Observational constraints on particle acidity using measurements and modelling of particles and gases

Abstract

In many parts of the world, the implementation of air quality regulations has led to significant decreases in SO emissions with minimal impact on NH emissions. In Canada and the United States, the molar ratio of NH : SO emissions has increased dramatically between 1990 and 2014. In many regions of North America, this will lead the molar ratio of NH : SO, where NH is the sum of particle phase NH and gas phase NH, and SO is the sum of particle phase HSO and SO, to exceed 2. A thermodynamic model (E-AIM model II) is used to investigate the sensitivity of particle pH, and the gas-particle partitioning of NH and inorganic nitrate, to the atmospheric NH : SO ratio. Steep increases in pH and the gas fraction of NH are found as NH : SO varies from below 1 to above 2. The sensitivity of the gas fraction of nitrate also depends strongly on temperature. The results show that if NH : SO exceeds 2, and the gas and particle phase NH are in equilibrium, the particle pH will be above 2. Observations of the composition of particulate matter and gas phase NH from two field campaigns in southern Canada in 2007 and 2012 have median NH : SO ratios of 3.8 and 25, respectively. These campaigns exhibited similar amounts of NH, but very different particle phase loadings. Under these conditions, the pH values calculated using the observations as input to the E-AIM model were in the range of 1-4. The pH values were typically higher at night because the higher relative humidity increased the particle water content, diluting the acidity. The assumption of equilibration between the gas and particle phase NH was evaluated by comparing the observed and modelled gas fraction of NH. In general, E-AIM was able to reproduce the partitioning well, suggesting that the dominant constituents contributing to particle acidity were measured, and that the estimated pH values were realistic. These results suggest that regions of the world where the ratio of NH : SO emissions is beginning to exceed 2 on a molar basis may be experiencing rapid increases in aerosol pH of 1-3 pH units. This could have important consequences for the rates of condensed phase reactions that are acid-catalyzed.

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

“…The above results suggest that the reverse mode calculations (only using aerosol quantity as model inputs) are strongly affected by the ion balance and hence the ionic measurement errors are very likely to lead to unreliable estimates of particle pH (Murphy et al, 2017;Hennigan et al, 2015). Furthermore, an equivalent ratio of near unity may not indicate that fine particles of winter haze have a pH of around 7 or close to 7 Cheng et al, 2016;Ma et al, 2017).…”

“…The samples with negative ion balance (cations < anions) usually project pH values below 2 (highly acidic), whereas those with positive ion balance (cations > anions) are identified with pH values above 7.4 (neutral or basic). These features have been demonstrated also by Hennigan et al (2015) and Murphy et al (2017) using different observational datasets. Since 25 the inputs to reverse mode calculations include only aerosol phase measurements of ions, the predicted pH values depend largely on the ion balance (Hennigan et al, 2015).…”

“…(2-5) respectively. Although they are straightforward to calculate, a few recent studies (Murphy et al, 2017;Hennigan et al, 2015;Guo et al, 2016) have demonstrated that the ion balance and equivalent ratio calculated from ambient particle measurements…”

“…A few studies have determined the pH of laboratory generated particles using colorimetry/spectrometry and Raman microspectroscopy (Li and Jang, 2012;Rindelaub et al, 2016;Craig et al, 2017), but such techniques have not been applied to ambient particles partly due to their much more complex chemical and physical 15 properties. Therefore, an indirect (or proxy) method-thermodynamic equilibrium modeling-has been widely used to estimate particle pH for many regions of the world (Bougiatioti et al, 2016;Guo et al, 2017a;Parworth et al, 2017;Murphy et al, 2017;Yao et al, 2007;Zhang et al, 2007). A number of thermodynamic models have been developed, subject to the principle of minimizing the Gibbs energy of the multi-phase aerosol system, leading to a computationally intensive optimization problem.…”

Chemical Characterisation of Water-soluble Ions in Atmospheric Particulate Matter on the East Coast of Peninsular Malaysia

“…365 To obtain error bars for the strong acidity predictions (Fig. 8) (Murphy et al, 2017). %RSDtot was estimated by combining the error of the recovery process for each ion (%RSDrec , Table S1) and the error of the instrument for each ion (%RSDins , Table S2).…”