In some regions, reducing aerosol ammonium nitrate
(NH4NO3) concentrations may substantially improve air quality.
This can be accomplished by reductions in precursor emissions, such as
nitrogen ...oxides (NOx) to lower nitric acid (HNO3)
that partitions to the aerosol, or reductions in ammonia (NH3) to
lower particle pH and keep HNO3 in the gas phase. Using the
ISORROPIA-II thermodynamic aerosol model and detailed observational data sets,
we explore the sensitivity of aerosol NH4NO3 to gas-phase
NH3 and NOx controls for a number of contrasting
locations, including Europe, the United States, and China. NOx control
is always effective, whereas the aerosol response to NH3 control is
highly nonlinear and only becomes effective at a thermodynamic sweet
spot. The analysis provides a conceptual framework and fundamental
evaluation on the relative value of NOx versus NH3
control and demonstrates the relevance of pH as an air quality parameter. We
find that, regardless of the locations examined, it is only when ambient
particle pH drops below an approximate critical value of 3 (slightly higher
in warm and slightly lower in cold seasons) that NH3 reduction
leads to an effective response in PM2.5 mass. The required amount of
NH3 reduction to reach the critical pH and efficiently decrease
NH4NO3 at different sites is assessed. Owing to the linkage
between NH3 emissions and agricultural productivity, the substantial
NH3 reduction required in some locations may not be feasible.
Finally, controlling NH3 emissions to increase aerosol acidity and
evaporate NH4NO3 will have other effects, beyond reduction of
PM2.5 NH4NO3, such as increasing aerosol toxicity and
potentially altering the deposition patterns of nitrogen and trace nutrients.
Hydroxyl (OH) and peroxy radicals (HO2 and RO2) were measured in the Pearl River Delta, which is one of the most polluted areas in China, in autumn 2014. The radical observations were complemented by ...measurements of OH reactivity (inverse OH lifetime) and a comprehensive set of trace gases including carbon monoxide (CO), nitrogen oxides (NOx=NO, NO2) and volatile organic compounds (VOCs). OH reactivity was in the range from 15 to 80 s−1, of which about 50 % was unexplained by the measured OH reactants. In the 3 weeks of the campaign, maximum median radical concentrations were 4.5×106 cm−3 for OH at noon and 3×108 and 2.0×108 cm−3 for HO2 and RO2, respectively, in the early afternoon. The completeness of the daytime radical measurements made it possible to carry out experimental budget analyses for all radicals (OH, HO2, and RO2) and their sum (ROx). The maximum loss rates for OH, HO2, and RO2 reached values between 10 and 15 ppbv h−1 during the daytime. The largest fraction of this can be attributed to radical interconversion reactions while the real loss rate of ROx remained below 3 ppbv h−1. Within experimental uncertainties, the destruction rates of HO2 and the sum of OH, HO2, and RO2 are balanced by their respective production rates. In case of RO2, the budget could be closed by attributing the missing OH reactivity to unmeasured VOCs. Thus, the presumption of the existence of unmeasured VOCs is supported by RO2 measurements. Although the closure of the RO2 budget is greatly improved by the additional unmeasured VOCs, a significant imbalance in the afternoon remains, indicating a missing RO2 sink. In case of OH, the destruction in the morning is compensated by the quantified OH sources from photolysis (HONO and O3), ozonolysis of alkenes, and OH recycling (HO2+NO). In the afternoon, however, the OH budget indicates a missing OH source of 4 to 6 ppbv h−1. The diurnal variation of the missing OH source shows a similar pattern to that of the missing RO2 sink so that both largely compensate each other in the ROx budget. These observations suggest the existence of a chemical mechanism that converts RO2 to OH without the involvement of NO, increasing the RO2 loss rate during the daytime from 5.3 to 7.4 ppbv h−1 on average. The photochemical net ozone production rate calculated from the reaction of HO2 and RO2 with NO yields a daily integrated amount of 102 ppbv ozone, with daily integrated ROx primary sources being 22 ppbv in this campaign. The produced ozone can be attributed to the oxidation of measured (18 %) and unmeasured (60 %) hydrocarbons, formaldehyde (14 %), and CO (8 %). An even larger integrated net ozone production of 140 ppbv would be calculated from the oxidation rate of VOCs with OH if HO2 and all RO2 radicals react with NO. However, the unknown RO2 loss (evident in the RO2 budget) causes 30 ppbv less ozone production than would be expected from the VOC oxidation rate.
