Nitryl chloride (ClNO2) plays potentially important roles in atmospheric chemistry, but its abundance and effect are not fully understood due to the small number of ambient observations of ClNO2 to ...date. In late autumn 2013, ClNO2 was measured with a chemical ionization mass spectrometer (CIMS) at a mountain top (957 m above sea level) in Hong Kong. During 12 nights with continuous CIMS data, elevated mixing ratios of ClNO2 (>400 parts per trillion by volume) or its precursor N2O5 (>1000 pptv) were observed on six nights, with the highest ever reported ClNO2 (4.7 ppbv, 1 min average) and N2O5 (7.7 ppbv, 1 min average) in one case. Backward particle dispersion calculations driven by winds simulated with a mesoscale meteorological model show that the ClNO2/N2O5‐laden air at the high‐elevation site was due to transport of urban/industrial pollution north of the site. The highest ClNO2/N2O5 case was observed in a later period of the night and was characterized with extensively processed air and with the presence of nonoceanic chloride. A chemical box model with detailed chlorine chemistry was used to assess the possible impact of the ClNO2 in the well‐processed regional plume on next day ozone, as the air mass continued to downwind locations. The results show that the ClNO2 could enhance ozone by 5–16% at the ozone peak or 11–41% daytime ozone production in the following day. This study highlights varying importance of the ClNO2 chemistry in polluted environments and the need to consider this process in photochemical models for prediction of ground‐level ozone and haze.
Key Points
First observation of ClNO2 in the planetary boundary layer of China
Combined high‐resolution meteorological and measurement‐constrained chemical models in data analysis
ClNO2 enhances daytime ozone peak by 5‐16% in well‐processed PRD air
•Well-defined seasonal patterns of AOC, kOH and ROx production were found in rural Hong Kong.•kOH was dominated by oxidation of NO2 and OVOCs in later summer and of CO and OVOCs in autumn and ...winter.•Major primary ROx sources were photolysis of O3 and HONO in late summer and photolysis of O3 and HCHO in autumn and winter.
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Atmospheric oxidizing capacity (AOC), dominated by the hydroxyl radical (OH), is an important index of the self-cleaning capacity of atmosphere and plays a vital role in the tropospheric chemistry. To better understand the key processes governing the chemistry of rural atmosphere of southern China, we analyzed the oxidation capacity and radical chemistry at a regional background site in Hong Kong from 23 August to 22 December 2012, which covered the summer, autumn and winter seasons. A chemical box model built on the latest Master Chemical Mechanism (v3.3) was used to elucidate the OH reactivity and sources of ROX radicals (ROX=OH+HO2+RO2). The AOC showed a clear seasonal pattern with stronger intensity in late summer compared to autumn and winter. Reactions with NO2 (30%) and oxygenated volatile organic compounds (OVOCs) (31%) together dominated the OH loss in summer, while reactions with CO (38% in autumn and 39% in winter) and OVOCs (34% in autumn and 25% in winter) made larger contributions in autumn and winter. Photolysis of O3 (36%–47%) presented the major ROX source during all three seasons. The second largest ROx source was HONO photolysis (25%) in summer compared to HCHO photolysis in autumn (20%) and winter (21%). Besides, photolysis of other OVOCs was another important primary source of ROx radicals with average contributions of 14%, 13% and 20% for the summer, autumn and winter cases, respectively. Overall, the present study evaluates the oxidizing capacity of the rural atmosphere of South China and elucidates the varying characteristics of photochemical processes in different air masses.
Although nitrous acid (HONO) plays an important role in the chemistry of polluted atmospheres, its atmospheric abundances and sources are still not well understood. This paper reports ambient ...measurements of HONO taken over four select months in different seasons at a suburban site in Hong Kong. The data were analyzed to elucidate the seasonal characteristics, emission ratios and rates of heterogeneous production. The monthly averaged HONO concentrations ranged from 0.35 ± 0.30 ppbv in late spring (May) to 0.93 ± 0.67 ppbv in late autumn (November). The similar variation patterns of HONO, NOx, and traffic flow from midnight to rush hours suggest that the HONO concentration was strongly influenced by vehicle emissions. The emission ratios (HONO/NOx) were derived from an analysis of 21 fresh plumes (NO/NOx > 0.80), with the range of 0.5–1.6%. The large variability in the emission ratios is attributed to the reaction of NO2 on black carbon (BC) emitted from vehicles, based on a strong correlation between the HONO/NOx and concurrently measured BC. The mean conversion rate of NO2 to HONO on ground surface during nighttime estimated on nine select days was 0.52 × 10−2h−1, which is relatively low compared with other reported values. This paper highlights a large variability in vehicle emission ratios and heterogeneous conversions of NO2 at ground surface. Photochemical models must consider this variability to better simulate the primary sources of HONO and subsequent photochemistry in the lower part of the troposphere.
•Seasonal variation of HONO is reported for the first time in the PRD region.•Direct emission from vehicles is an important source of HONO during rush hours.•HONO emission ratios from vehicles positively correlate with freshly emitted BC.•A low average conversion rate of NO2 to HONO at ground (0.52% h−1) is derived.
