pH of Aerosols in a Polluted Atmosphere: Source Contributions to Highly Acidic Aerosol

Abstract

Acidity (pH) plays a key role in the physical and chemical behavior of PM. However, understanding of how specific PM sources impact aerosol pH is rarely considered. Performing source apportionment of PM allows a unique link of sources pH of aerosol from the polluted city. Hourly water-soluble (WS) ions of PM were measured online from December 25th, 2014 to June 19th, 2015 in a northern city in China. Five sources were resolved including secondary nitrate (41%), secondary sulfate (26%), coal combustion (14%), mineral dust (11%), and vehicle exhaust (9%). The influence of source contributions to pH was estimated by ISORROPIA-II. The lowest aerosol pH levels were found at low WS-ion levels and then increased with increasing total ion levels, until high ion levels occur, at which point the aerosol becomes more acidic as both sulfate and nitrate increase. Ammonium levels increased nearly linearly with sulfate and nitrate until approximately 20 μg m, supporting that the ammonium in the aerosol was more limited by thermodynamics than source limitations, and aerosol pH responded more to the contributions of sources such as dust than levels of sulfate. Commonly used pH indicator ratios were not indicative of the pH estimated using the thermodynamic model.

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

“…The studies using the standard ISORROPIA model with the stable state assumption have predicted unrealistic pH values of around 7 and should be reevaluated He et al, 2017). Previous ISORROPIA calculations using the metastable state assumption 15 obtained average pH values from 4.1 to 5.4 (He et al, 2017;Liu et al, 2017a;Shi et al, 2017;Cheng et al, 2016;Guo et al, 2017b;Tan et al, 2018), agreeing reasonably with our results (an average pH value of 4.6). It is noted that concentrations of Ca 2+ and Mg 2+ were not measured in this study and a sensitivity test suggests that including these crustal cations in thermodynamic calculations would increase predicted pH values by about 0.1 unit (Fig.…”

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

“…During the less polluted period, average concentrations of SO 4 2− (8.9 μg/m 3 in episode A) were higher than those of NO 3 − (6.2 μg/m 3 in episode A), while over the entire period, average concentrations of SO 4 2− (11 μg/m 3 ) were lower than those of NO 3 − (13 μg/m 3 ). Our previous work also showed that when sulfate was dominant, pH could be low (<4; Shi et al, 2017).…”

“…In this work, x ij is the measured concentration of the j th WS ions in the i th sample, f pj is the concentration of the j th WS ions in p th source, and g ip is the contribution of p th source to TWI for the i th sample. More detailed information can be found in our previous works (Shi et al, ) and the supporting information.…”

Observation of nitrate dominant PM&lt;sub&gt;2.5&lt;/sub&gt; and particle pH elevation in urban Beijing during the winter of 2017

“…can predict H + , ALWC, and partitioning of volatile/semi volatile components, such as 65 ammonia (Fountoukis and Nenes, 2007;Clegg et al, 2008). These models' ability to describe physiochemical properties of airborne particles was validated by various studies Guo et al, 2016;Shi et al, 2017;Tao and Murphy, 2019;Murphy et al, 2017). However, several publications using the same method gave https://doi.org/10.5194/acp-2019-541 Preprint.…”