An observation-based model coupled to the Master Chemical Mechanism (V3.3.1)
and constrained by a full suite of observations was developed to study
atmospheric oxidation capacity (AOC), OH ...reactivity, OH chain length and
HOx (=OH+HO2) budget for three different ozone (O3)
concentration levels in Shanghai, China. Five months of observations from 1 May to 30 September 2018 showed that the air quality level is lightly
polluted or worse (Ambient Air Quality Index, AQI, of > 100) for
12 d, of which ozone is the primary pollutant for 10 d, indicating
ozone pollution was the main air quality challenge in Shanghai during
summer of 2018. The levels of ozone and its precursors, as well as
meteorological parameters, revealed the significant differences among
different ozone levels, indicating that the high level of precursors is the
precondition of ozone pollution, and strong radiation is an essential
driving force. By increasing the input JNO2 value by 40 %, the
simulated O3 level increased by 30 %–40 % correspondingly under the
same level of precursors. The simulation results show that AOC, dominated by
reactions involving OH radicals during the daytime, has a positive
correlation with ozone levels. The reactions with non-methane volatile
organic compounds (NMVOCs; 30 %–36 %), carbon monoxide (CO; 26 %–31 %) and nitrogen dioxide (NO2; 21 %–29 %) dominated
the OH reactivity under different ozone levels in Shanghai. Among the
NMVOCs, alkenes and oxygenated VOCs (OVOCs) played a key role in OH
reactivity, defined as the inverse of the OH lifetime. A longer OH chain
length was found in clean conditions primarily due to low NO2 in the
atmosphere. The high level of radical precursors (e.g., O3, HONO and
OVOCs) promotes the production and cycling of HOx, and the daytime
HOx primary source shifted from HONO photolysis in the morning to
O3 photolysis in the afternoon. For the sinks of radicals, the reaction
with NO2 dominated radical termination during the morning rush hour,
while the reactions of radical–radical also contributed to the sinks of
HOx in the afternoon. Furthermore, the top four species contributing to
ozone formation potential (OFP) were HCHO, toluene, ethylene and
m/p-xylene. The concentration ratio (∼23 %) of these four
species to total NMVOCs is not proportional to their contribution
(∼55 %) to OFP, implying that controlling key VOC species
emission is more effective than limiting the total concentration of VOC in
preventing and controlling ozone pollution.
Atmospheric oxidation capacity is the basis for
converting freshly emitted substances into secondary products and is
dominated by reactions involving hydroxyl radicals (OH) during daytime. In
this ...study, we present in situ measurements of ROx radical (hydroxy OH,
hydroperoxy HO2, and organic peroxy RO2) precursors and products;
the measurements are carried out in four Chinese megacities (Beijing,
Shanghai, Guangzhou, and Chongqing) during photochemically polluted seasons.
The atmospheric oxidation capacity is evaluated using an observation-based
model and radical chemistry precursor measurements as input. The radical
budget analysis illustrates the importance of HONO and HCHO photolysis,
which account for ∼50 % of the total primary radical
sources. The radical propagation is efficient due to abundant NO in urban
environments. Hence, the production rate of secondary pollutants, that is,
ozone (and fine-particle precursors (H2SO4, HNO3, and
extremely low volatility organic compounds, ELVOCs) is rapid, resulting in
secondary air pollution. The ozone budget demonstrates its high production
in urban areas; also, its rapid transport to downwind areas results in rapid
increase in local ozone concentrations. The O3–NOx–VOC (volatile
organic compound) sensitivity tests show that ozone production is
VOC-limited and that alkenes and aromatics should be mitigated first for
ozone pollution control in the four studied megacities. In contrast,
NOx emission control (that is, a decrease in NOx) leads to more
severe ozone pollution. With respect to fine-particle pollution, the role of
the HNO3–NO3 partitioning system is investigated using a thermal
dynamic model (ISORROPIA 2). Under high relative humidity (RH) and
ammonia-rich conditions, nitric acid converts into nitrates. This study
highlights the efficient radical chemistry that maintains the atmospheric
oxidation capacity in Chinese megacities and results in secondary pollution
characterized by ozone and fine particles.
