Heterogeneous reactions of mineral dust aerosol with trace gases in the atmosphere could directly and indirectly affect tropospheric oxidation capacity, in addition to aerosol composition and ...physicochemical properties. In this article we provide a comprehensive and critical review of laboratory studies of heterogeneous uptake of OH, NO3, O3, and their directly related species as well (including HO2, H2O2, HCHO, HONO, and N2O5) by mineral dust particles. The atmospheric importance of heterogeneous uptake as sinks for these species is assessed (i) by comparing their lifetimes with respect to heterogeneous reactions with mineral dust to lifetimes with respect to other major loss processes and (ii) by discussing relevant field and modeling studies. We have also outlined major open questions and challenges in laboratory studies of heterogeneous uptake by mineral dust and discussed research strategies to address them in order to better understand the effects of heterogeneous reactions with mineral dust on tropospheric oxidation capacity.
As nitrous acid (HONO) photolysis is an important source of hydroxyl radical (OH), apportionment of the ambient HONO sources is necessary to better understand atmospheric oxidation. Based on the data ...HONO-related species and various parameters measured during the one–month campaign at Wangdu (a rural site in North China plain) in summer 2014, a box model was adopted with input of current literature parametrizations for various HONO sources (nitrogen dioxide heterogeneous conversion, photoenhanced conversion, photolysis of adsorbed nitric acid and particulate nitrate, acid displacement, and soil emission) to reveal the relative importance of each source at the rural site. The simulation results reproduced the observed HONO production rates during noontime in general but with large uncertainty from both the production and destruction terms. NO2 photoenhanced conversion and photolysis of particulate nitrate were found to be the two major mechanisms with large potential of HONO formation but the associated uncertainty may reduce their importance to be nearly negligible. Soil nitrite was found to be an important HONO source during fertilization periods, accounted for (80 ± 6)% of simulation HONO during noontime. For some episodes of the biomass burning, only the NO2 heterogeneous conversion to HONO was promoted significantly. In summary, the study of the HONO budget is still far from closed, which would require a significant effort on both the accurate measurement of HONO and the determination of related kinetic parameters for its production pathways.
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.
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.
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.
Amplified Trace Gas Removal in the Troposphere Hofzumahaus, Andreas; Rohrer, Franz; Lu, Keding ...
Science (American Association for the Advancement of Science),
06/2009, Volume:
324, Issue:
5935
Journal Article
Peer reviewed
The degradation of trace gases and pollutants in the troposphere is dominated by their reaction with hydroxyl radicals (OH). The importance of OH rests on its high reactivity, its ubiquitous ...photochemical production in the sunlit atmosphere, and most importantly on its regeneration in the oxidation chain of the trace gases. In the current understanding, the recycling of OH proceeds through HO₂ reacting with NO, thereby forming ozone. A recent field campaign in the Pearl River Delta, China, quantified tropospheric OH and HO₂ concentrations and turnover rates by direct measurements. We report that concentrations of OH were three to five times greater than expected, and we propose the existence of a pathway for the regeneration of OH independent of NO, which amplifies the degradation of pollutants without producing ozone.
This study reveals aerosol liquid water content (ALWC) in PM2.5 ranged from 2% up to 74%, and the associated secondary inorganic fraction rose from 24% to 55%, while ambient relative humidity (RH) ...increased from 15% to 83% in the atmosphere over Beijing. Unexpectedly, the secondary inorganic fraction in PM2.5 increased with an increase in the ambient RH, which is a meteorological parameter independent of anthropogenic activities, indicating the presence of a feedback mechanism driven by Henry’s law and thermodynamic equilibrium. During haze episodes, simultaneously elevated RH levels and anthropogenic secondary inorganic mass concentrations resulted in an abundant ALWC. The condensed water could act as an efficient medium for multiphase reactions, thereby facilitating the transformation of reactive gaseous pollutants into particles and accelerating the formation of heavy haze. ALWC was well correlated with the mass concentrations of both nitrate and sulfate, indicating both nitrate and sulfate salts play key roles in determining ALWC. Coincident with a significant reduction in SO2 emissions throughout China, nitrates will become a dominant anthropogenic inorganic salt driving ALWC. Thus, the abundance of ALWC and its effects on the aerosol chemistry and climate should be reconsidered.
