Fine particle pH and gas-particle phase partitioning of inorganic species in Pasadena, California, during the 2010 CalNex campaign

pH is a fundamental aerosol property that affects ambient particle concentration and composition, linking pH to all aerosol environmental impacts. Here, PM1 and PM2.5 pH are calculated based on data from measurements during the California Research at the Nexus of Air Quality and Climate Change (CalNex) study from 15 May to 15 June 2010 in Pasadena, CA. Particle pH and water were predicted with the ISORROPIA-II thermodynamic model and validated by comparing predicted to measured gas-particle partitioning of inorganic nitrate, ammonium, and chloride. The study mean ± standard deviation PM1 pH was 1.9 ± 0.5 for the SO4 2--NO3 --NH4 +-HNO3-NH3 system. For PM2.5, internal mixing of sea salt components (SO4 2--NO3 --NH4 +-Na+-Cl--K+-HNO3-NH3-HCl system) raised the bulk pH to 2.7 ± 0.3 and improved predicted nitric acid partitioning with PM2.5 components. The results show little effect of sea salt on PM1 pH, but significant effects on PM2.5 pH. A mean PM1 pH of 1.9 at Pasadena was approximately one unit higher than what we have reported in the southeastern US, despite similar temperature, relative humidity, and sulfate ranges, and is due to higher total nitrate concentrations (nitric acid plus nitrate) relative to sulfate, a situation where particle water is affected by semi-volatile nitrate concentrations. Under these conditions nitric acid partitioning can further promote nitrate formation by increasing aerosol water, which raises pH by dilution, further increasing nitric acid partitioning and resulting in a significant increase in fine particle nitrate and pH. This study provides insights into the complex interactions between particle pH and nitrate in a summertime coastal environment and a contrast to recently reported pH in the eastern US in summer and winter and the eastern Mediterranean. All studies have consistently found highly acidic PM1 with pH generally below 3. © Author(s) 2017.

Fine-particle water and pH in the southeastern United States

Hairun Guo, Athanasios Nenes

Particle water and pH are predicted using meteorological observations (relative humidity (RH), temperature (T)), gas/particle composition, and thermodynamic modeling (ISORROPIA-II). A comprehensive uncertainty analysis is included, and the model is validated. We investigate mass concentrations of particle water and related particle pH for ambient fine-mode aerosols sampled in a relatively remote Alabama forest during the Southern Oxidant and Aerosol Study (SOAS) in summer and at various sites in the southeastern US during different seasons, as part of the Southeastern Center for Air Pollution and Epidemiology (SCAPE) study. Particle water and pH are closely linked; pH is a measure of the particle H+ aqueous concentration and depends on both the presence of ions and amount of particle liquid water. Levels of particle water, in turn, are determined through water uptake by both the ionic species and organic compounds. Thermodynamic calculations based on measured ion concentrations can predict both pH and liquid water but may be biased since contributions of organic species to liquid water are not considered. In this study, contributions of both the inorganic and organic fractions to aerosol liquid water were considered, and predictions were in good agreement with measured liquid water based on differences in ambient and dry light scattering coefficients (prediction vs. measurement: slope = 0.91, intercept = 0.5 μg mg-3, R2 = 0.75). ISORROPIA-II predictions were confirmed by good agreement between predicted and measured ammonia concentrations (slope = 1.07, intercept = g-0.12 μg mg-3, R2 = 0.76). Based on this study, organic species on average contributed 35% to the total water, with a substantially higher contribution (50%) at night. However, not including contributions of organic water had a minor effect on pH (changes pH by 0.15 to 0.23 units), suggesting that predicted pH without consideration of organic water could be sufficient for the purposes of aqueous secondary organic aerosol (SOA) chemistry. The mean pH predicted in the Alabama forest (SOAS) was 0.94 ± 0.59 (median 0.93). pH diurnal trends followed liquid water and were driven mainly by variability in RH; during SOAS nighttime pH was near 1.5, while daytime pH was near 0.5. pH ranged from 0.5 to 2 in summer and 1 to 3 in the winter at other sites. The systematically low pH levels in the southeast may have important ramifications, such as significantly influencing acid-catalyzed reactions, gas-aerosol partitioning, and mobilization of redox metals and minerals. Particle ion balances or molar ratios, often used to infer pH, do not consider the dissociation state of individual ions or particle liquid water levels and do not correlate with particle pH. © Author(s) 2015. CC Attribution 3.0 License.