Nitro-aromatic compounds (NACs), as important contributors to the light absorption by brown carbon, have been widely observed in various ambient atmospheres; however, their formation in the urban ...atmosphere was little studied. In this work, we report an intensive field study of NACs in summer 2016 at an urban Beijing site, characterized by both high-NOx and anthropogenic VOC dominated conditions. We investigated the factors that influence NAC formation (e.g., NO2, VOC precursors, RH and photolysis) through quantification of eight NACs, along with major components in fine particulate matter, selected volatile organic compounds, and gases. The average total concentration of the quantified NACs was 6.63 ng m-3, higher than those reported in other summertime studies (0.14–6.44 ng m-3). 4-Nitrophenol (4NP, 32.4 %) and 4-nitrocatechol (4NC, 28.5 %) were the top two most abundant NACs, followed by methyl-nitrocatechol (MNC), methyl-nitrophenol (MNP), and dimethyl-nitrophenol (DMNP). The oxidation of toluene and benzene in the presence of NOx was found to be a more dominant source of NACs than primary biomass burning emissions. The NO2 concentration level was found to be an important factor influencing the secondary formation of NACs. A transition from low- to high-NOx regimes coincided with a shift from organic- to inorganic-dominated oxidation products. The transition thresholds were NO2∼20 ppb for daytime andNO2∼25 ppb for nighttime conditions. Under low-NOx conditions, NACs increased with NO2, while the NO3- concentrations and (NO3-)/NACs ratios were lower, implying organic-dominated products. Under high-NOx conditions, NAC concentrations did not further increase with NO2, while theNO3- concentrations and (NO3-)/NACs ratios showed increasing trends, signaling a shift from organic- to inorganic-dominated products. Nighttime enhancements were observed for 3M4NC and 4M5NC, while daytime enhancements were noted for 4NP, 2M4NP, and DMNP, indicating different formation pathways for these two groups of NACs. Our analysis suggested that the aqueous-phase oxidation was likely the major formation pathway of 4M5NC and 3M5NC, while photo-oxidation of toluene and benzene in the presence of NO2 could be more important for the formation of nitrophenol and its derivatives. Using the (3M4NC+4M5NC) / 4NP ratios as an indicator of the relative contribution of aqueous-phase and gas-phase oxidation pathways to NAC formation, we observed that the relative contribution of aqueous-phase pathways increased at elevated ambient RH and remained constant at RH > 30 %. We also found that the concentrations of VOC precursors (e.g., toluene and benzene) and aerosol surface area acted as important factors in promoting NAC formation, and photolysis as an important loss pathway for nitrophenols.
Volatile organic compounds (VOCs) play important roles in the tropospheric
atmosphere. In this study, VOCs were measured at an urban site in Guangzhou, one of the megacities in the Pearl River Delta ...(PRD), using a gas chromatograph–mass spectrometer/flame ionization detection (GC–MS/FID) and a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS). Diurnal profile analyses show that stronger chemical removal by OH radicals for more reactive hydrocarbons occurs during the daytime, which is used to estimate the daytime average OH radical concentration. In comparison, diurnal profiles of oxygenated volatile organic compounds (OVOCs) indicate evidence of contributions from secondary formation. Detailed source analyses of OVOCs, using a photochemical age-based parameterization method, suggest important contributions from both primary emissions and secondary formation for measured OVOCs. During the campaign, around 1700 ions were detected in PTR-ToF-MS mass spectra, among which there were 462 ions with noticeable concentrations. VOC signals from these ions are quantified based on the sensitivities of available VOC species. OVOC-related ions dominated PTR-ToF-MS mass spectra, with an average contribution of 73 % ± 9 %. Combining measurements from PTR-ToF-MS and GC–MS/FID, OVOCs contribute 57 % ± 10 % to the total concentration of VOCs. Using concurrent measurements of OH reactivity, OVOCs measured by PTR-ToF-MS contribute greatly to the OH reactivity (19 % ± 10 %). In comparison, hydrocarbons account for 21 % ± 11 % of OH reactivity. Adding up the contributions from inorganic gases (48 % ± 15 %), ∼ 11 % (range of 0 %–19 %) of the OH reactivity remains `missing”, which is well within the combined uncertainties between the measured and calculated OH reactivity. Our results demonstrate the important roles of OVOCs in the emission and evolution budget of VOCs in the urban atmosphere.
