Sulfate formation is dominated by manganese-catalyzed oxidation of SO2 on aerosol surfaces during haze events

The formation mechanism of aerosol sulfate during wintertime haze events in China is still largely unknown. As companions, SO2 and transition metals are mainly emitted from coal combustion. Here, we argue that the transition metal-catalyzed oxidation of SO2 on aerosol surfaces could be the dominant sulfate formation pathway and investigate this hypothesis by integrating chamber experiments, numerical simulations and in-field observations. Our analysis shows that the contribution of the manganese-catalyzed oxidation of SO2 on aerosol surfaces is approximately one to two orders of magnitude larger than previously known routes, and contributes 69.2% ± 5.0% of the particulate sulfur production during haze events. This formation pathway could explain the missing source of sulfate and improve the understanding of atmospheric chemistry and climate change.

Direct and catalytic contribution of formaldehyde to particulate matter?

<p>Formaldehyde (HCHO) is produced mainly via photochemical oxidation of volatile organic compounds as well as direct emissions mainly from combustion processes. HCHO has a high vapor pressure but as a result of the hydration of the aldehyde group, it has a Henry’s law constant that allows it to partition into cloud droplets. We present results of two different pathways through which HCHO may contribute to the mass of particulate matter: Formation of hydroxymethanesulfonate (HMS) from reaction of HCHO with dissolved sulfur dioxide (SO<sub>2aq</sub>) and formation of sulfate by reaction of HCHO with hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) to form hydroxyl methyl hydroperoxide (HMHP), which in turn can oxidize SO<sub>2aq</sub> to sulfate and reform HCHO. The former pathway contributes to both the carbon and sulfur component of particulate matter whereas the latter contributes to the sulfur particulate budget and suggests a catalytic role of formaldehyde.</p><p>We combine laboratory kinetics studies of these reactions with model simulations using GEOS-Chem. The model simulations are analyzed at regional and global scales under present day and simplified preindustrial conditions, in which all anthropogenic emissions are set to zero. The analysis suggests that, depending on conditions, these processes may have significant impact on the sulfur particulate matter budget, specifically the rate of particulate sulfur formation. The results also suggest that under conditions that favor HMS formation, HMS may be the most abundant single organic molecule contributing particulate matter carbon.</p>

On Aerosol Liquid Water and Sulfate Associations: The Potential for Fine Particulate Matter Biases

In humid locations of the Eastern U.S., sulfate is a surrogate for aerosol liquid water (ALW), a poorly measured particle constituent. Regional and seasonal variation in ALW–sulfate relationships offers a potential explanation to reconcile epidemiology and toxicology studies regarding particulate sulfur and health endpoints. ALW facilitates transfer of polar species from the gas phase to the particle phase and affects particle pH and metal oxidation state. Though abundant and a potential indicator of adverse health endpoints, ALW is largely removed in most particulate matter measurement techniques, including in routine particulate matter (PM2.5) networks that use federal reference method (FRM) monitors, which are used in epidemiology studies. We find that in 2004, a typical year in the available record, ambient ALW mass is removed during sampling and filter equilibration to standard laboratory conditions at most (94%) sites, up to 85% of the ambient water mass. The removal of ALW can induce the evaporation of other semi-volatile compounds present in PM2.5, such as ammonium nitrate and numerous organics. This produces an artifact in the PM mass measurements that is, importantly, not uniform in space or time. This suggests that PM2.5 epidemiology studies that exclude ALW are biased. This work provides a plausible explanation to resolve multi-decade discrepancies regarding ambient sulfate and health impacts in some epidemiological and toxicological studies.

Particulate sulfur in the upper troposphere and lowermost stratosphere – sources and climate forcing

This study is based on fine-mode aerosol samples collected in the upper troposphere (UT) and the lowermost stratosphere (LMS) of the Northern Hemisphere extratropics during monthly intercontinental flights at 8.8–12 km altitude of the IAGOS-CARIBIC platform in the time period 1999–2014. The samples were analyzed for a large number of chemical elements using the accelerator-based methods PIXE (particle-induced X-ray emission) and PESA (particle elastic scattering analysis). Here the particulate sulfur concentrations, obtained by PIXE analysis, are investigated. In addition, the satellite-borne lidar aboard CALIPSO is used to study the stratospheric aerosol load. A steep gradient in particulate sulfur concentration extends several kilometers into the LMS, as a result of increasing dilution towards the tropopause of stratospheric, particulate sulfur-rich air. The stratospheric air is diluted with tropospheric air, forming the extratropical transition layer (ExTL). Observed concentrations are related to the distance to the dynamical tropopause. A linear regression methodology handled seasonal variation and impact from volcanism. This was used to convert each data point into stand-alone estimates of a concentration profile and column concentration of particulate sulfur in a 3 km altitude band above the tropopause. We find distinct responses to volcanic eruptions, and that this layer in the LMS has a significant contribution to the stratospheric aerosol optical depth and thus to its radiative forcing. Further, the origin of UT particulate sulfur shows strong seasonal variation. We find that tropospheric sources dominate during the fall as a result of downward transport of the Asian tropopause aerosol layer (ATAL) formed in the Asian monsoon, whereas transport down from the Junge layer is the main source of UT particulate sulfur in the first half of the year. In this latter part of the year, the stratosphere is the clearly dominating source of particulate sulfur in the UT during times of volcanic influence and under background conditions.

Particulate sulfur in the upper troposphere and lowermost stratosphere – sources and climate forcing

This study is based on fine mode aerosol samples collected in the upper troposphere (UT) and the lowermost stratosphere (LMS) of the northern hemisphere extratropics during monthly intercontinental flights of the IAGOS-CARIBIC platform in the time period 1999–2014. The samples were analyzed for a large number of chemical elements using the accelerator-based methods PIXE (particle-induced X-ray emission) and PESA (particle elastic scattering analysis). Here the particulate sulfur concentrations, obtained by PIXE analysis, are investigated. A steep gradient in particulate sulfur concentration extends several kilometers into the LMS, affected by increasing dilution of particulate sulfur-rich by stratospheric air towards the tropopause. Observed concentrations are related to the distance to the dynamical tropopause. A linear regression methodology revealing seasonal variation and impact from volcanism is used to convert each data point to standalone estimates of a concentration profile and column concentration of particulate sulfur in a 3 km altitude band above the tropopause. We find distinct responses to volcanic eruptions and a significant contribution to the stratospheric aerosol optical depth and radiative forcing of this lowest part of the LMS. Further, the origin of UT particulate sulfur shows a strong seasonal variation. We find that tropospheric sources dominate during summer and fall, whereas these sources make a small contribution during winter and spring. In these latter seasons the stratosphere is the clearly dominating source of particulate sulfur in the UT during moderate volcanic influence as well as background conditions.