Sebastian Schmitt

Bio: Sebastian Schmitt is an academic researcher from Forschungszentrum Jülich. The author has contributed to research in topics: Aerosol & Benzene. The author has an hindex of 10, co-authored 16 publications receiving 432 citations.

Secondary organic aerosol reduced by mixture of atmospheric vapours

Abstract: 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.

Fast Photochemistry in Wintertime Haze: Consequences for Pollution Mitigation Strategies

Abstract: 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 va...

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

Abstract: . 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.

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

Abstract: . 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.

Mutual promotion between aerosol particle liquid water and particulate nitrate enhancement leads to severe nitrate-dominated particulate matter pollution and low visibility

Abstract: . 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 ( p NO 3 - ) over the NCP. In this study, we observed a polluted episode with the particulate nitrate mass fraction in nonrefractory PM 1 (NR-PM 1 ) being up to 44 % during wintertime in Beijing. Based on this typical p NO 3 - -dominated haze event, the linkage between aerosol water uptake and p NO 3 - 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 p NO 3 - 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.

Secondary Organic Aerosol Formation from Anthropogenic Air Pollution: Rapid and Higher than Expected

Abstract: [1] The atmospheric chemistry of volatile organic compounds (VOCs) in urban areas results in the formation of ‘photochemical smog’, including secondary organic aerosol (SOA). State-of-the-art SOA models parameterize the results of simulation chamber experiments that bracket the conditions found in the polluted urban atmosphere. Here we show that in the real urban atmosphere reactive anthropogenic VOCs (AVOCs) produce much larger amounts of SOA than these models predict, even shortly after sunrise. Contrary to current belief, a significant fraction of the excess SOA is formed from first-generation AVOC oxidation products. Global models deem AVOCs a very minor contributor to SOA compared to biogenic VOCs (BVOCs). If our results are extrapolated to other urban areas, AVOCs could be responsible for additional 3–25 Tg yr−1 SOA production globally, and cause up to −0.1 W m−2 additional top-of-the-atmosphere radiative cooling.

New particle formation in forests inhibited by isoprene emissions

Abstract: 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.

Environmental effects of ozone depletion, UV radiation and interactions with climate change: UNEP Environmental Effects Assessment Panel, update 2017

Abstract: This assessment, by the United Nations Environment Programme (UNEP) Environmental Effects Assessment Panel (EEAP), one of three Panels informing the Parties to the Montreal Protocol, provides an update, since our previous extensive assessment (Photochem. Photobiol. Sci., 2019, 18, 595-828), of recent findings of current and projected interactive environmental effects of ultraviolet (UV) radiation, stratospheric ozone, and climate change. These effects include those on human health, air quality, terrestrial and aquatic ecosystems, biogeochemical cycles, and materials used in construction and other services. The present update evaluates further evidence of the consequences of human activity on climate change that are altering the exposure of organisms and ecosystems to UV radiation. This in turn reveals the interactive effects of many climate change factors with UV radiation that have implications for the atmosphere, feedbacks, contaminant fate and transport, organismal responses, and many outdoor materials including plastics, wood, and fabrics. The universal ratification of the Montreal Protocol, signed by 197 countries, has led to the regulation and phase-out of chemicals that deplete the stratospheric ozone layer. Although this treaty has had unprecedented success in protecting the ozone layer, and hence all life on Earth from damaging UV radiation, it is also making a substantial contribution to reducing climate warming because many of the chemicals under this treaty are greenhouse gases.