Atmospheric chemistry research has been growing rapidly in China in the last 25 years since the concept of the “air pollution complex” was first proposed by Professor Xiaoyan TANG in 1997. For papers ...published in 2021 on air pollution (only papers included in the Web of Science Core Collection database were considered), more than 24 000 papers were authored or co-authored by scientists working in China. In this paper, we review a limited number of representative and significant studies on atmospheric chemistry in China in the last few years, including studies on (1) sources and emission inventories, (2) atmospheric chemical processes, (3) interactions of air pollution with meteorology, weather and climate, (4) interactions between the biosphere and atmosphere, and (5) data assimilation. The intention was not to provide a complete review of all progress made in the last few years, but rather to serve as a starting point for learning more about atmospheric chemistry research in China. The advances reviewed in this paper have enabled a theoretical framework for the air pollution complex to be established, provided robust scientific support to highly successful air pollution control policies in China, and created great opportunities in education, training, and career development for many graduate students and young scientists. This paper further highlights that developing and low-income countries that are heavily affected by air pollution can benefit from these research advances, whilst at the same time acknowledging that many challenges and opportunities still remain in atmospheric chemistry research in China, to hopefully be addressed over the next few decades.
Gaseous nitrous acid (HONO) is an important precursor of tropospheric hydroxyl radicals (OH). OH is responsible for atmospheric self-cleansing and controls the concentrations of greenhouse gases like ...methane and ozone. Due to lack of measurements, vertical distributions of HONO and its sources in the troposphere remain unclear. Here, we present a set of observations of HONO and its budget made onboard a Zeppelin airship. In a sunlit layer separated from Earth's surface processes by temperature inversion, we found high HONO concentrations providing evidence for a strong gas-phase source of HONO consuming nitrogen oxides and potentially hydrogen oxide radicals. The observed properties of this production process suggest that the generally assumed impact of HONO on the abundance of OH in the troposphere is substantially overestimated.
In the summer of 2018, a comprehensive field campaign, with measurements on
HONO and related parameters, was conducted at the foot (150 m a.s.l.) and
the summit of Mt. Tai (1534 m a.s.l.) in the ...central North China Plain
(NCP). With the implementation of a 0-D box model, the HONO budget with six
additional sources and its role in radical chemistry at the foot station
were explored. We found that the model default source, NO + OH, could only
reproduce 13 % of the observed HONO, leading to a strong unknown source
strength of up to 3 ppbv h−1. Among the additional sources, the
NO2 uptake on the ground surface dominated (∼ 70 %)
nighttime HONO formation, and its photo-enhanced reaction dominated
(∼ 80 %) daytime HONO formation. Their contributions were
sensitive to the mixing layer height (MLH) used for the parameterizations,
highlighting the importance of a reasonable MLH for exploring ground-level
HONO formation in 0-D models and the necessity of gradient measurements. A
ΔHONO/ΔNOx ratio of 0.7 % for direct emissions from
vehicle exhaust was inferred, and a new method to quantify its contribution
to the observations was proposed and discussed. Aerosol-derived sources,
including the NO2 uptake on the aerosol surface and the particulate
nitrate photolysis, did not lead to significant HONO formation, with their
contributions lower than NO + OH. HONO photolysis in the early morning initialized the daytime photochemistry
at the foot station. It was also a substantial radical source throughout the
daytime, with contributions higher than O3 photolysis to OH initiation.
Moreover, we found that OH dominated the atmospheric oxidizing capacity in
the daytime, while modeled NO3 appeared to be significant at night.
Peaks of modeled NO3 time series and average diurnal variation reached
22 and 9 pptv, respectively. NO3-induced reactions contribute 18 % of
nitrate formation potential (P(HNO3)) and 11 % of the isoprene
(C5H8) oxidation throughout the whole day. At night, NO3
chemistry led to 51 % and 44 % of P(HNO3) or the C5H8
oxidation, respectively, implying that NO3 chemistry could
significantly affect nighttime secondary organic and inorganic aerosol
formation in this high-O3 region. Considering the severe O3
pollution in the NCP and the very limited NO3 measurements, we suggest
that besides direct measurements of HOx and primary HOx precursors
(O3, HONO, alkenes, etc.), NO3 measurements should be conducted to
understand the atmospheric oxidizing capacity and air pollution formation in
this and similar regions.
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