Enhanced aqueous formation and neutralization of fine atmospheric particles driven by extreme cold

Abstract

The prevailing view for aqueous secondary aerosol formation is that it occurs in clouds and fogs, owing to the large liquid water content compared to minute levels in fine particles. Our research indicates that this view may need reevaluation due to enhancements in aqueous reactions in highly concentrated small particles. Here, we show that low temperature can play a role through a unique effect on particle pH that can substantially modulate secondary aerosol formation. Marked increases in hydroxymethanesulfonate observed under extreme cold in Fairbanks, Alaska, demonstrate the effect. These findings provide insight on aqueous chemistry in fine particles under cold conditions expanding possible regions of secondary aerosol formation that are pH dependent beyond conditions of high liquid water.

Fig. 1.. Variation of PM_2.5 species and meteorological parameters during the ALPACA field study and characteristics of HMS relative to sulfate.

Time series of (A) PM_2.5 sulfate, PM_2.5 HMS, and temperature; (B) PM_2.5 organics, aerosol liquid water content (ALWC) calculated by ISORROPIA-Lite, and RH during the ALPACA campaign. The polluted period is highlighted in gray. HMS shown here is calculated as 70% of total S(IV) (see Materials and Methods). (C) Mass ratio of HMS to sulfate versus ambient temperature for PM_2.5 and TSPs. Data denoted by points with X’s that do not follow the trend with temperature correspond to a warm period of high RH when cloud/fog influences may have occurred. (D) Size distributions of HMS and sulfate collected with a multistage cascade impactor (54, 62) during the polluted period (10:00 a.m. 30 January to 9:00 a.m. 1 February), the lognormal fit with geometric mean particle diameter (Dpg; in micrometers) is shown for each).

Fig. 2.. Summary of observations and behavior of fine-particle pH in Fairbanks and relationship to TA and sulfate.

(A) Predicted pH time series during the campaign colored by molar ammonia partitioning (εTA), with the polluted period highlighted in gray with 30-min time resolution. (B) pH frequency distribution for the whole campaign and for the polluted period (29 January to 5 February). (C) Total ammonium (TA) versus total sulfate (TS) (i.e., HSO₄⁻ + SO₄²⁻), colored by pH, for the ALPACA campaign. Gray dashed line is 2:1 (a molar ratio of 2).

Fig. 3.. Sensitivity of fine-particle pH to TA and TS as a function of temperature.

(A) The importance of how TA/TS changes with temperature. This can be visualized as the pH change on the dotted vertical line where TS = 10 μg/m³ in plots (B) to (E). (B to E) The pH dependence on TA and TS at varying temperatures, with the white dashed line indicating where the molar TA/TS = 2. (B) 233 K, (C) 253 K, (D) 273 K, and (E) 293 K. Plots (B) to (E) all have the same x axis and y axis, and all use the same pH colorbar shown on the right. Mean campaign masses of chloride, PM_2.5 NVCs, and TN were used and held constant in all plots (tables S2 and S3). RH was held constant at 75%. The buffering pH range of sulfate decreases slightly as temperature decreases, while the ammonia buffering range increases markedly.

Fig. 4.. Fine-particle acidity at which ammonia gas-particle partitioning provides maximum buffering as a function of aerosol water and ambient temperature.

pH associated with peak ammonia buffering (εTA = 0.5) versus temperature and ALWC, plotted with mean values for various campaigns that calculated ALWC. Fairbanks, Alaska, is this campaign, with no organic water included in ALWC. Fairbanks* is this campaign at the lowest measured temperature and no organic water. NCP are conditions for severe winter haze in China (24, 32). Beijing, China, is typical wintertime conditions (63). Cabauw, The Netherlands, is the yearly mean conditions (29). Pasadena, California, is mean summertime conditions (31). Southeast (SE) US is mean summertime conditions in Centreville, Alabama (26, 30). Crete, Greece, is for fall conditions (64). Northeastern (NE) US is wintertime aircraft measurements (43).