High-Resolution Data Sets Unravel the Effects of Sources and Meteorological Conditions on Nitrate and Its Gas-Particle Partitioning

Abstract

Nitrate is one of the most abundant inorganic water-soluble ions in fine particulate matter (PM2.5). However, the formation mechanism of nitrate in the ambient atmosphere, especially the impacts of its semivolatility and the various existing forms of nitrogen, remain under-investigated. In this study, hourly ambient observations of speciated PM2.5 components (NO3 –, SO4 2–, etc.) were collected in Tianjin, China. Source contributions were analyzed by PMF/ME2 (Positive Matrix Factorization using the Multilinear Engine 2) program, and pH were estimated by ISORROPIA-II, to investigate the relationship between pH and nitrate. Five sources (factors) were resolved: secondary sulfate (SS), secondary nitrate (SN), dust, vehicle and coal combustion. SN and pH showed a triangle-shaped relationship. When SS was high, the fraction of nitrate partitioning into the aerosol phase exhibits a characteristic “S-curve” relationship with pH for different seasons. An index (I TL) is developed and combined with pH to explore the sensitive regions of “S-curve”. Controlling the emissions of anions (SO4 2–, Cl–), cations (Ca2+, Mg2+, etc.) and gases (NO x , NH3, SO2, etc.) will change pH, potentially reducing or increasing SN. The findings of this work provide an effective approach for exploring the formation mechanisms of nitrate under different influencing factors (sources, pH, and I RL).

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

“…NOR (average of 0.21) was higher on average than SOR (average of 0.09; Figure ), suggesting that there is faster nitrate generation than sulfate in winter (or, more precisely, faster oxidation of NO x versus SO 2 ), which can also be seen in the concentrations of nitrate and sulfate (Figure ). The NOR can be also be influenced by the change of partitioning between the particle and gas‐phase nitrate, which is affected by temperature and pH (Shi et al, ). Weak negative correlations between pH, SOR, and NOR were observed.…”

Observation of nitrate dominant PM<sub>2.5</sub> and particle pH elevation in urban Beijing during the winter of 2017

“…Considering that at the starting stage of PM2.5 pollutions, the relative humidity was usually lower (30% -50), the significant increase of hygroscopicity in nitrate rich particles might greatly enhance the uptake of precursor gases or promote secondary formations or precursor uptake processes e.g. ammonia partitioning and nitrate formation through partitioning or hydrolysis of N2O5 (Badger et al, 2006;Bertram and Thornton, 2009;Hodas et al, 2014;Shi et al, 2019). 330…”

“…Cheng et al conducted modeling work and suggest that Beijing's PM2.5 pH 70 ranged from 5.4-6.2, which is favorable for aqueous NO2 oxidation. Not only by modeling works, field observation and chamber study also support the NO2 oxidation's major contribution and address the importance of high ALWC as well as sufficient ammonia (Wang et al, 2016a;Chen et al, 2019). Meanwhile, particle pH during winter of 2015 to 2016 simulated by Liu et al with the same method was lower (3.0-4.9, average 4.2) and it was suggested that the acidic particle did not favor the NO2 oxidation mechanism.…”

The acidity of atmospheric particles and clouds

“…Other highly acidic pH values include 1.95 for a fog at Mt. Oyama in Japan (Mori et al, 1997); 1.94 for a fog in Dübendorf, Switzerland (Sigg et al, 1987); and 1.7 for a fog in Kahler Asten, Germany (Kroll and Winkler, 1988). Such acidic values are typically associated with large inputs of sulfuric and nitric acids, although hydrochloric acid has also been an important source of acidity in some urban areas (e.g., the Dübendorf fog).…”