An optical analytical instrument based on light emitting diode (LED) incoherent broadband cavity enhanced absorption spectroscopy (LED-IBBCEAS) was developed for simultaneous measurements of HONO and ...NO2 in ambient air. Detection limits (for signal-to-noise ratio, SNR = 2) of 0.6 ppbv for HONO and 2 ppbv for NO2 were achieved with an optimum acquisition time of 120 s. Field intercomparison campaign at a suburban site of Tung Chung in Hong Kong was carried out. Daytime and nighttime concentrations of HONO and NO2 were recorded and compared with data from a long path absorption photometer (LOPAP) and a NOx analyzer equipped with a blue light converter. The test of such a LED-IBBCEAS setup by the measurement intercomparison with the well established HONO and NO2 measurement instruments, for the first time, in a real atmospheric environment, demonstrated the feasibility of the proposed technique for the measurement of environmental HONO and NO2 concentrations in a manner that is free of chemical and spectral interference. During the measurement period from 12:32 on May 12th to 12:32 on May 14th, 2012, the quantitative assessments from these techniques showed well agreed data. High temporal correlations of HONO with NOx and CO were observed, and possible formation sources of HONO are discussed. Occasional inconsistencies in the quantitative assessment of HONO were observed during the in-field campaign. The problems encountered by the IBBCEAS technique applied to field observation and the further improvements are discussed.
•Measurements of ambient HONO and NO2 in a field campaign by IBBCEAS spectrometer.•Intercomparison of the measured HONO concentrations with data from a LOPAP.•Measurement of NO2 concentrations compared with a BLC based NOx analyzer.•Detection limits of 0.3 ppbv for HONO and 1 ppbv for NO2 achieved over 120 s.
The growth of newly formed particles through new particle formation (NPF) contributes a significant fraction to the cloud condensation nuclei, yet the driving mechanisms remain unclear, especially ...for polluted environments. To investigate the potential species contributing for nanoparticle growth in environments with significant anthropogenic influences, we measured the hygroscopicity of newly formed particles at 20–40 nm at a rural observational site in the North China Plain during winter 2018. Our results demonstrate that these particles were not very hygroscopic, with the mean hygroscopicity parameter κ of 0.13 ± 0.09. Clear differences in the inferred κ of the growing material responsible for the growth were observed among different events, indicating that even at the same region, the compounds driving particle growth may not be identical. This may be synergistically influenced by the NPF precursors, oxidants and meteorological conditions, suggesting complex mechanisms might co‐exist behind nanoparticle growth in polluted environments.
Plain Language Summary
Particles newly formed through nucleation grow large enough to become cloud condensation nuclei which influences climate. However, the driving mechanisms and the key contributing species for nanoparticle growth are poorly understood. Here, we present direct measurements of the hygroscopicity of particles through new particle formation (NPF) that occurred in a rural environment in the North China Plain (NCP) during wintertime of 2018. Relatively low hygroscopicity of these newly formed particles were observed, suggesting less water‐soluble compounds drove the growth there. We also found that the hygroscopicity of the growing material responsible for nanoparticle growth was not identical on different NPF days. Our results reveal that the compounds driving the growth varied with different NPF conditions, deepening our understanding of nanoparticle growth in polluted environments.
Key Points
Particles with sizes at 20–40 nm formed through new particle formation (NPF) are not very hygroscopic at rural North China Plain (NCP)
The hygroscopicity parameter κ of the growing material varied from ∼0 to ∼0.1 among NPF events
Precursors, oxidants and meteorological conditions may synergistically determine the chemical compounds responsible for particle growth
Nitrous acid (HONO) plays an important role in radical formation and photochemical oxidation processes in the boundary layer. However, its impact on the chemistry in a street canyon microenvironment ...has not been thoroughly investigated. In this study, we measured HONO in a street canyon in urban Hong Kong and used an observation-based box model (OBM) with the Master Chemical Mechanism (MCM v3.3.1) to investigate the contribution of HONO to local oxidation chemistry. The observed HONO mixing ratios were in the range of 0.4–13.9 ppbv, with an average of 3.91 ppbv in the daytime and 2.86 ppbv at night. A mean HONO/NOx emission ratio of 1.0% (±0.5%) from vehicle traffic was derived. OBM simulations constrained by the observed HONO showed that the maximum concentrations of OH, HO2, and RO2 reached 4.65 × 106, 4.40 × 106, and 1.83 × 106 molecules cm−3, which were 7.9, 5.0, and 7.5 times, respectively, the results in the case without HONO constrained. Photolysis of HONO contributed to 86.5% of the total primary radical production rates and led to efficient NO2 and O3 production under the condition of weak regional transport of O3. The formation of HNO3 contributed to 98.4% of the total radical termination rates. Our results suggest that HONO could significantly increase the atmospheric oxidation capacity in a street canyon and enhance the secondary formation of HNO3 and HCHO, which can damage outdoor building materials and pose health risks to pedestrians.