Particulate nitrate (NO3-) is one of the dominant components of fine particles in China, especially during pollution episodes,
and has a significant impact on human health, air quality, and climate. ...Here
a comprehensive field campaign that focuses on the atmospheric oxidation
capacity and aerosol formation and their effects in the Yangtze River Delta
(YRD) was conducted from May to June 2019 at a regional site in Changzhou,
Jiangsu Province in China. The concentrations of NO3-, OH radicals, N2O5, NO2, O3, and relevant parameters were measured simultaneously. We showed a high NO3- mass concentration with
10.6 ± 8.9 µg m−3 on average, which accounted for 38.3 % of total water-soluble particulate components and 32.0 % of total
PM2.5, followed by the proportion of sulfate, ammonium, and chloride by
26.0 %, 18.0 %, and 2.0 %, respectively. This result confirmed that
the heavy nitrate pollution in eastern China happened not only in winter, but also in summer. This study's high nitrate oxidation ratio (NOR)
emphasized the solid atmospheric oxidation and fast nitrate formation
capacity in the YRD. It was found that OH + NO2 during daytime dominated nitrate formation on clean days, while N2O5 hydrolysis vastly
enhanced and became comparable with that of OH + NO2 during polluted
days (67.2 % and 30.2 %, respectively). The updated observed-constraint Empirical Kinetic Modeling Approach (EKMA) was used to assess the kinetic
controlling factors of both local O3 and NO3- productions,
which indicated that the O3-targeted scheme (VOCs : NOx = 2:1)
is adequate for mitigating the O3 and nitrate pollution coordinately during summertime in this region. Our results promote the understanding of
nitrate pollution mechanisms and mitigation based on field observation and
model simulation and call for more attention to nitrate pollution in the
summertime.
Aerosol nitrate has become the most abundant compound during aerosol pollution in eastern China. The Chinese government implemented a stringent policy during 2013–2017 to tackle aerosol pollution. ...However, the response of nitrate to nitrogen oxides (NOx) reduction is unclear owing to the limitation of long‐term measurement. Here, we performed a 9‐year continuous measurement of aerosol compositions in Shanghai and confirmed a decrease in most species except nitrate. The contribution of nitrate to fine particulate matter (PM2.5) increased significantly, reaching up to 35% in pollution episodes after 2017. This is in contrast to the evident reduction in NOx emissions. We found that the elevated dinitrogen pentoxide (N2O5) hydrolysis is responsible for the observed nitrate trend. Increased ozone and decreased nitrogen dioxide (NO) facilitated the formation of N2O5, and increased nitrate proportion promoted the uptake of N2O5 and eventually enhanced the conversion efficiency of NO2 to nitrate. Our results highlight the importance of synergic control of aerosol and ozone pollution.
Plain Language Summary
To tackle severe particulate matter (PM) pollution, the Chinese government tried to reduce the primary emission of pollutants, especially a stringent policy during 2013 and 2017, named Air Pollution Prevention and Control Action Plan. A particular need is to understand the respond of the chemical composition of PM2.5 to primary emission reductions, which supports the PM2.5 pollution control more effectively and scientifically. In this work, we performed a 9‐year online measurement of PM2.5 and its chemical compositions in the Yangtze River Delta from 2011 to 2019. We confirmed a significant decreasing trend for most species except nitrate, which remained unchanged in contrast to the evident reduction of nitrogen oxides. After ruling out the potential influences of meteorological parameters, we demonstrated the elevated dinitrogen pentoxide (N2O5) hydrolysis is responsible for the observed nitrate trend. Increased ozone and decreased nitrogen dioxide (NO) promoted the formation of N2O5, and increased nitrate proportion promoted the uptake of N2O5 and eventually promoted the conversion efficiency of NO2 to nitrate. Overall, the primary emission reduction strategies feedback to the elevated formation of nitrate mainly via the increased O3 production, highlighting the need for the synergic control of aerosol and atmospheric oxidation capacity in mitigating PM2.5 pollutions.
