Fine particle pH for Beijing winter haze as inferred from different thermodynamic equilibrium models

Abstract

Abstract. pH is an important property of aerosol particles but is difficult to measure directly. Several studies have estimated the pH values for fine particles in North China winter haze using thermodynamic models (i.e., E-AIM and ISORROPIA) and ambient measurements. The reported pH values differ widely, ranging from close to 0 (highly acidic) to as high as 7 (neutral).In order to understand the reason for this discrepancy, we calculated pH values using these models with different assumptions 25 with regard to model inputs and particle phase states. We find that the large discrepancy is due primarily to differences in the model assumptions adopted in previous studies. Calculations using only aerosol phase composition as inputs (i.e., reverse mode) are sensitive to the measurement errors of ionic species and inferred pH values exhibit a bimodal distribution with peaks between −2 and 2 and between 7 and 10. Calculations using total (gas plus aerosol phase) measurements as inputs (i.e., forward mode) are affected much less by the measurement errors, and results are thus superior to those obtained from the reverse mode 30 calculations. Forward mode calculations in this and previous studies collectively indicate a moderately acidic condition (pH from about 4 to about 5) for fine particles in North China winter haze, indicating further that ammonia plays an important role in determining this property. The differences in pH predicted by the forward mode E-AIM and ISORROPIA calculations may be attributed mainly to differences in estimates of activity coefficients for hydrogen ions. The phase state assumed, which can be either stable (solid plus liquid) or metastable (only liquid), does not significantly impact pH predictions of ISORROPIA. 35Atmos. Chem. Phys. Discuss., https://doi

Aerosol pH Dynamics During Haze Periods in an Urban Environment in China: Use of Detailed, Hourly, Speciated Observations to Study the Role of Ammonia Availability and Secondary Aerosol Formation and Urban Environment

“…In addition to ISORROPIA‐II, we used E‐AIM to calculate the pH and the H + activity coefficients for some samples to compare the pH estimated by different models and to evaluate the effect of the H + activity coefficient on pH. For the same samples, we found the mean pH from E‐AIM was 4.1, a little bit lower than from ISORROPIA‐II (mean pH = 4.7), and the influence of H + activity coefficients on pH was weaker than that of H + molality (log 10 γ(H + ) = 0.32 ± 0.22, γ(H + ) is the activity coefficient of H + ), which agrees well with other recent assessments (Jia et al, ; Song et al, ).…”

“…Aerosol acidity, as characterized by pH (hydrogen ion activity expressed on a logarithmic scale), influences aerosol growth and concentrations via secondary aerosol formation and gas‐aerosol partitioning (Jang et al, ; H. Y. Li et al, ; Meskhidze et al, ; Pattatyus et al, ; Surratta et al, ). Fine particulate matter, much of which can be acidic (Lawal et al, ; Silvern et al, ; Song et al, , ; Weber et al, ), is also associated with harmful effects on human health. Particulate matter can also have negative environmental effects, such as reduced visibility and damage to ecological systems and historical monuments (Bouwman et al, ; Cheng et al, ; Huo et al, ; Ren et al, ; Tang et al, ; W. Wang, Liu, et al, ).…”

“…Specifically, the effect of the availability and behavior of NH 3 (g) and NH 4 + on aerosol pH is still poorly understood. In this study, we used ISORROPIA‐II (Fountoukis & Nenes, ; Guo et al, ; Song et al, ), a gas‐aerosol thermodynamics equilibrium model and detailed hourly field observations to investigate the relationships among aerosol pH, sources of emissions of WS ions and their precursors (especially NH 3 (g) and NH 4 + ), and secondary formation of inorganic aerosols. ISORROPIA‐II has been applied in many past studies and has proven to be effective in calculating aerosol water content and pH (Guo et al, ; Shi et al, ).…”

Contribution of Hydroxymethane Sulfonate to Ambient Particulate Matter: A Potential Explanation for High Particulate Sulfur During Severe Winter Haze in Beijing

“…Measurements from fog/low cloud water in Beijing showed a pH of 5.2–6.2 between 1999 and 2006 (Jiang et al, ). More recent observations infer an aerosol pH of ~4–5 during haze events (Guo et al, ; Liu et al, ; Song et al, ), which implies a somewhat higher pH in cloud droplets due to ion exchange between cloud and aerosol during cloud processing (Ervens, ; Liu et al, ). In addition, precipitation in Beijing reveals pH values of ~5.5–6 (Huo et al, ; Tang et al, ; Zhu et al, ).…”

Redistribution of PM2.5‐associated nitrate and ammonium during outdoor‐to‐indoor transport

“…T was varied by ±5K (288K and 298K) and RH by ±20% (30% and 70%), respectively. Although particle‐phase change (solid crystals may precipitate out from liquid phase) is not considered when RH drops below 30%‐40%, Song et al have shown that the metastable (a single liquid phase) and stable (solid + liquid phases) solutions predict similar pH.…”