Secondary organic aerosol contributes to the atmospheric particle burden with implications for air quality and climate. Biogenic volatile organic compounds emitted from plants are important secondary organic aerosol precursors with isoprene emissions dominating globally. However, its yield of particle mass from oxidation is generally modest compared to that of other terpenoids. Here we show that isoprene, carbon monoxide and methane can suppress the instantaneous mass and the overall mass yield derived from monoterpenes in mixtures. We find that isoprene scavenges hydroxyl radicals preventing reaction with monoterpenes and the resulting isoprene peroxy radicals scavenge highly oxygenated monoterpene products. These effects reduced 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. Highly reactive, modest aerosol yield compounds are not necessarily net producers and their oxidation can suppress both particle number and mass.

Abstract. Recent studies have recognised highly oxygenated organic molecules (HOMs) in the atmosphere as important in the formation of secondary organic aerosol (SOA). A large number of studies have focused on HOM formation from oxidation of biogenically emitted monoterpenes. However, HOM formation from anthropogenic vapours has so far received much less attention. Previous studies have identified the importance of aromatic volatile organic compounds (VOCs) for SOA formation. In this study, we investigated several aromatic compounds, benzene (C6H6), toluene (C7H8), and naphthalene (C10H8), for their potential to form HOMs upon reaction with hydroxyl radicals (OH). We performed flow tube experiments with all three VOCs and focused in detail on benzene HOM formation in the Jülich Plant Atmosphere Chamber (JPAC). In JPAC, we also investigated the response of HOMs to NOx and seed aerosol. Using a nitrate-based chemical ionisation mass spectrometer (CI-APi-TOF), we observed the formation of HOMs in the flow reactor oxidation of benzene from the first OH attack. However, in the oxidation of toluene and naphthalene, which were injected at lower concentrations, multi-generation OH oxidation seemed to impact the HOM composition. We tested this in more detail for the benzene system in the JPAC, which allowed for studying longer residence times. The results showed that the apparent molar benzene HOM yield under our experimental conditions varied from 4.1 % to 14.0 %, with a strong dependence on the OH concentration, indicating that the majority of observed HOMs formed through multiple OH-oxidation steps. The composition of the identified HOMs in the mass spectrum also supported this hypothesis. By injecting only phenol into the chamber, we found that phenol oxidation cannot be solely responsible for the observed HOMs in benzene experiments. When NOx was added to the chamber, HOM composition changed and many oxygenated nitrogen-containing products were observed in CI-APi-TOF. Upon seed aerosol injection, the HOM loss rate was higher than predicted by irreversible condensation, suggesting that some undetected oxygenated intermediates also condensed onto seed aerosol, which is in line with the hypothesis that some of the HOMs were formed in multi-generation OH oxidation. Based on our results, we conclude that HOM yield and composition in aromatic systems strongly depend on OH and VOC concentration and more studies are needed to fully understand this effect on the formation of HOMs and, consequently, SOA. We also suggest that the dependence of HOM yield on chamber conditions may explain part of the variability in SOA yields reported in the literature and strongly advise monitoring HOMs in future SOA studies.

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.

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 Jülich 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  <  30 ppb, [BVOC]0 ∕ [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.

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 (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.

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 : < 1to 20 ppb, SO 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.

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; In this study, the NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; dependence of secondary organic aerosol (SOA) formation from &amp;#946;-pinene photooxidation was comprehensively investigated in the J&amp;#252;lich Plant Atmosphere Chamber. Consistent with the results of previous NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; studies we found increases of SOA yields at low NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; conditions ([NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;]&lt;sub&gt;0&lt;/sub&gt; &lt; 30 ppb, [BVOC]&lt;sub&gt;0&lt;/sub&gt;/[NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;]&lt;sub&gt;0&lt;/sub&gt; &gt; 10 ppbC ppb&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;). Furthermore, increasing [NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;] at high NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; conditions ([NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;]&lt;sub&gt;0&lt;/sub&gt; &gt; 30 ppb, [BVOC]&lt;sub&gt;0&lt;/sub&gt;/[NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;]&lt;sub&gt;0&lt;/sub&gt; ~ 10 to ~ 2.6 ppbC ppb&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;) suppressed the SOA yield. The increase of SOA yield at low NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; conditions was attributed to increase of OH concentration, most probably by OH recycling in NO + HO&lt;sub&gt;2&lt;/sub&gt; &amp;#8594; NO&lt;sub&gt;2&lt;/sub&gt; + OH reaction. Separate measurements without NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; 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 [NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;]. Measuring the NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; dependence of SOA yields at lower [NO]/[NO&lt;sub&gt;2&lt;/sub&gt;] ratio showed less pronounced increase in both; OH concentration and SOA yield. This result was consistent to our assumption of OH recycling by NO and to SOA yields being dependent on OH concentrations. It furthermore indicated that NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; dependencies vary for different NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; compositions. A substantial fraction of the NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;-induced decrease of SOA yields at high NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; conditions was caused by NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;-induced suppression of new particle formation (NPF). This was shown by probing the NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; dependence of SOA formation in the presence of seed particles. After eliminating the effect of NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;-induced suppression of NPF and NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; induced changes of OH concentrations, the overall effect of NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; on the SOA yield from &amp;#946;-pinene photooxidation was moderate. Comparing with &amp;#946;-pinene experiments, the SOA formation from &amp;#945;-pinene photooxidation was only suppressed by increasing NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;. However, basic mechanisms of the NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; impacts were the same as that of &amp;#946;-pinene.&lt;/p&gt;