Key Points
A 9‐year observation of fine particulate matter (PM2.5) chemical compositions in Shanghai from 2011 to 2019 confirmed a decrease in most species except nitrate
The conversion rate of nitrogen dioxide (NO2) to nitrate was promoted by dinitrogen pentoxide hydrolysis, resulting in a nonlinear response of nitrate to NO2 reduction
Increased ozone concentration, decreased NO concentration, and increased nitrate proportion played a key role in enhancing nitrate formation
•178 nitrate modeling studies in China published during 2007–2021 were reviewed.•Model performance on nitrate and possible reasons for model-observation biases were summarized.•The contribution of ...N2O5 heterogeneous chemistry to nitrate varies from 21.0% to 51.6% among studies.•Decreased SO2 emission, enhanced AOC, and weakened deposition account for increasing PM2.5 nitrate fraction.•Multiple-pollutant control strategies involving NH3, NOx, and VOCs are needed to mitigate nitrate pollution.
Particulate nitrate (pNO3) is now becoming the principal component of PM2.5 during severe winter haze episodes in many cities of China. To gain a comprehensive understanding of the key factors controlling pNO3 formation and driving its trends, we reviewed the recent pNO3 modeling studies which mainly focused on the formation mechanism and recent trends of pNO3 as well as its responses to emission controls in China. The results indicate that although recent chemical transport models (CTMs) can reasonably capture the spatial–temporal variations of pNO3, model-observation biases still exist due to large uncertainties in the parameterization of dinitrogen pentoxide (N2O5) uptake and ammonia (NH3) emissions, insufficient heterogeneous reaction mechanism, and the predicted low sulfate concentrations in current CTMs. The heterogeneous hydrolysis of N2O5 dominates nocturnal pNO3 formation, however, the contribution to total pNO3 varies among studies, ranging from 21.0% to 51.6%. Moreover, the continuously increasing PM2.5 pNO3 fraction in recent years is mainly due to the decreased sulfur dioxide emissions, the enhanced atmospheric oxidation capacity (AOC), and the weakened nitrate deposition. Reducing NH3 emissions is found to be the most effective control strategy for mitigating pNO3 pollution in China. This review suggests that more field measurements are needed to constrain the parameterization of heterogeneous N2O5 and nitrogen dioxide (NO2) uptake. Future studies are also needed to quantify the relationships of pNO3 to AOC, O3, NOx, and volatile organic compounds (VOCs) in different regions of China under different meteorological conditions. Research on multiple-pollutant control strategies involving NH3, NOX, and VOCs is required to mitigate pNO3 pollution, especially during severe winter haze events.
Organosulfates (OSs) with ambiguous formation mechanisms are a potential source of missing secondary organic aerosol (SOA) in current atmospheric models. In this study, we chemically characterized ...OSs and nitrooxy-OSs (NOSs) formed under the influence of biogenic emissions and anthropogenic pollutants (e.g., NOx, SO42−) in summer in Beijing. An ultrahigh-resolution mass spectrometer equipped with an electrospray ionization source was applied to examine the overall molecular composition of S-containing organics. The number and intensities of S-containing organics, the majority of which could be assigned as OSs and NOSs, increased significantly during pollution episodes, which indicated their importance for SOA accumulation. To further investigate the distribution and formation of OSs and NOSs, high-performance liquid chromatography coupled with mass spectrometry was employed to quantify 10 OSs and 3 NOS species. The total concentrations of quantified OSs and NOSs were 41.4 and 13.8 ng m−3, respectively. Glycolic acid sulfate was the most abundant species among all the quantified species, followed by monoterpene NOSs (C10H16NO7S−). The total concentration of three isoprene OSs was 14.8 ng m−3 and the isoprene OSs formed via the HO2 channel were higher than those formed via the NO ∕ NO2 channel. The OS concentration coincided with the increase in acidic sulfate aerosols, aerosol acidity, and liquid water content (LWC), indicating the acid-catalyzed aqueous-phase formation of OSs in the presence of acidic sulfate aerosols. When sulfate dominated the accumulation of secondary inorganic aerosols (SIAs; sulfate, nitrate, and ammonium; SO42− ∕ SIA > 0.5), OS formation would obviously be promoted as the increasing of acidic sulfate aerosols, aerosol LWC, and acidity (pH < 2.8). Otherwise, acid-catalyzed OS formation would be limited by lower aerosol acidity when nitrate dominated the SIA accumulation. The nighttime enhancement of monoterpene NOSs suggested their formation via the nighttime NO3-initiated oxidation of monoterpene under high-NOx conditions. However, isoprene NOSs are presumed to form via acid-catalyzed chemistry or reactive uptake of oxidation products of isoprene. This study provides direct observational evidence and highlights the secondary formation of OSs and NOSs via the interaction between biogenic precursors and anthropogenic pollutants (NOx, SO2, and SO42−). The results imply that future reduction in anthropogenic emissions can help to reduce the biogenic SOA burden in Beijing or other areas impacted by both biogenic emissions and anthropogenic pollutants.
