It has been suggested that volatile organic compounds (VOCs) are involved in organic aerosol formation, which in turn affects radiative forcing and climate. The most abundant VOCs emitted by terrestrial vegetation are isoprene and its derivatives, such as monoterpenes and sesquiterpenes. New particle formation in boreal regions is related to monoterpene emissions and causes an estimated negative radiative forcing of about -0.2 to -0.9 W m-2. The annual variation in aerosol growth rates during particle nucleation events correlates with the seasonality of monoterpene emissions of the local vegetation, with a maximum during summer. The frequency of nucleation events peaks, however, in spring and autumn. Here we present evidence from simulation experiments conducted in a plant chamber that isoprene can significantly inhibit new particle formation. The process leading to the observed decrease in particle number concentration is linked to the high reactivity of isoprene with the hydroxyl radical (OH). The suppression is stronger with higher concentrations of isoprene, but with little dependence on the specific VOC mixture emitted by trees. A parameterization of the observed suppression factor as a function of isoprene concentration suggests that the number of new particles produced depends on the OH concentration and VOCs involved in the production of new particles undergo three to four steps of oxidation by OH. Our measurements simulate conditions that are typical for forested regions and may explain the observed seasonality in the frequency of aerosol nucleation events, with a lower number of nucleation events during summer compared to autumn and spring. Biogenic emissions of isoprene are controlled by temperature and light, and if the relative isoprene abundance of biogenic VOC emissions increases in response to climate change or land use change, the new particle formation potential may decrease, thus damping the aerosol negative radiative forcing effect.

New particle formation in forests inhibited by isoprene emissions

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). Adding reactive gases such as isoprene to mixtures lowers the production of secondary organic aerosol in the atmosphere, thus reducing the atmospheric particulate burden, with implications for human health and climate.

/pdf/secondary-organic-aerosol-reduced-by-mixture-of-atmospheric-113t9ukam9.pdf

Secondary organic aerosol reduced by mixture of atmospheric vapours

. Anthropogenic emissions such as NO x and SO 2 influence the biogenic
secondary organic aerosol (SOA) formation, but detailed mechanisms and
effects are still elusive. We studied the effects of NO x and SO 2 on
the SOA formation from the photooxidation of α -pinene and limonene at
ambient relevant NO x and SO 2 concentrations (NO x : 2 : < 0.05 to 15 ppb). In these experiments,
monoterpene oxidation was dominated by OH oxidation. We found that SO 2 
induced nucleation and enhanced SOA mass formation. NO x strongly
suppressed not only new particle formation but also SOA mass yield. However,
in the presence of SO 2 which induced a high number concentration of
particles after oxidation to H 2 SO 4 , the suppression of the mass
yield of SOA by NO x was completely or partly compensated for. This indicates
that the suppression of SOA yield by NO x was largely due to the
suppressed new particle formation, leading to a lack of particle surface for
the organics to condense on and thus a significant influence of vapor wall
loss on SOA mass yield. By compensating for the suppressing effect on
nucleation of NO x , SO 2 also compensated for the suppressing effect
on SOA yield. Aerosol mass spectrometer data show that increasing NO x 
enhanced nitrate formation. The majority of the nitrate was organic nitrate
(57–77 %), even in low-NO x conditions ( ∼  1 ppb). Organic nitrate contributed 7–26 % of total organics assuming a
molecular weight of 200 g mol −1 . SOA from α -pinene photooxidation at
high NO x had a generally lower hydrogen to carbon ratio (H  ∕  C), compared to
low NO x . The NO x dependence of the chemical composition can be
attributed to the NO x dependence of the branching ratio of the RO 2 
loss reactions, leading to a lower fraction of organic hydroperoxides and
higher fractions of organic nitrates at high NO x . While NO x 
suppressed new particle formation and SOA mass formation, SO 2 can
compensate for such effects, and the combining effect of SO 2 and
NO x may have an important influence on SOA formation affected by
interactions of biogenic volatile organic compounds (VOCs) with anthropogenic
emissions.

/pdf/effects-of-no-x-and-so-2-on-the-secondary-organic-aerosol-r3khthk0wa.pdf

Effects of NO x and SO 2 on the secondary organic aerosol formation from photooxidation of α -pinene and limonene

Fine particulate matter (PM2.5), largely composed of secondary organic aerosol (SOA), is currently one of the most intractable environmental problems in China. As crucial precursors for SOA, understanding the formation propensity of various volatile organic compound (VOC) species and sources is useful for pollution control. In this work, we estimated the SOA formation potential (SOAP) of anthropogenic VOC emissions based on an improved speciated VOC emission inventory and investigated its distribution in China. According to our estimates, toluene had the largest SOAP, followed by n-dodecane, m-/p-xylene, styrene, n-decane, and n-undecane, while passenger cars, chemical fiber manufacturing, asphalt paving, and building coating were the top five SOAP-contributing sources nationwide. The spatial distribution of SOAP in China shows a distinct pattern of high values in the southeast and low values in the northwest. Beijing–Tianjin–Hebei and surroundings, the Yangtze River Delta, Pearl River Delta, and Sichuan–...