Trees can significantly impact the urban air chemistry by the uptake and emission of reactive biogenic volatile organic compounds (BVOCs), which are involved in ozone and particle formation. Here we ...present the emission potentials of "constitutive" (cBVOCs) and "stress-induced" BVOCs (sBVOCs) from the dominant broadleaf woody plant species in the megacity of Beijing. Based on the municipal tree census and cuvette BVOC measurements on leaf level, we built an inventory of BVOC emissions, and assessed the potential impact of BVOCs on secondary organic aerosol (SOA) formation in 2005 and 2010, i.e., before and after realizing the large tree-planting program for the 2008 Olympic Games. We found that sBVOCs, such as fatty acid derivatives, benzenoids, and sesquiterpenes, constituted a significant fraction ( ∼ 40 %) of the total annual BVOC emissions, and we estimated that the overall annual BVOC budget may have doubled from ∼ 4.8 × 109 g C year−1 in 2005 to ∼ 10.3 × 109 g C year−1 in 2010 due to the increase in urban greening, while at the same time the emission of anthropogenic VOCs (AVOCs) decreased by 24 %. Based on the BVOC emission assessment, we estimated the biological impact on SOA mass formation potential in Beijing. Constitutive and stress-induced BVOCs might produce similar amounts of secondary aerosol in Beijing. However, the main contributors of SOA-mass formations originated from anthropogenic sources (> 90 %). This study demonstrates the general importance to include sBVOCs when studying BVOC emissions. Although the main problems regarding air quality in Beijing still originate from anthropogenic activities, the present survey suggests that in urban plantation programs, the selection of low-emitting plant species has some potential beneficial effects on urban air quality.
As has been the case in North America and western Europe,
the SO2 emissions have substantially reduced in the North China Plain (NCP) in
recent years. Differential rates of reduction in SO2 and NOx
...concentrations result in the frequent occurrence of particulate matter pollution dominated by nitrate
(pNO3-) over the NCP. In this
study, we observed a polluted episode with the particulate nitrate mass
fraction in nonrefractory PM1 (NR-PM1) being up to 44 % during
wintertime in Beijing. Based on this typical pNO3--dominated haze
event, the linkage between aerosol water uptake and pNO3-
enhancement, further impacting on visibility degradation, has been
investigated based on field observations and theoretical calculations.
During haze development, as ambient relative humidity (RH) increased from
∼10 % to 70 %, the aerosol particle liquid water
increased from ∼1 µg m−3 at the beginning to
∼75 µg m−3 in the fully developed haze period. The
aerosol liquid water further increased the aerosol surface area and volume,
enhancing the condensational loss of N2O5 over particles. From the
beginning to the fully developed haze, the condensational loss of
N2O5 increased by a factor of 20 when only considering aerosol
surface area and volume of dry particles, while increasing by a factor of 25 when
considering extra surface area and volume due to water uptake. Furthermore,
aerosol liquid water favored the thermodynamic equilibrium of HNO3 in
the particle phase under the supersaturated HNO3 and NH3 in the
atmosphere. All the above results demonstrated that pNO3- is
enhanced by aerosol water uptake with elevated ambient RH during haze
development, in turn facilitating the aerosol take-up of water due to the
hygroscopicity of particulate nitrate salt. Such mutual promotion between
aerosol particle liquid water and particulate nitrate enhancement can
rapidly degrade air quality and halve visibility within 1 d. Reduction
of nitrogen-containing gaseous precursors, e.g., by control of traffic
emissions, is essential in mitigating severe haze events in the NCP.
Recent studies have recognised highly oxygenated organic molecules (HOMs) in the atmosphere as important in the formation of secondary organic aerosol (SOA). A large number of studies have focused on ...HOM formation from oxidation of biogenically emitted monoterpenes. However, HOM formation from anthropogenic vapours has so far received much less attention. Previous studies have identified the importance of aromatic volatile organic compounds (VOCs) for SOA formation. In this study, we investigated several aromatic compounds, benzene (C6H6), toluene (C7H8), and naphthalene (C10H8), for their potential to form HOMs upon reaction with hydroxyl radicals (OH). We performed flow tube experiments with all three VOCs and focused in detail on benzene HOM formation in the Jülich Plant Atmosphere Chamber (JPAC). In JPAC, we also investigated the response of HOMs to NOx and seed aerosol. Using a nitrate-based chemical ionisation mass spectrometer (CI-APi-TOF), we observed the formation of HOMs in the flow reactor oxidation of benzene from the first OH attack. However, in the oxidation of toluene and naphthalene, which were injected at lower concentrations, multi-generation OH oxidation seemed to impact the HOM composition. We tested this in more detail for the benzene system in the JPAC, which allowed for studying longer residence times. The results showed that the apparent molar benzene HOM yield under our experimental conditions varied from 4.1 % to 14.0 %, with a strong dependence on the OH concentration, indicating that the majority of observed HOMs formed through multiple OH-oxidation steps. The composition of the identified HOMs in the mass spectrum also supported this hypothesis. By injecting only phenol into the chamber, we found that phenol oxidation cannot be solely responsible for the observed HOMs in benzene experiments. When NOx was added to the chamber, HOM composition changed and many oxygenated nitrogen-containing products were observed in CI-APi-TOF. Upon seed aerosol injection, the HOM loss rate was higher than predicted by irreversible condensation, suggesting that some undetected oxygenated intermediates also condensed onto seed aerosol, which is in line with the hypothesis that some of the HOMs were formed in multi-generation OH oxidation. Based on our results, we conclude that HOM yield and composition in aromatic systems strongly depend on OH and VOC concentration and more studies are needed to fully understand this effect on the formation of HOMs and, consequently, SOA. We also suggest that the dependence of HOM yield on chamber conditions may explain part of the variability in SOA yields reported in the literature and strongly advise monitoring HOMs in future SOA studies.