Gaseous amines have recently been identified as the key precursors for frequent new particle formation in polluted urban atmospheres. An open question that arises is why amines are abundant in urban ...areas despite the absence of agricultural emissions. Here, using the Vocus Proton‐Transfer‐Reaction Time‐of‐Flight Mass‐Spectrometry Mobile Laboratory, we present highly time‐resolved and ultrasensitive measurements of amines to elucidate their atmospheric abundance and source distribution over the Yangtze River Delta city cluster. Our results show strong spatiotemporal heterogeneity in atmospheric C2‐ and C3‐amine levels across various landscapes. Unexpectedly, we found that urban areas were ubiquitous hotspots of amines while amine source strength from agricultural areas were rather low. Corroborated by source measurements and source tracers, all of the evidence suggest that nonagricultural emissions are the most important sources responsible for the observed patterns of urban atmospheric amines, implying significant consequences for the global amine emission inventory.
Plain Language Summary
Recently, the cluster formation of sulfuric acid‐dimethylamine‐H2O has been identified as an important mechanism to explain the frequent formation of new particles in urban polluted atmospheres. However, the interest in gaseous amines across the aerosol scientific community contrasts sharply with the extremely sparse information on their sources and atmospheric abundance. In this study, we present the first mobile measurements showing that the source strength of amines from agricultural areas is modest, thus questing the long‐held assumption that the agricultural sources of amines are similar to those of ammonia. In the case of C2 amines, industrial processes are considered to be an important emission pathway. The dominance of nonagricultural sources, such as fuel combustion, in C3 amines is likely to assign these volatile amine compounds a key function in enhancing new particle formation in polluted atmospheres.
Key Points
Mobile measurements are a powerful approach to map major sources of gaseous amines
In highly urbanized areas, nonagricultural emission sources dominate atmospheric amines
C2 and C3 amines are mainly originated from industrial and mobile sources, respectively
Vehicular emissions are an important source for volatile organic compounds (VOCs) in urban and downwind regions. In this study, we conducted a chassis dynamometer study to investigate VOC emissions ...from vehicles using gasoline, diesel, and liquefied petroleum gas (LPG) as fuel. Time-resolved VOC emissions from vehicles are chemically characterized by a
proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) with
high frequency. Our results show that emission factors of VOCs generally
decrease with the improvement of emission standards for gasoline vehicles,
whereas variations in emission factors for diesel vehicles with emission
standards are more diverse. Mass spectrum analysis of the PTR-ToF-MS suggests that cold starts significantly influence VOC emissions of gasoline vehicles, while the influences are less important for diesel vehicles. Large differences in VOC emissions between gasoline and diesel vehicles are observed with emission factors of most VOC species from diesel vehicles being higher than gasoline vehicles, especially for most oxygenated volatile organic compounds (OVOCs) and heavier aromatics. These results indicate quantification of heavier species by the PTR-ToF-MS may be important in the characterization of vehicular exhausts. Our results suggest that VOC pairs (e.g., C14 aromatics / toluene ratio) could potentially provide good indicators for distinguishing emissions from gasoline and diesel vehicles. The fractions of OVOCs in total VOC emissions are determined by combining measurements of hydrocarbons from canisters and online observations of the PTR-ToF-MS. We show that OVOCs contribute 9.4 % ± 5.6 % of total VOC emissions for gasoline vehicles, while the fractions are significantly higher for diesel vehicles (52 %–71 %), highlighting the importance of detecting these OVOC species in diesel emissions. Our study demonstrated that the large number of OVOC species measured by the PTR-ToF-MS are important in the characterization of VOC emissions from vehicles.
PDF
