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Highly time-resolved measurements of SO4 2− , NO3 − , and NH4 + in PM2.5 were simultaneously performed at an urban site and downwind rural site in Beijing during the 2008 Olympics to investigate the impacts of control measures and regional transport. The mean concentrations (± standard deviations) of SO4 2− , NO3 − , and NH4 + were 18.23 (± 19.96), 9.47 (± 11.41), and 9.70 (± 8.92) µg/m 3 , respectively, at the rural site. These concentrations were comparable to those of 20.74 (± 20.36), 8.83 (± 9.51), and 10.85 (± 8.99) µg/m 3 at the urban site. Clear diurnal variations of SO4 2− , NO3 − , and NH4 + were observed at both sites, and were related to meteorological conditions, primary emissions, and regional transport. The effectiveness of the control measures on SO4 2− , NO3 − , and NH4 + was evaluated by comparing the urban site concentrations during three periods: before the full-scale control, after the full-scale control but before the Olympics, and during the Olympics. The high pollution observed after the full-scale control was attributed to regional transport from the sector south of Beijing. The samples in the air masses from the northwest were selected to minimize the influences of meteorological factors and regional transport, and the results showed a clear reduction of SO4 2− , NO3 − , and NH4 + concentrations (approximately 35%– 69%) after the full-scale control began, suggesting the effectiveness of the control measures in reducing the local secondary inorganic aerosols. A widespread pollution episode was observed during August 3–10 at the rural site, with regional transport being identified as the main contributor. Secondary transformation evidently occurred during August 3–4 and contributed more than 50% of the rural secondary ion concentrations. During August 5–10, the whole region experienced a stable and well-developed plume.

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.