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; Anthropogenic emissions such as NO&lt;sub&gt;x&lt;/sub&gt; and SO&lt;sub&gt;2&lt;/sub&gt; influence the biogenic secondary organic aerosol (SOA) formation, but detailed mechanisms and effects are still elusive. We studied the effects of NO&lt;sub&gt;x&lt;/sub&gt; and SO&lt;sub&gt;2&lt;/sub&gt; on the SOA formation from photooxidation of &amp;#945;-pinene and limonene at ambient relevant NO&lt;sub&gt;x&lt;/sub&gt; and SO&lt;sub&gt;2&lt;/sub&gt; concentrations (NO&lt;sub&gt;x&lt;/sub&gt;: &lt;&amp;#8201;1&amp;#8201;ppb to 20&amp;#8201;ppb, SO&lt;sub&gt;2&lt;/sub&gt;: &lt;&amp;#8201;0.05&amp;#8201;ppb to 15&amp;#8201;ppb). In these experiments, monoterpene oxidation was dominated by OH oxidation. We found that SO&lt;sub&gt;2&lt;/sub&gt; induced nucleation and enhanced SOA mass formation. NO&lt;sub&gt;x&lt;/sub&gt; strongly suppressed not only new particle formation but also SOA mass yield. However, in the presence of SO&lt;sub&gt;2&lt;/sub&gt; which induced high number concentration of particles after oxidation to H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;, the mass yield of SOA at high NO&lt;sub&gt;x&lt;/sub&gt; was comparable to that at low NO&lt;sub&gt;x&lt;/sub&gt;. This indicates that the suppression of SOA yield by NO&lt;sub&gt;x&lt;/sub&gt; was mainly due to the suppressed new particle formation, leading to a lack of particle surface for the organics to condense on. By compensating the suppressing effect on nucleation of NO&lt;sub&gt;x&lt;/sub&gt;, SO&lt;sub&gt;2&lt;/sub&gt; also compensated the suppressing effect on SOA yield. Aerosol mass spectrometer data show that increasing NO&lt;sub&gt;x&lt;/sub&gt; enhanced nitrate formation. The majority of the nitrate was organic nitrate (57&amp;#8201;%&amp;#8211;77&amp;#8201;%), even in low NO&lt;sub&gt;x&lt;/sub&gt; conditions (&lt;~&amp;#8201;1&amp;#8201;ppb). Organic nitrate contributed 7&amp;#8201;%&amp;#8211;26&amp;#8201;% of total organics assuming a molecular weight of 200&amp;#8201;g/mol. SOA from &amp;#945;-pinene photooxidation at high NO&lt;sub&gt;x&lt;/sub&gt; had generally lower hydrogen to carbon ratio (H&amp;#8201;/&amp;#8201;C), compared with at low NO&lt;sub&gt;x&lt;/sub&gt;. The NO&lt;sub&gt;x&lt;/sub&gt; dependence of the chemical composition can be attributed to the NO&lt;sub&gt;x&lt;/sub&gt; dependence of the branching ratio of the RO&lt;sub&gt;2&lt;/sub&gt; loss reactions, leading to lower fraction of organic hydroperoxide and higher fractions of organic nitrate at high NO&lt;sub&gt;x&lt;/sub&gt;. While NO&lt;sub&gt;x&lt;/sub&gt; suppressed new particle formation and SOA mass formation, SO&lt;sub&gt;2&lt;/sub&gt; can compensate such effects, and the combining effect of SO&lt;sub&gt;2&lt;/sub&gt; and NO&lt;sub&gt;x&lt;/sub&gt; may have important influence on SOA formation affected by interactions of biogenic volatile organic compounds (VOC) with anthropogenic emissions.&lt;/p&gt;