Characterizing the thermodynamic and chemical composition factors controlling PM2.5 nitrate: Insights gained from two years of online measurements in Hong Kong. (December 2015)

• Record Type: Journal Article
• Authors: Griffith, Stephen M.; Huang, X.H. Hilda; Louie, P.K.K.; Yu, Jian Zhen
• Is Part Of: Atmospheric environment. Volume 122(2015)
• Page Start: 864
• Page End: 875
• Publication Date: 2015-12
• Subjects: PM2.5 nitrate characterization -- Seasonal trends -- Nitrate episodes -- Free NH4+ -- MARGA; Air -- Pollution -- Periodicals; Air -- Pollution -- Meteorological aspects -- Periodicals; 551.51
• DOI: 10.1016/j.atmosenv.2015.02.009
• Languages: English
• ISSNs: 1352-2310

Abstract

Abstract: In this study, we investigated the seasonal, diurnal, and episodic characteristics of aerosol nitrate concentrations in PM2.5 at a suburban receptor site in Hong Kong using an hourly MARGA sampled dataset. At the site, large spikes in the NO3 − concentration have been observed in all seasons, and are easily overlooked in datasets examining 24 h average concentrations. As a key component to PM2.5, nitrate constituted between 5 and 12% of the mass concentration on average per month, but contributed up to 25% during some episodic cases spanning only a few hours. Seasonal variations of PM2.5 nitrate concentrations at the site were driven by temperature and excess [NH4 + ] in the aerosol, defined as the amount of ammonium in excess of that required for satisfying [NH4 + ]/[SO4 2− ] = 1.5. The vast majority of winter nitrate data was associated with ammonium-rich aerosols ([NH4 + ]/[SO4 2− ] > 1.5), with the diurnal variation tracking the availability of excess [NH4 + ]. Distinctly different than winter conditions, the summer nitrate data was in ammonium-poor regime and tracked nitric acid concentrations, a photochemical tracer. A regression analysis of measured nitrate with the excess [NH4 + ] shows good correlation in spring, summer and winter (R 2 : 0.72–0.81), with slopes greater than 0.7 indicating the majority of excess NH4 + is associated with PM2.5 nitrate. It was found that measured nitrate exceeded excess [NH4 + ] in samples of low excess [NH4 + ] availability, leading to our finding that nitric acid attaching to sea salt and crustal particles in the fine mode is a non-negligible route (constituting up to ∼20% of the PM2.5 nitrate in this study) to assimilate nitrate into the PM2.5 aerosol. Accounting for both this minor route and the ammonia + nitric acid route may prove useful in modeling efforts to capture PM2.5 nitrate measurement fluctuations, particularly during events of a large influx of alkali particles, such as dust storms. Graphical abstract: Highlights: Seasonal, diurnal, and episodic PM2.5 nitrate trends are characterized. PM2.5 nitrate seasonal and diurnal trends track NH4 + availability and temperature. Episodic nitrate events occur throughout sampling period but persist more in winter. HNO3 adding to dust/sea salt particles is found to be important to PM2.5 nitrate.