Evidence of high PM2.5 strong acidity in ammonia-rich atmosphere of Guangzhou, China: Transition in pathways of ambient ammonia to form aerosol ammonium at [NH₄⁺]/[SO₄^2-]=1.5

In this study, 24-h PM_2.5 samples were collected using Harvard Honeycomb denuder/filter-pack system during different seasons in 2006 and 2007 at an urban site in Guangzhou, China. The particles collected in this study were generally acidic (average strong acidity ([H⁺]) ~70 nmol m^-3). Interestingly, aerosol sulfate was not fully neutralized in the ammonia-rich atmosphere (NH₃~30 ppb) and even when NH₄⁺]/[SO₄^2-] was larger than 2. Consequently, strong acidity ([H⁺]) as high as 170 nmol m^-3 was observed in these samples. The kinetic rate of neutralization of acidity (acidic sulfate) by ambient ammonia was significantly higher than the rate of formation of ammonium nitrate involving HNO₃ and NH₃ for [NH₄⁺]/[SO₄^2-]≤1.5 and much lower for NH₄⁺]/[SO₄^2-]>1.5. Therefore, higher nitrate principally formed via homogeneous gas phase reactions involving ammonia and nitric acid were observed for [NH₄⁺]/[SO₄^2-]>1.5. However, little nitrate, probably formed via heterogeneous processes e.g. reaction of HNO₃ with sea salt or crustal species, was observed for [NH₄⁺]/[SO₄^2-]≤1.5. These demonstrate a clear transition in the pathways of ambient ammonia to form aerosol ammonium at [NH₄⁺]/[SO₄^2-]=1.5 and evidently explain the observed high acidity due to the unneutralized sulfate in the ammonium-rich aerosol (NH₄⁺]/[SO₄^2-]>1.5). In fact, the measured acidity was almost similar to the excess acid defined as the acid that remains at [NH₄⁺]/[SO₄^2-]=1.5 due to the un-neutralized fraction of sulfate ([H⁺]=0.5[SO₄^2-]). The presence of high excess acid and ammonium nitrate significantly lowered the deliquescence relative humidity of ammonium sulfate (from 80% to 40%) in the ammonium-rich samples. © 2010 Elsevier B.V.