Photochemical Oxidation of Water-Soluble Organic Carbon (WSOC) on Mineral Dust and Enhanced Organic Ammonium Formation

Water-soluble organic carbon (WSOC), which is closely related to biogenic emissions, is of great importance in the atmosphere for its ubiquitous existence and rich abundance. Levoglucosan, a typical WSOC, is usually considered to be stable and thus used as a tracer of biomass burning. However, we found that levoglucosan can be photo-oxidized on mineral dust, with formic acid, oxalic acid, glyoxylic acid, 2,3-dioxopropanoic acid, dicarbonic acid, performic acid, mesoxalaldehyde, 2-hydroxymalonaldehyde, carbonic formic anhydride, and 1,3-dioxolane-2,4-dione detected as main products. Further, we observed the heterogeneous uptake of NH₃ promoted by the carboxylic acids stemming from the photocatalytic oxidation (PCO) of levoglucosan. The mineral-dust-initiated PCO of levoglucosan and enhanced heterogeneous uptake of NH₃, which are highly influenced by irradiation and moisture conditions, were for the first time revealed. The reaction mechanisms and pathways were studied in detail by diffuse reflection infrared Fourier transform spectroscopy (DRIFTS), high-pressure photon ionization time-of-flight mass spectrometry (HPPI-ToF-MS) and flow reactor systems. Diverse WSOC constituents were studied as well, and the reactivity toward NH₃ is related to the number of hydroxyl groups of the WSOC molecules. This work reveals a new precursor of secondary organic aerosols and provides experimental evidence of the existence of organic ammonium salts in atmospheric particles.

Gas-phase ammonia and PM2.5 ammonium in a busy traffic area of Nanjing, China

The gas-phase ammonia (NH3) and fine particle PM2.5 ammonium (pNH4(+)) (collectively, NHx) were monitored between July 2013 and August 2014 in a busy traffic area of Nanjing, China. Results showed that PM2.5 concentration was 66.7 μg m(-3), and NH3 concentration was 6.66 μg m(-3). In the PM2.5, the concentration of pNH4(+) was 3.04 μg m(-3), SO4(2-) (pSO4(2-)) was 10.16 μg m(-3), and NO3(-) (pNO3(-)) was 1.60 μg m(-3). The significant correlation curves from the tests of PM2.5 revealed that molar ratio of pNH4(+) and pSO4(2-) was approximately 2, which could be (NH4)2SO4. Particulate NH4(+) primarily associated with pSO4(2-), which accounted for 4.54% of total PM2.5 mass. The PM2.5 observed acidic and the NH3 in the atmosphere neutralized acidic species, mainly in a sulfate form. The traffic intensity in the region was partially related to the formation of PM2.5 and NH3, suggesting that traffic pollution may be an important source of PM2.5. The reaction between NHx and acidic species was assumed to the secondary PM2.5. The neutralization and photochemical property of NHx were discussed.

Effect of ammonia on the volatility of organic diacids

The effect of ammonia on the partitioning of two dicarboxylic acids, oxalic (C2) and adipic (C6) is determined. Measurements by a tandem differential mobility analysis system and a thermodenuder (TD-TDMA) system are used to estimate the saturation vapor pressure and enthalpy of vaporization of ammonium oxalate and adipate. Ammonia dramatically lowered the vapor pressure of oxalic acid, by several orders of magnitude, with an estimated vapor pressure of 1.7 ± 0.8 × 10(–6) Pa at 298 K. The vapor pressure of ammonium adipate was 2.5 ± 0.8 × 10(–5) Pa at 298 K, similar to that of adipic acid. These results suggest that the dominance of oxalate in diacid concentrations measured in ambient aerosol could be attributed to the salt formation with ammonia.

Uncertainties in PM2.5 gravimetric and speciation measurements and what we can learn from them

The U.S. Environmental Protection Agency (EPA) and the federal land management community (National Park Service, United States Fish and Wildlife Service, United States Forest Service, and Bureau of Land Management) operate extensive particle speciation monitoring networks that are similar in design but are operated for different objectives. Compliance (mass only) monitoring is also carried out using federal reference method (FRM) criteria at approximately 1000 sites. The Chemical Speciation Network (CSN) consists of approximately 50 long-term-trend sites, with about another 250 sites that have been or are currently operated by state and local agencies. The sites are located in urban or suburban settings. The Interagency Monitoring of Protected Visual Environments (IMPROVE) monitoring network consists of about 181 sites, approximately 170 of which are in nonurban areas. Each monitoring approach has its own inherent monitoring limitations and biases. Determination of gravimetric mass has both negative and positive artifacts. Ammonium nitrate and other semivolatiles are lost during sampling, whereas, on the other hand, measured mass includes particle-bound water. Furthermore, some species may react with atmospheric gases, further increasing the positive mass artifact. Estimating aerosol species concentrations requires assumptions concerning the chemical form of various molecular compounds, such as nitrates and sulfates, and organic material and soil composition. Comparing data collected in the various monitoring networks allows for assessing uncertainties and biases associated with both negative and positive artifacts of gravimetric mass determinations, assumptions of chemical composition, and biases between different sampler technologies. All these biases are shown to have systematic seasonal characteristics. Unaccounted-for particle-bound water tends to be higher in the summer, as does nitrate volatilization. The ratio of particle organic mass divided by organic carbon mass (Roc) is higher during summer and lower during the winter seasons in both CSN and IMPROVE networks, and Roc is lower in urban than non-urban environments.

Measurements of oxalic acid, oxalates, malonic acid, and malonates in atmospheric particulates

This study systematically examined effects of analytical approaches on resultant concentrations of oxalic acid, oxalates, malonic acid, and malonates. Results demonstrated that employing separate water extraction and THF extraction is required to properly quantify dicarboxylic acids vs dicarboxylates using IC or GC-MS. Applications of the recommended methods to analyze PM2.5 collected in Singapore showed that concentrations of oxalate ranged from 361.4 to 481.4 ng m(-3), which were 10-14.7 times higher than that of oxalic acid. Unlike that of oxalates, malonate concentrations (10.5-23.4 ng m(-3)) were no more than half of malonic acid concentration (43.8-53.9 ng m(-3)) in PM2.5. Concentration ratios of oxalate-to-oxalic acid and malonate-to-malonic acid obtained from this work were applied to reported literature data; as a first approximation, in urban environments similar to that in Singapore, quantifiable oxalic acid, oxalates, malonic acid, and malonates in PM2.5 could range from 7.6 to 68.0, 82.2 to 732.8, 6.3 to 150, and 1.3 to 60 ng m(-3), respectively. Because photooxidation properties and hygroscopicity of dicarboxylic acids can substantially differ from that of dicarboxylates, more studies are needed to quantify ambient oxalic acid and malonic acid vs oxalates and malonates.