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+]/[SO42–] = 1.5

In this study, 24-h PM2.5 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+]/[SO42−] 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+]/[SO42−] ≤ 1.5 and much lower for NH4+]/[SO42−] > 1.5. Therefore, higher nitrate principally formed via homogeneous gas phase reactions involving ammonia and nitric acid were observed for [NH4+]/[SO42−] > 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+]/[SO42−] ≤ 1.5. These demonstrate a clear transition in the pathways of ambient ammonia to form aerosol ammonium at [NH4+]/[SO42−] = 1.5 and evidently explain the observed high acidity due to the unneutralized sulfate in the ammonium-rich aerosol (NH4+]/[SO42−] > 1.5). In fact, the measured acidity was almost similar to the excess acid defined as the acid that remains at [NH4+]/[SO42−] = 1.5 due to the un-neutralized fraction of sulfate ([H+] = 0.5[SO42−]). 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.