Spatial Distribution of Secondary Organic Aerosol Formation Potential in China Derived from Speciated Anthropogenic Volatile Organic Compound Emissions.

. The global secondary organic aerosol (SOA) budget is highly uncertain, with
global annual SOA production rates, estimated from global models, ranging
over an order of magnitude and simulated SOA concentrations underestimated
compared to observations. In this study, we use a global composition-climate
model (UKCA) with interactive chemistry and aerosol microphysics to provide
an in-depth analysis of the impact of each VOC source on the global SOA
budget and its seasonality. We further quantify the role of each source on
SOA spatial distributions, and evaluate simulated seasonal SOA concentrations
against a comprehensive set of observations. The annual global SOA production
rates from monoterpene, isoprene, biomass burning, and anthropogenic
precursor sources is 19.9, 19.6, 9.5, and 24.6  Tg (SOA) a−1 ,
respectively. When all sources are included, the SOA production rate from all
sources is 73.6  Tg (SOA) a−1 , which lies within the range of
estimates from previous modelling studies. SOA production rates and SOA
burdens from biogenic and biomass burning SOA sources peak during Northern
Hemisphere (NH) summer. In contrast, the anthropogenic SOA production rate is
fairly constant all year round. However, the global anthropogenic SOA burden
does have a seasonal cycle which is lowest during NH summer, which is
probably due to enhanced wet removal. Inclusion of the new SOA sources also
accelerates the ageing by condensation of primary organic aerosol (POA),
making it more hydrophilic, leading to a reduction in the POA lifetime. With
monoterpene as the only source of SOA, simulated SOA and total organic
aerosol (OA) concentrations are underestimated by the model when compared to
surface and aircraft measurements. Model agreement with observations improves
with all new sources added, primarily due to the inclusion of the
anthropogenic source of SOA, although a negative bias remains. A further
sensitivity simulation was performed with an increased anthropogenic SOA
reaction yield, corresponding to an annual global SOA production rate of
70.0  Tg (SOA) a−1 . Whilst simulated SOA concentrations improved
relative to observations, they were still underestimated in urban
environments and overestimated further downwind and in remote environments.
In contrast, the inclusion of SOA from isoprene and biomass burning did not
improve model–observations biases substantially except at one out of two
tropical locations. However, these findings may reflect the very limited
availability of observations to evaluate the model, which are primarily
located in the NH mid-latitudes where anthropogenic emissions are high. Our
results highlight that, within the current uncertainty limits in SOA sources
and reaction yields, over the NH mid-latitudes, a large anthropogenic SOA
source results in good agreement with observations. However, more
observations are needed to establish the importance of biomass burning and
biogenic sources of SOA in model agreement with observations.

/pdf/the-impact-of-biogenic-anthropogenic-and-biomass-burning-2k18rg5nj0.pdf

The impact of biogenic, anthropogenic, and biomass burning volatile organic compound emissions on regional and seasonal variations in secondary organic aerosol

. In this study, the NOx dependence of secondary organic aerosol (SOA) formation from photooxidation of the biogenic volatile organic compound (BVOC) β-pinene was comprehensively investigated in the Julich Plant Atmosphere Chamber. Consistent with the results of previous NOx studies we found increases of SOA yields with increasing [NOx] at low-NOx conditions ([NOx]0     10 ppbC ppb−1). Furthermore, increasing [NOx] at high-NOx conditions ([NOx]0  >  30 ppb, [BVOC]0 ∕ [NOx]0  ∼  10 to  ∼  2.6 ppbC ppb−1) suppressed the SOA yield. The increase of SOA yield at low-NOx conditions was attributed to an increase of OH concentration, most probably by OH recycling in NO + HO2  →  NO2 + OH reaction. Separate measurements without NOx addition but with different OH primary production rates confirmed the OH dependence of SOA yields. After removing the effect of OH concentration on SOA mass growth by keeping the OH concentration constant, SOA yields only decreased with increasing [NOx]. Measuring the NOx dependence of SOA yields at lower [NO] ∕ [NO2] ratio showed less pronounced increase in both OH concentration and SOA yield. This result was consistent with our assumption of OH recycling by NO and to SOA yields being dependent on OH concentrations. Our results furthermore indicated that NOx dependencies vary for different NOx compositions. A substantial fraction of the NOx-induced decrease of SOA yields at high-NOx conditions was caused by NOx-induced suppression of new particle formation (NPF), which subsequently limits the particle surface where low volatiles condense. This was shown by probing the NOx dependence of SOA formation in the presence of seed particles. After eliminating the effect of NOx-induced suppression of NPF and NOx-induced changes of OH concentrations, the remaining effect of NOx on the SOA yield from β-pinene photooxidation was moderate. Compared to β-pinene, the SOA formation from α-pinene photooxidation was only suppressed by increasing NOx. However, basic mechanisms of the NOx impacts were the same as that of β-pinene.

/pdf/impact-of-no-x-and-oh-on-secondary-organic-aerosol-formation-22m5wggzub.pdf

Mehrnaz Sarrafzadeh

Papers

Impact of NO x and OH on secondary organic aerosol formation from β -pinene photooxidation