the proposed method is that the ambient S A can be obtained solely based on measurements of a three-wavelength humidified nephelometer system, facilitating real-time measurements of ambient S A and promoting studies in aerosol heterogeneous reactions. • Humidified nephelometer system performs great in predicting aerosol surface area. • Proposed method can be used in different regions. • Aerosol hygroscopic growth impacts significantly on aerosol surface area. [ABSTRACT FROM AUTHOR]

Abstract The light scattering enhancement factor f (RH), defined as the ratio of the light scattering coefficient (σ S P ) observed under an elevated relative humidity (RH) conditions to that under dry conditions, is a crucial parameter for estimating aerosol direct radiative effects and atmospheric visibility. In this paper, a new f (RH) parameterization scheme considering both the influence of aerosol chemical compositions and that of particle number size distribution (PNSD) is proposed based on in situ measurements in the North China Plain. The development of this parameterization scheme includes three steps. First, aerosol hygroscopicity parameter κ is parameterized with aerosol chemical compositions. Then, the ratio between f (RH) fitted parameter κ s c a (f (R H) = 1 + κ s c a R H 100 − R H ) and κ is introduced to correlate f (RH) with κ. The ratio, influenced mostly by PNSD, is described as a function of scattering Ångström exponent (SAE) because the SAE can represent the predominant size of aerosol particles to some extent. Finally, f (RH) cycle is parameterized with κ s c a . Validation results show that a good consistency between the parameterized and measured f (RH = 80%) is achieved and the correlation coefficient is 0.80. This parameterization scheme between aerosol chemistry and f (RH) can be used in chemical transport models for reducing uncertainties in estimations of direct aerosol radiative forcing and air quality assessments. Highlights • A relationship between the f (RH) fitted parameter (κ sca) and the aerosol hygroscopicity parameter (κ f(RH)) is found. • The ratio of κ sca and κ f(RH) increases linearly with the increase of the scattering Ångström exponent. • A new parameterization of f (RH) considering the aerosol chemical compositions is developed. [ABSTRACT FROM AUTHOR]