In contrast to summer smog, the contribution of photochemistry to the formation of winter haze in northern mid-to-high latitude is generally assumed to be minor due to reduced solar UV and water ...vapor concentrations. Our comprehensive observations of atmospheric radicals and relevant parameters during several haze events in winter 2016 Beijing, however, reveal surprisingly high hydroxyl radical oxidation rates up to 15 ppbv/h, which is comparable to the high values reported in summer photochemical smog and is two to three times larger than those determined in previous observations during winter in Birmingham (Heard et al. Geophys. Res. Lett. 2004, 31, (18)), Tokyo (Kanaya et al. J. Geophys. Res.: Atmos. 2007, 112, (D21)), and New York (Ren et al. Atmos. Environ. 2006, 40, 252–263). The active photochemistry facilitates the production of secondary pollutants. It is mainly initiated by the photolysis of nitrous acid and ozonolysis of olefins and maintained by an extremely efficiently radical cycling process driven by nitric oxide. This boosted radical recycling generates fast photochemical ozone production rates that are again comparable to those during summer photochemical smog. The formation of ozone, however, is currently masked by its efficient chemical removal by nitrogen oxides contributing to the high level of wintertime particles. The future emission regulations, such as the reduction of nitrogen oxide emissions, therefore are facing the challenge of reducing haze and avoiding an increase in ozone pollution at the same time. Efficient control strategies to mitigate winter haze in Beijing may require measures similar as implemented to avoid photochemical smog in summer.
Secondary organic aerosol contributes to the atmospheric particle burden with implications for air quality and climate. Biogenic volatile organic compounds such as terpenoids emitted from plants are ...important secondary organic aerosol precursors with isoprene dominating the emissions of biogenic volatile organic compounds globally. However, the particle mass from isoprene oxidation is generally modest compared to that of other terpenoids. Here we show that isoprene, carbon monoxide and methane can each suppress the instantaneous mass and the overall mass yield derived from monoterpenes in mixtures of atmospheric vapours. We find that isoprene 'scavenges' hydroxyl radicals, preventing their reaction with monoterpenes, and the resulting isoprene peroxy radicals scavenge highly oxygenated monoterpene products. These effects reduce the yield of low-volatility products that would otherwise form secondary organic aerosol. Global model calculations indicate that oxidant and product scavenging can operate effectively in the real atmosphere. Thus highly reactive compounds (such as isoprene) that produce a modest amount of aerosol are not necessarily net producers of secondary organic particle mass and their oxidation in mixtures of atmospheric vapours can suppress both particle number and mass of secondary organic aerosol. We suggest that formation mechanisms of secondary organic aerosol in the atmosphere need to be considered more realistically, accounting for mechanistic interactions between the products of oxidizing precursor molecules (as is recognized to be necessary when modelling ozone production).
