The most important chemical cleaning agent of the atmosphere is the hydroxyl radical, OH. It determines the oxidizing power of the atmosphere, and thereby controls the removal of nearly all gaseous ...atmospheric pollutants. The atmospheric supply of OH is limited, however, and could be overcome by consumption due to increasing pollution and climate change, with detrimental feedback effects. To date, the high variability of OH concentrations has prevented the use of local observations to monitor possible trends in the concentration of this species. Here we present and analyse long-term measurements of atmospheric OH concentrations, which were taken between 1999 and 2003 at the Meteorological Observatory Hohenpeissenberg in southern Germany. We find that the concentration of OH can be described by a surprisingly linear dependence on solar ultraviolet radiation throughout the measurement period, despite the fact that OH concentrations are influenced by thousands of reactants. A detailed numerical model of atmospheric reactions and measured trace gas concentrations indicates that the observed correlation results from compensations between individual processes affecting OH, but that a full understanding of these interactions may not be possible on the basis of our current knowledge of atmospheric chemistry. As a consequence of the stable relationship between OH concentrations and ultraviolet radiation that we observe, we infer that there is no long-term trend in the level of OH in the Hohenpeissenberg data set.
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.
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.
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 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.
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
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.
We derive an analytical expression for the deposition velocity of H
2
on soil, v
d
, which explicitly describes the dependence of v
d
on the average soil moisture,
, and temperature, T. It is based ...on the solution of the equation for vertical diffusion in a two-layer soil model consisting of a dry top layer of a depth, δ, free of H
2
removal, and a moist deeper layer with a uniform H
2
removal rate constant. The dependence of δ on
is derived from modelled vertical profiles of Θ
w
. The resulting dependence of v
d
on
is compared to the previously used v
d
(
) derived from a single-layer model, i.e. one without a dry top layer, and with a set of v
d
from field measurements. The implications are discussed.
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.
In a recent paper we derived an analytical expression for the deposition velocity of molecular hydrogen on soil that includes the action of a dry top soil layer without H2 removal (Ehhalt and Rohrer, ...2013). With this short note we would like to expand that expresion to include a production of H2 within the soil. Citation: Tellus B 2013, 65, 20620, http://dx.doi.org/10.3402/tellusb.v65i0.20620
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