Water can be a major component of aerosol particles, also serving as a medium for aqueous-phase reactions. In this study, a novel method is presented to calculate the aerosol liquid water content at high relative humidity based on measurements of aerosol hygroscopic growth factor, particle number size distribution and relative humidity in the Haze in China (HaChi) summer field campaign (July–August 2009) in the North China Plain. The aerosol liquid water content calculated using this method agreed well with that calculated using a thermodynamic equilibrium model (ISORROPIA II) at high relative humidity (>60%) with a correlation coefficient of 0.96. At low relative humidity (<60%), an underestimation was found in the calculated aerosol liquid water content by the thermodynamic equilibrium model. This discrepancy mainly resulted from the ISORROPIA II model, which only considered limited aerosol chemical compositions. The mean and maximum values of aerosol liquid water content during the HaChi campaign reached 1.69 × 10−4 g m−3 and 9.71 × 10−4 g m−3, respectively. A distinct diurnal variation of the aerosol liquid water content was found, with lower values during daytime and higher ones at night. The aerosol liquid water content depended strongly on the relative humidity. The aerosol liquid water content in the accumulation mode dominated the total aerosol liquid water content.

As has been the case in North America and western Europe, the SO₂ emissions have substantially reduced in the North China Plain (NCP) in recent years. Differential rates of reduction in SO₂ and NO_x concentrations result in the frequent occurrence of particulate matter pollution dominated by nitrate ($p{\mathrm{NO}}_{\mathrm{3}}^{-}$ ) over the NCP. In this study, we observed a polluted episode with the particulate nitrate mass fraction in nonrefractory PM₁ (NR-PM₁) being up to 44 % during wintertime in Beijing. Based on this typical $p{\mathrm{NO}}_{\mathrm{3}}^{-}$ -dominated haze event, the linkage between aerosol water uptake and $p{\mathrm{NO}}_{\mathrm{3}}^{-}$ enhancement, further impacting on visibility degradation, has been investigated based on field observations and theoretical calculations. During haze development, as ambient relative humidity (RH) increased from ∼10 % to 70 %, the aerosol particle liquid water increased from ∼1 µg m⁻³ at the beginning to ∼75 µg m⁻³ in the fully developed haze period. The aerosol liquid water further increased the aerosol surface area and volume, enhancing the condensational loss of N₂O₅ over particles. From the beginning to the fully developed haze, the condensational loss of N₂O₅ increased by a factor of 20 when only considering aerosol surface area and volume of dry particles, while increasing by a factor of 25 when considering extra surface area and volume due to water uptake. Furthermore, aerosol liquid water favored the thermodynamic equilibrium of HNO₃ in the particle phase under the supersaturated HNO₃ and NH₃ in the atmosphere. All the above results demonstrated that $p{\mathrm{NO}}_{\mathrm{3}}^{-}$ is enhanced by aerosol water uptake with elevated ambient RH during haze development, in turn facilitating the aerosol take-up of water due to the hygroscopicity of particulate nitrate salt. Such mutual promotion between aerosol particle liquid water and particulate nitrate enhancement can rapidly degrade air quality and halve visibility within 1 d. Reduction of nitrogen-containing gaseous precursors, e.g., by control of traffic emissions, is essential in mitigating severe haze events in the NCP.

The aim of this investigation was to obtain a better understanding of the variability of the cloud condensation nuclei (CCN) activity during new particle formation (NPF) events in an anthropogenically polluted atmosphere of the North China Plain (NCP). We investigated the size-resolved activation ratio as well as particle number size distribution, hygroscopicity, and volatility during a 4-week intensive field experiment in summertime at a regional atmospheric observatory in Xianghe. Interestingly, based on a case study, two types of NPF events were found, in which the newly formed particles exhibited either a higher or a lower hygroscopicity. Therefore, the CCN activity of newly formed particles in different NPF events was largely different, indicating that a simple parameterization of particle CCN activity during NPF events over the NCP might lead to poor estimates of CCN number concentration (N_CCN). For a more accurate estimation of the potential N_CCN during NPF events, the variation of CCN activity has to be taken into account. Considering that a fixed activation ratio curve or critical diameter are usually used to calculate N_CCN, the influence of the variation of particle CCN activity on the calculation of N_CCN during NPF events was evaluated based on the two parameterizations. It was found that N_CCN might be underestimated by up to 30 % if a single activation ratio curve (representative of the region and season) were to be used in the calculation; and might be underestimated by up to 50 % if a fixed critical diameter (representative of the region and season) were used. Therefore, we suggest not using a fixed critical diameter in the prediction of N_CCN in NPF. If real-time CCN activity data are not available, using a proper fixed activation ratio curve can be an alternative but compromised choice.

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.

Aerosol hygroscopicity is crucial for understanding roles of aerosol particles in atmospheric chemistry and aerosol climate effects. Light-scattering enhancement factor f(RH, λ) is one of the parameters describing aerosol hygroscopicity, which is defined as f(RH, λ) = σ_sp(RH, λ)∕σ_sp(dry, λ), where σ_sp(RH, λ) or σ_sp(dry, λ) represents σ_sp at wavelength λ under certain relative humidity (RH) or dry conditions. Traditionally, an overall hygroscopicity parameter κ can be retrieved from measured f(RH, λ), hereinafter referred to as κ_f(RH), by combining concurrently measured particle number size distribution (PNSD) and mass concentration of black carbon. In this paper, a new method is proposed to directly derive κ_f(RH) based only on measurements from a three-wavelength humidified nephelometer system. The advantage of this newly proposed approach is that κ_f(RH) can be estimated without any additional information about PNSD and black carbon. This method is verified with measurements from two different field campaigns. Values of κ_f(RH) estimated from this new method agree very well with those retrieved by using the traditional method: all points lie near the 1 : 1 line and the square of correlation coefficient between them is 0.99. The verification results demonstrate that this newly proposed method of deriving κ_f(RH) is applicable at different sites and in seasons of the North China Plain and might also be applicable in other regions around the world.

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, V_a(dry), with a machine learning method called random forest model based on measurements of the dry nephelometer. The estimated V_a(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 V_a(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.