Although regional haze adversely affects human health and possibly counteracts global warming from increasing levels of greenhouse gases, the formation and radiative forcing of regional haze on climate remain uncertain. By combining field measurements, laboratory experiments, and model simulations, we show a remarkable role of black carbon (BC) particles in driving the formation and trend of regional haze. Our analysis of long-term measurements in China indicates declined frequency of heavy haze events along with significantly reduced SO2, but negligibly alleviated haze severity. Also, no improving trend exists for moderate haze events. Our complementary laboratory experiments demonstrate that SO2 oxidation is efficiently catalyzed on BC particles in the presence of NO2 and NH3, even at low SO2 and intermediate relative humidity levels. Inclusion of the BC reaction accounts for about 90–100% and 30–50% of the sulfate production during moderate and heavy haze events, respectively. Calculations using a radiative transfer model and accounting for the sulfate formation on BC yield an invariant radiative forcing of nearly zero W m−2 on the top of the atmosphere throughout haze development, indicating small net climatic cooling/warming but large surface cooling, atmospheric heating, and air stagnation. This BC catalytic chemistry facilitates haze development and explains the observed trends of regional haze in China. Our results imply that reduction of SO2 alone is insufficient in mitigating haze occurrence and highlight the necessity of accurate representation of the BC chemical and radiative properties in predicting the formation and assessing the impacts of regional haze.
High levels of ultrafine particles (UFPs; diameter of less than 50 nm) are frequently produced from new particle formation under urban conditions, with profound implications on human health, weather, and climate. However, the fundamental mechanisms of new particle formation remain elusive, and few experimental studies have realistically replicated the relevant atmospheric conditions. Previous experimental studies simulated oxidation of one compound or a mixture of a few compounds, and extrapolation of the laboratory results to chemically complex air was uncertain. Here, we show striking formation of UFPs in urban air from combining ambient and chamber measurements. By capturing the ambient conditions (i.e., temperature, relative humidity, sunlight, and the types and abundances of chemical species), we elucidate the roles of existing particles, photochemistry, and synergy of multipollutants in new particle formation. Aerosol nucleation in urban air is limited by existing particles but negligibly by nitrogen oxides. Photooxidation of vehicular exhaust yields abundant precursors, and organics, rather than sulfuric acid or base species, dominate formation of UFPs under urban conditions. Recognition of this source of UFPs is essential to assessing their impacts and developing mitigation policies. Our results imply that reduction of primary particles or removal of existing particles without simultaneously limiting organics from automobile emissions is ineffective and can even exacerbate this problem. new particle formation | nucleation | ultrafine particles | growth | organics
Secondary organic aerosol (SOA) represents a major constituent of tropospheric fine particulate matter, with profound implications for human health and climate. However, the chemical mechanisms leading to SOA formation remain uncertain, and atmospheric models consistently underpredict the global SOA budget. Small α-dicarbonyls, such as methylglyoxal, are ubiquitous in the atmosphere because of their significant production from photooxidation of aromatic hydrocarbons from traffic and industrial sources as well as from biogenic isoprene. Current experimental and theoretical results on the roles of methylglyoxal in SOA formation are conflicting. Using quantum chemical calculations, we show cationic oligomerization of methylglyoxal in aqueous media. Initial protonation and hydration of methylglyoxal lead to formation of diols/tetrol, and subsequent protonation and dehydration of diols/tetrol yield carbenium ions, which represent the key intermediates for formation and propagation of oligomerization. On the other hand, our results reveal that the previously proposed oligomerization via hydration for methylglyoxal is kinetically and thermodynamically implausible. The carbenium ion-mediated mechanism occurs barrierlessly on weakly acidic aerosols and cloud/fog droplets and likely provides a key pathway for SOA formation from biogenic and anthropogenic emissions.
Large amounts of
small α-dicarbonyls (glyoxal and methylglyoxal)
are produced in the atmosphere from photochemical oxidation of biogenic
isoprene and anthropogenic aromatics, but the fundamental mechanisms
leading to secondary organic aerosol (SOA) and brown carbon (BrC)
formation remain elusive. Methylglyoxal is commonly believed to be
less reactive than glyoxal because of unreactive methyl substitution,
and available laboratory measurements showed negligible aerosol growth
from methylglyoxal. Herein, we present experimental results to demonstrate
striking oligomerization of small α-dicarbonyls leading to SOA
and BrC formation on sub-micrometer aerosols. Significantly more efficient
growth and browning of aerosols occur upon exposure to methylglyoxal
than glyoxal under atmospherically relevant concentrations and in
the absence/presence of gas-phase ammonia and formaldehyde, and nonvolatile
oligomers and light-absorbing nitrogen-heterocycles are identified
as the dominant particle-phase products. The distinct aerosol growth
and light absorption are attributed to carbenium ion-mediated nucleophilic
addition, interfacial electric field-induced attraction, and synergetic
oligomerization involving organic/inorganic species, leading to surface-
or volume-limited reactions that are dependent on the reactivity and
gaseous concentrations. Our findings resolve an outstanding discrepancy
concerning the multiphase chemistry of small α-dicarbonyls and
unravel a new avenue for SOA and BrC formation from atmospherically
abundant, ubiquitous carbonyls and ammonia/ammonium sulfate.
Photooxidation of volatile organic compounds (VOCs) produces secondary organic aerosol (SOA) and lightabsorbing brown carbon (BrC) via multiple reaction steps/ pathways, reflecting significant chemical complexity relevant to gaseous oxidation and subsequent gas-to-particle conversion. Toluene is an important VOC under urban conditions, but the fundamental chemical mechanism leading to SOA formation remains uncertain. Here, we elucidate multigeneration SOA production from toluene by simultaneously tracking the evolutions of gas-phase oxidation and aerosol formation in a reaction chamber. Large size increase and browning of monodisperse submicrometer seed particles occur shortly after initiating oxidation by hydroxyl radical (OH) at 10−90% relative humidity (RH). The evolution in gaseous products and aerosol properties (size/density/optical properties) and chemical speciation of aerosol-phase products indicate that the aerosol growth and browning result from earlier generation products consisting dominantly of dicarbonyl and carboxylic functional groups. While volatile dicarbonyls engage in aqueous reactions to yield nonvolatile oligomers and lightabsorbing nitrogen heterocycles/heterochains (in the presence of NH 3 ) at high RH, organic acids contribute to aerosol carboxylates via ionic dissociation or acid−base reaction in a wide RH range. We conclude that toluene contributes importantly to SOA/BrC formation from dicarbonyls and organic acids because of their prompt and high yields from photooxidation and unique functionalities for participation in aerosol-phase reactions.
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