Anthropogenic emissions such as NO_x and SO₂ influence the biogenic secondary organic aerosol (SOA) formation, but detailed mechanisms and effects are still elusive. We studied the effects of NO_x and SO₂ on the SOA formation from the photooxidation of α-pinene and limonene at ambient relevant NO_x and SO₂ concentrations (NO_x: < 1to 20 ppb, SO₂: < 0.05 to 15 ppb). In these experiments, monoterpene oxidation was dominated by OH oxidation. We found that SO₂ 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 SO₂ which induced a high number concentration of particles after oxidation to H₂SO₄, 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, SO₂ 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 RO₂ 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, SO₂ can compensate for such effects, and the combining effect of SO₂ and NO_x may have an important influence on SOA formation affected by interactions of biogenic volatile organic compounds (VOCs) with anthropogenic emissions.

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 NO_x 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 NO_x 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 this study, the NO_x 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 NO_x studies we found increases of SOA yields with increasing [NO_x] at low-NO_x conditions ([NO_x]₀ < 30 ppb, [BVOC]₀ ∕ [NO_x]₀ > 10 ppbC ppb⁻¹). Furthermore, increasing [NO_x] at high-NO_x conditions ([NO_x]₀ > 30 ppb, [BVOC]₀ ∕ [NO_x]₀ ∼ 10 to ∼ 2.6 ppbC ppb⁻¹) suppressed the SOA yield. The increase of SOA yield at low-NO_x conditions was attributed to an increase of OH concentration, most probably by OH recycling in NO + HO₂ → NO₂ + OH reaction. Separate measurements without NO_x 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_x]. Measuring the NO_x 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 NO_x dependencies vary for different NO_x compositions. A substantial fraction of the NO_x-induced decrease of SOA yields at high-NO_x conditions was caused by NO_x-induced suppression of new particle formation (NPF), which subsequently limits the particle surface where low volatiles condense. This was shown by probing the NO_x dependence of SOA formation in the presence of seed particles. After eliminating the effect of NO_x-induced suppression of NPF and NO_x-induced changes of OH concentrations, the remaining effect of NO_x 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 NO_x. However, basic mechanisms of the NO_x impacts were the same as that of β-pinene.

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 NO_x concentrations result in the frequent occurrence of particulate matter pollution dominated by nitrate ($p{\mathrm{NO}}_{\mathrm{3}}^{-}$ ) 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 $p{\mathrm{NO}}_{\mathrm{3}}^{-}$ -dominated haze event, the linkage between aerosol water uptake and $p{\mathrm{NO}}_{\mathrm{3}}^{-}$ 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 $p{\mathrm{NO}}_{\mathrm{3}}^{-}$ 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.