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The ability of aerosol particles to uptake water (hygroscopic growth) is an important determinant of aerosol optical properties and radiative effects. Aerosol hygroscopic growth is traditionally measured by humidified tandem differential mobility analyzers (HTDMA), in which size-selected dry particles are exposed to elevated relative humidity (RH), and the size distribution of humidified particles is subsequently measured using a scanning mobility particle sizer. As a scanning mobility particle sizer can measure only one particle size at a time, HTDMA measurements are time consuming, and ambient measurements are often limited to a single RH level. Pinterich et al. (2017b) showed that fast measurements of aerosol hygroscopic growth are possible using a humidity-controlled fast integrated mobility spectrometer (HFIMS). In HFIMS, the size distribution of humidified particles is rapidly captured by a water-based fast integrated mobility spectrometer (WFIMS), leading to a factor of ∼10 increase in measurement time resolution. In this study we present a prototype HFIMS that extends fast hygroscopic growth measurements to a wide range of atmospherically relevant RH values, allowing for more comprehensive characterizations of aerosol hygroscopic growth. A dual-channel humidifier consisting of two humidity conditioners in parallel is employed such that aerosol RH can be quickly stepped among different RH levels by sampling from alternating conditioners. The measurement sequence is also optimized to minimize the transition time between different particle sizes. The HFIMS is capable of measuring aerosol hygroscopic growth of six particle diameters under five RH levels ranging from 20 % to 85 % (30 separate measurements) every 25 min. The performance of this HFIMS is characterized and validated using laboratory-generated ammonium sulfate aerosol standards. Measurements of ambient aerosols are shown to demonstrate the capability of HFIMS to capture the rapid evolution of aerosol hygroscopic growth and its dependence on both size and RH. [ABSTRACT FROM AUTHOR]

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 spatiotemporal variability of aerosol pH and sulfate formation during winter in the North China Plain (NCP), we have developed a new aerosol water chemistry (AWAC) module 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 spatiotemporal variation in the 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 increasing 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 the surface level were typically greater than 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 the 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. Sensitivity tests 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 dominant TMI pathway regimes to dominant NO2/O3 pathway regimes. [ABSTRACT FROM AUTHOR]

Understanding the formation mechanism of severe haze is crucial for the development of efficient pollution control strategy. Recently, multiphase reactions in aerosol water has been for the first time suggested as an important source of sulfate aerosol during severe haze (Zheng et al. 2015; Cheng et al. 2016; Wang et al. 2016). Though several oxidation mechanisms have been recognized, the dominant oxidation pathway is still under debate reflecting a missing consensus. Based on a model survey with Weather Research and Forecasting model coupled with Chemistry (WRF/Chem), we investigated the variability of aerosol pH and regimes of sulfate formation through multiphase oxidations during the haze episodes in January of 2013. Our results show a large spatial and temporal variability in the aerosol pH and sulfate formation regimes. Surface aerosol pH shows a clear diurnal variation with low pH during daytime and high pH during nighttime for most cases. Aerosol pH tends to decrease with increasing altitude in the boundary layer. For the scenario best reproduces the observations in Beijing, NO2, O3 and TMI reactions all can dominate the oxidation pathways of sulfate in northern and middle Beijing-Tianjin-Hebei (BTH) areas. H2O2 pathway has the greatest contribution over the most southern BTH areas. With the increasing height, O3 pathway and gas phase oxidation by OH radicals become more important. Moreover, our study also suggests that, emissions of crustal particles, NH3 and soluble iron/manganese have great impacts on aqueous phase chemistry, and should be better constrained in future studies. [ABSTRACT FROM AUTHOR]