Copernicus GmbH2015

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

Hairun Guo, Athanasios Nenes

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 µgm-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 5000m 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. © 2016. The Authors.

Wiley-Blackwell2016

Marginal direct climate forcing by atmospheric aerosols

Athanasios Nenes

Previous research on the direct effect of atmospheric aerosols on climate has estimated the average radiative forcing per unit sulfate mass, and has used this average to calculate the magnitude and spatial distribution of sulfate forcing. In this paper, we posit that radiative forcing is often a nonlinear function of sulfate mass concentration. In contrast to measures of average forcing, we introduce the concept of 'marginal forcing', which is defined as the change in radiative forcing for an incremental change in sulfate concentration. A multi-component, size-resolved aerosol box model is used, which couples an aerosol chemical equilibrium model with a model for calculating radiative forcing based on Mie theory. The results for a typical nonurban continental aerosol show that total aerosol mass and radiative forcing are nonlinear functions of sulfate concentration. This nonlinearity is mainly due to the chemical interaction of sulfate with volatile inorganic components of the aerosol (ammonium, nitrate, and water). As a result, the marginal forcing varies significantly as a function of sulfate concentration; from - 550 to + 20 W (g SO4 2-)-1 at a relative humidity (RH) of 80%. Estimates of marginal forcing are strongly sensitive to RH. Absolute marginal forcing also decreases significantly with total nitrate concentration, increases with total ammonia concentration, and generally increases with temperature. We estimate that the bias in assuming a constant average forcing may cause overestimates in local continental aerosol radiative forcing by up to 50%, and in the marginal forcing by a factor of two or more. This bias is greatest at intermediate sulfate concentration, high RH, high total nitrate concentration, low total ammonia concentration( ≤ 2 μg m-3), and low temperature. Previous research on the direct effect of atmospheric aerosols on climate has estimated the average radiative forcing per unit sulfate mass, and has used this average to calculate the magnitude and spatial distribution of sulfate forcing. In this paper, we posit that radiative forcing is often a nonlinear function of sulfate mass concentration. In contrast to measures of average forcing, we introduce the concept of `marginal forcing', which is defined as the change in radiative forcing for an incremental change in sulfate concentration. A multi-component, size-resolved aerosol box model is used, which couples an aerosol chemical equilibrium model with a model for calculating radiative forcing based on Mie theory. The results for a typical nonurban continental aerosol show that total aerosol mass and radiative forcing are nonlinear functions of sulfate concentration. This nonlinearity is mainly due to the chemical interaction of sulfate with volatile inorganic components of the aerosol (ammonium, nitrate, and water). As a result, the marginal forcing varies significantly as a function of sulfate concentration, from -550 to +20 W(g SO4 2-)-1 at a relative humidity (RH) of 80%. Estimates of marginal forcing are strongly sensitive to RH. Absolute marginal forcing also decreases significantly with total nitrate concentration, increases with total ammonia concentration, and generally increases with temperature. We estimate that the bias in assuming a constant average forcing may cause overestimates in local continental aerosol radiative forcing by up to 50%, and in the marginal forcing by a factor of two or more. This bias is greatest at intermediate sulfate concentration, high RH, high total nitrate concentration, low total ammonia concentration (≥2 μg m-3), and low temperature.

Elsevier Sci Ltd1998

Fine-particle water and pH in the southeastern United States

Hairun Guo, Athanasios Nenes

Copernicus GmbH2015

Marginal direct climate forcing by atmospheric aerosols

Athanasios Nenes

Elsevier Sci Ltd1998