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.
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.
Amplified Trace Gas Removal in the Troposphere Hofzumahaus, Andreas; Rohrer, Franz; Lu, Keding ...
Science (American Association for the Advancement of Science),
06/2009, Letnik:
324, Številka:
5935
Journal Article
Recenzirano
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.
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
In 2014, a large, comprehensive field campaign was conducted in the densely populated North China Plain. The measurement site was located in a botanic garden close to the small town Wangdu, without ...major industry but influenced by regional transportation of air pollution. The loss rate coefficient of atmospheric hydroxyl radicals (OH) was quantified by direct measurements of the OH reactivity. Values ranged between 10 and 20 s−1 for most of the daytime. Highest values were reached in the late night with maximum values of around 40 s−1. OH reactants mainly originated from anthropogenic activities as indicated (1) by a good correlation between measured OH reactivity and carbon monoxide (linear correlation coefficient R2 = 0.33) and (2) by a high contribution of nitrogen oxide species to the OH reactivity (up to 30 % in the morning). Total OH reactivity was measured by a laser flash photolysis–laser-induced fluorescence instrument (LP-LIF). Measured values can be explained well by measured trace gas concentrations including organic compounds, oxygenated organic compounds, CO and nitrogen oxides. Significant, unexplained OH reactivity was only observed during nights, when biomass burning of agricultural waste occurred on surrounding fields. OH reactivity measurements also allow investigating the chemical OH budget. During this campaign, the OH destruction rate calculated from measured OH reactivity and measured OH concentration was balanced by the sum of OH production from ozone and nitrous acid photolysis and OH regeneration from hydroperoxy radicals within the uncertainty of measurements. However, a tendency for higher OH destruction compared to OH production at lower concentrations of nitric oxide is also observed, consistent with previous findings in field campaigns in China.
Daytime concentrations of HONO, NOx, OH and photolysis frequencies were measured during the ECHO field campaign in a mixed deciduous forest near Jülich, West‐Germany, in summer 2003. Midday ...measurements show clear evidence for a large, yet unexplained daytime source of HONO (∼500 pptv/h), which represents an important net source of OH radicals due to ongoing HONO photolysis. The evidence for a large HONO daytime source is for the first time completely constrained by measured parameters, needed to determine the daytime budget of HONO. The large contribution of 33% to the primary OH production during noon at the top of the forest canopy suggests that the unexplained source of HONO could have an important impact on the photochemical transformation of biogenically emitted volatile organic compounds (VOCs) by OH into partly oxidized VOCs and secondary organic aerosols during their release from forest regions into the troposphere.
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.
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 and
RO2 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.
This paper presents the measurements of OH and HO2 radical concentrations as well as photolysis frequencies of different molecules during the Berliner Ozone (BERLIOZ) field experiment in July/August ...1998 at the rural site Pabstthum about 50 km NW of Berlin. Radical concentrations were measured using laser‐induced fluorescence (LIF) spectroscopy, while filter radiometers and a scanning spectroradiometer were used to obtain photolysis frequencies. The radical data set covers the time period from 20 July to 6 August and consists of more than 6000 simultaneous measurements of OH and HO2 with a typical time resolution of about 90 s. The maximum OH and HO2 daytime concentrations were 8 × 106 and 8 × 108 cm−3, respectively. While nighttime values of OH were usually below the detection limit of our instrument (3.5 × 105 cm−3), HO2 did show significant concentrations throughout most of the nights (on average 3 × 107 cm−3). The OH concentration was mainly controlled by solar UV radiation and showed a high linear correlation with J(O1D). A deviation from this general behavior was observed around dawn and dusk, when OH concentrations well above the detection limit were observed, although J(O1D) was essentially zero. A comparison with data sets from previous campaigns revealed that even though the linear correlation is found in other environments as well the slope OH/J(O1D) differs significantly. The diurnal cycles of HO2 were less dependent on the solar actinic flux but were predominantly influenced by NO. During episodes of high NO, HO2 remained below the detection limit (1 × 107 cm−3) but started to rise rapidly as soon as NO started to decrease.
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