•Higher daytime HONO level than that at night was found in a street canyon.•Vehicle emission ratio of HONO/NOx was 1.0% (±0.5%).•HONO photolysis contributed to 86.5% of total primary radical production rates.•High daytime HONO enhanced the secondary formation of HNO3 and HCHO.•HONO accelerated Ox production under conditions with weak regional O3.
Abstract
The role of new particle formation (NPF) events and their contribution to haze formation through subsequent growth in polluted megacities is still controversial. To improve the understanding ...of the sources, meteorological conditions, and chemistry behind air pollution, we performed simultaneous measurements of aerosol composition and particle number size distributions at ground level and at 260 m in central Beijing, China, during a total of 4 months in 2015–2017. Our measurements show a pronounced decoupling of gas-to-particle conversion between the two heights, leading to different haze processes in terms of particle size distributions and chemical compositions. The development of haze was initiated by the growth of freshly formed particles at both heights, whereas the more severe haze at ground level was connected directly to local primary particles and gaseous precursors leading to higher particle growth rates. The particle growth creates a feedback loop, in which a further development of haze increases the atmospheric stability, which in turn strengthens the persisting apparent decoupling between the two heights and increases the severity of haze at ground level. Moreover, we complemented our field observations with model analyses, which suggest that the growth of NPF-originated particles accounted up to ∼60% of the accumulation mode particles in the Beijing–Tianjin–Hebei area during haze conditions. The results suggest that a reduction in anthropogenic gaseous precursors, suppressing particle growth, is a critical step for alleviating haze although the number concentration of freshly formed particles (3–40 nm) via NPF does not reduce after emission controls.
Nitrous acid (HONO) plays important roles in tropospheric chemistry, but its source(s) are not completely understood. Here we analyze measurements of HONO, nitrogen dioxide (NO2), and related ...parameters at a coastal site in Hong Kong during September–December 2012. The nocturnal NO2‐to‐HONO conversion rates were estimated in air masses passing over land and sea surfaces. The conversion rates in the “sea cases” (3.17–3.36 × 10−2 h−1) were significantly higher than those in the “land cases” in our study (1.20–1.30 × 10−2 h−1) and in previous studies by others. These results suggest that air‐sea interactions may be a significant source of atmospheric HONO and need to be considered in chemical transport models.
Key Points
Strong HONO production in sea‐coming air massesAir‐sea interactions may significantly affect atmospheric chemistryChemical transport models may need to consider HONO production on the sea
Nitrate radical (NO3) and dinitrogen pentoxide (N2O5) play crucial roles in the nocturnal atmosphere. To quantify their impacts, we deployed a thermal-dissociation chemical ionization mass ...spectrometry (TD-CIMS), to measure their concentration, as well as ClNO2 at a coastal background site in the southern of China during the late autumn of 2012. Moderate levels of NO3, N2O5 and high concentration of ClNO2 were observed during the study period, indicating active NOx-O3 chemistry in the region. Distinct features of NO3, N2O5 and ClNO2 mixing ratios were observed in different airmasses. Further analysis revealed that the N2O5 heterogeneous reaction was the dominant loss of N2O5 and NO3, which showed higher loss rate compared to that in other coastal sites. Especially, the N2O5 loss rates could reach up to 0.0139 s−1 when airmasses went across the sea. The fast heterogeneous loss of N2O5 led to rapid NOx loss which could be comparable to the daytime process through NO2 oxidization by OH, and on the other hand, to rapid nitrate aerosol formation. In summary, our results revealed that the N2O5 hydrolysis could play significant roles in regulating the air quality by reducing NOx but forming nitrate aerosols.
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•N2O5 and NO3 were observed with distinct features in different airmasses.•The fast hydrolysis of N2O5 dominated the loss of N2O5 and NO3, particularly when continental air went across the sea.•The fast loss of N2O5 can contribute to a significant fraction of NOx removal and nitrate aerosol formation.
Nitrate radical (NO
) and dinitrogen pentoxide (N
O
) play crucial roles in the nocturnal atmosphere. To quantify their impacts, we deployed a thermal-dissociation chemical ionization mass ...spectrometry (TD-CIMS), to measure their concentration, as well as ClNO
at a coastal background site in the southern of China during the late autumn of 2012. Moderate levels of NO
, N
O
and high concentration of ClNO
were observed during the study period, indicating active NO
-O
chemistry in the region. Distinct features of NO
, N
O
and ClNO
mixing ratios were observed in different airmasses. Further analysis revealed that the N
O
heterogeneous reaction was the dominant loss of N
O
and NO
, which showed higher loss rate compared to that in other coastal sites. Especially, the N
O
loss rates could reach up to 0.0139 s
when airmasses went across the sea. The fast heterogeneous loss of N
O
led to rapid NO
loss which could be comparable to the daytime process through NO
oxidization by OH, and on the other hand, to rapid nitrate aerosol formation. In summary, our results revealed that the N
O
hydrolysis could play significant roles in regulating the air quality by reducing NO
but forming nitrate aerosols.
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