Fine particle pH and the partitioning of nitric acid during winter in the northeastern United States

Abstract

Particle pH is a critical but poorly constrained quantity that affects many aerosol processes and properties, including aerosol composition, concentrations, and toxicity. We assess PM1 pH as a function of geographical location and altitude, focusing on the northeastern U.S., based on aircraft measurements from the Wintertime Investigation of Transport, Emissions, and Reactivity campaign (1 February to 15 March 2015). Particle pH and water were predicted with the ISORROPIA‐II thermodynamic model and validated by comparing predicted to observed partitioning of inorganic nitrate between the gas and particle phases. Good agreement was found for relative humidity (RH) above 40%; at lower RH observed particle nitrate was higher than predicted, possibly due to organic‐inorganic phase separations or nitrate measurement uncertainties associated with low concentrations (nitrate < 1 µg m−3). Including refractory ions in the pH calculations did not improve model predictions, suggesting they were externally mixed with PM1 sulfate, nitrate, and ammonium. Sample line volatilization artifacts were found to be minimal. Overall, particle pH for altitudes up to 5000 m ranged between −0.51 and 1.9 (10th and 90th percentiles) with a study mean of 0.77 ± 0.96, similar to those reported for the southeastern U.S. and eastern Mediterranean. This expansive aircraft data set is used to investigate causes in variability in pH and pH‐dependent aerosol components, such as PM1 nitrate, over a wide range of temperatures (−21 to 19°C), RH (20 to 95%), inorganic gas, and particle concentrations and also provides further evidence that particles with low pH are ubiquitous.

The effects of isoprene and NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; on secondary organic aerosols formed through reversible and irreversible uptake to aerosol water

“…However, it is worth noting that particle acidity is not likely a factor in the relative split between reversible and irreversible aqSOA formed from isoprene. Studies predict that particles in the eastern USA are highly acidic throughout the year Battaglia et al, 2017;Guo et al, 2016Guo et al, , 2015, and acidity is not a limiting factor in isoprene SOA formation during the summer Xu et al, 2015). The implication from our observations is that reversible aqSOA from isoprene forms even in the presence of such persistently acidic particles.…”

Profiling aerosol optical, microphysical and hygroscopic properties in ambient conditions by combining in situ and remote sensing

“…The model performance has been evaluated against comprehensive ambient datasets over a wide range of acidities, RH and temperatures (Fountoukis and Nenes, 2007;Fountoukis et al, 2009;Hennigan et al, 2015;Guo et al, 2015;Weber et al, 2016;Guo et al, 2016). In our analysis, we also consider the contribution of hygroscopic organics to the water uptake of the aerosol using the approach of Guo et al (2015).…”

“…Both organic and inorganic compounds contained within aerosol can drive the formation of a liquid aerosol phase (e.g., Guo et al, 2016). Aerosol water uptake changes the particle size and refractive index with profound implications for radiative transfer and cloud formation (e.g., Quinn et al, 2005).…”

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

“…(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…”