The first wintertime in situ measurements of hydroxyl (OH), hydroperoxy (HO2) and organic peroxy (RO2) radicals (ROx=OH+HO2+RO2) in combination with observations of total reactivity of OH radicals, ...kOH in Beijing are presented. The field campaign “Beijing winter finE particle STudy – Oxidation, Nucleation and light Extinctions” (BEST-ONE) was conducted at the suburban site Huairou near Beijing from January to March 2016. It aimed to understand oxidative capacity during wintertime and to elucidate the secondary pollutants' formation mechanism in the North China Plain (NCP). OH radical concentrations at noontime ranged from 2.4×106cm-3 in severely polluted air (kOH∼27s-1) to 3.6×106cm-3 in relatively clean air (kOH∼5s-1). These values are nearly 2-fold larger than OH concentrations observed in previous winter campaigns in Birmingham, Tokyo, and New York City. During this campaign, the total primary production rate of ROx radicals was dominated by the photolysis of nitrous acid accounting for 46 % of the identified primary production pathways for ROx radicals. Other important radical sources were alkene ozonolysis (28 %) and photolysis of oxygenated organic compounds (24 %). A box model was used to simulate the OH, HO2 and RO2 concentrations based on the observations of their long-lived precursors. The model was capable of reproducing the observed diurnal variation of the OH and peroxy radicals during clean days with a factor of 1.5. However, it largely underestimated HO2 andRO2 concentrations by factors up to 5 during pollution episodes. The HO2 and RO2 observed-to-modeled ratios increased with increasing NO concentrations, indicating a deficit in our understanding of the gas-phase chemistry in the high NOx regime. The OH concentrations observed in the presence of large OH reactivities indicate that atmospheric trace gas oxidation by photochemical processes can be highly effective even during wintertime, thereby facilitating the vigorous formation of secondary pollutants.
Anthropogenic nitrogen oxides may influence the cloud condensation nuclei (CCN) activity of biogenic secondary organic aerosols (SOA) in both daytime photooxidation and nighttime NO3 oxidation, which ...has significant implications for the climatic impact of SOA. We investigated the influence of NOx on the CCN activity of monoterpene‐derived SOA in OH oxidation and in NO3 oxidation. In OH oxidation, NOx had little influence on the hygroscopic parameter κ of organic aerosol (κOrg), which was attributed to the minor fraction of organic nitrates (ON) in SOA (<24%), resulted from the low branching ratio of RO2 + NO to form ON. In contrast, in NO3 oxidation κOrg was much reduced compared to OH/O3 oxidation due to a dominant fraction of ON. We report κ of MT‐derived ON formed in photo‐oxidation and NO3 oxidation (0.029–0.052) for the first time to our knowledge, which may be used to improve model simulations of CCN concentrations.
Plain Language Summary
Anthropogenic nitrogen oxides may influence the cloud formation ability of biogenic secondary organic aerosols (SOA) in both daytime and nighttime, which has implications to understand the climatic impact of SOA. However, the influence remains unclear. We found that for monoterpenes, a major class of precursors of biogenic SOA, NOx had little influence on the cloud formation ability of SOA in the daytime oxidation. In contrast, in the nighttime oxidation of monoterpenes by NO3, an important oxidant formed from NOx at night‐time, SOA had much lower cloud formation ability than that in the photo‐oxidation. The difference was attributed to the different fractions of organic nitrates (ON) in SOA. We also determined the κ of monoterpene‐derived ON for the first time to our knowledge.
Key Points
In daytime OH oxidation NOx had little influences on the cloud condensation nuclei (CCN) activity of MT‐SOA
In nighttime NO3 oxidation MT‐SOA had much lower CCN activity compared with those formed via OH or O3 oxidation
We report the κ of monoterpene‐derived organic nitrates (0.029–0.052) for the first time to our knowledge
Despite the recent decrease in pollution events in Chinese urban areas, the World Health Organization air quality guideline values are still exceeded. Observations from monitoring networks show a ...stronger decrease of organic aerosol directly emitted to the atmosphere relative to secondary organic aerosol (SOA) generated from oxidation processes. Here, the uptake of water‐soluble gas‐phase oxidation products is reported as a major SOA contribution to particulate pollution in Beijing, triggered by the increase of aerosol liquid water. In pollution episodes, this pathway is enough to explain the increase in SOA mass, with formaldehyde, acetaldehyde, glycolaldehyde, formic acid, and acetic acid alone explaining 15%–25% of the SOA increase. Future mitigation strategies to reduce non‐methane volatile organic compound emissions should be considered to reduce organic particulate pollution in China.
Plain Language Summary
In the rapidly developing Chinese economy, air pollution from particulate matter (PM) is a major human health risk factor. We show that secondary organic aerosol (SOA) generated from oxidation processes represent 50%–80% of the organic PM in Beijing. We find that non‐equilibrium dissolution of C1−C2 carbonyl compounds to particles is a major pathway of SOA formation during pollution events. These compounds are ubiquitous products in the chemical oxidation of hydrocarbons; thus, the reduction of a single volatile organic compound precursor would not reduce the organic PM, but rather a broad reduction of the organic reactivity is required.
Key Points
Secondary organic aerosol generated from oxidation processes dominates organic particulate pollution in Beijing
Non‐equilibrium dissolution of carbonyl compounds to particles is a major pathway of SOA formation during haze episodes
A broad reduction of the gas‐phase organic reactivity is required to reduce secondary organic aerosol formation in haze events
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