In the summer of 2017, heavy ozone pollution swamped most of the North China Plain (NCP), with the maximum regional average of daily maximum 8 h ozone concentration (MDA8) reaching almost 120 ppbv. In light of the continuing reduction of anthropogenic emissions in China, the underlying mechanisms for the occurrences of these regional extreme ozone episodes are elucidated from two perspectives: meteorology and biogenic emissions. The significant positive correlation between MDA8 ozone and temperature, which is amplified during heat waves concomitant with stagnant air and no precipitation, supports the crucial role of meteorology in driving high ozone concentrations. We also find that biogenic emissions are enhanced due to factors previously not considered. During the heavy ozone pollution episodes in June 2017, biogenic emissions driven by high vapor pressure deficit (VPD), land cover change and urban landscape yield an extra mean MDA8 ozone of 3.08, 2.79 and 4.74 ppbv, respectively, over the NCP, which together contribute as much to MDA8 ozone as biogenic emissions simulated using the land cover of 2003 and ignoring VPD and urban landscape. In Beijing, the biogenic emission increase due to urban landscape has a comparable effect on MDA8 ozone to the combined effect of high VPD and land cover change between 2003 and 2016. In light of the large effect of urban landscape on biogenic emission and the subsequent ozone formation, the types of trees may be cautiously selected to take into account of the biogenic volatile organic compound (BVOC) emission during the afforestation of cities. This study highlights the vital contributions of heat waves, land cover change and urbanization to the occurrence of extreme ozone episodes, with significant implications for ozone pollution control in a future when heat wave frequency and intensity are projected to increase under global warming.

The current poor understanding of soluble iron (Fe) yield in atmospheric aerosols leaves two observational facts having not yet been correctly simulated in numerical models: the high Fe solubility in aerosols with low Fe content and, hence, the wide range of observed Fe solubility. Our observation at Qingdao, a coastal city of China, revealed that soluble Fe was produced along with aerosol acidification much more efficiently in fog than under other weather conditions. The median Fe solubility in fog aerosols, 5.81%, was 3.3 times of that in haze aerosols, 5.2 times of that in clear days, and 21.5 times of that in dust aerosols. Involving fog processing in models may reduce the discrepancy in the atmospheric flux of soluble Fe to the ocean between numerical simulations and field observations.

Utilizing a recent observational dataset of particulate matter with diameters less than 2.5 µm (PM_2.5) in North China, this study reveals a distinct seesaw feature of abnormally high and low PM_2.5 concentrations in the adjacent two months of December 2015 and January 2016, accompanied by distinct meteorological modulations. The seesaw pattern is postulated to be linked to a super El Niño and the Arctic Oscillation (AO). During the mature phase of El Niño in December 2015, the weakened East Asian winter monsoon (EAWM) and the associated low-level southerly wind anomaly reduced planetary boundary layer (PBL) height, favoring strong haze formation. This circulation pattern was completely reversed in the following month, in part due to a sudden phase change of the AO from positive to negative and the beginning of a decay of the El Niño, which enhanced the southward shift of the upper tropospheric jet from December to January relative to climatology, leading to an enhanced EAWM and substantially lower haze formation. This sub-seasonal change in circulation is also robustly found in 1982–1983 and 1997–1998, implicative of a general physical mechanism dynamically linked to El Niño and the AO. Numerical experiments using the Weather Research and Forecasting (WRF) Community Multiscale Air Quality (CMAQ) model were used to test the modulation of the meteorological conditions on haze formation. With the same emission, simulations for three super El Niño periods (1983, 1997 and 2015) robustly show higher PM_2.5 concentrations under the mature phase of the super El Niño, but substantially lower PM_2.5 concentrations during the decay phase of El Niño (and the sudden AO phase change), further verifying the modulation effect of the sub-seasonal circulation anomaly on PM_2.5 concentrations in North China.

We measured the particle number concentration, size distribution, and new particle formation (NPF) events in the marine atmosphere during six cruise campaigns over the marginal seas of China in 2011–2016 and one campaign from the marginal seas to the Northwest Pacific Ocean (NWPO) in 2014. We observed relatively frequent NPF events in the atmosphere over the marginal seas of China, i.e., on 23 out of 126 observational days, with the highest frequency of occurrence in fall, followed by spring and summer. In total, 22 out of 23 NPF events were found to be associated with the long-range transport of continental pollutants based on 24 h air mass back trajectories and pre-existing particle number concentrations, which largely exceeded the clean marine background, leaving one much weaker NPF event that was likely induced by oceanic precursors alone, as supported by multiple independent pieces of evidence. Although the long-range transport signal of continental pollutants can be clearly observed in the remote marine atmosphere over the NWPO, NPF events were observed on only 2 out of 36 days. The nucleation-mode particles (<30 nm), however, accounted for as high as 35 %±13 % of the total particle number concentration during the NWPO cruise campaign, implying the existence of many undetected NPF events in the near-sea-level atmosphere or above. To better characterize NPF events, we introduce a term called the net maximum increase in the nucleation-mode particle number concentration (NMINP) and correlate it with the formation rate of new particles (FR). We find a moderately good linear correlation between NMINP and FR at FR≤8 cm⁻³ s⁻¹, but no correlation exists at FR>8 cm⁻³ s⁻¹. The possible mechanisms are argued in terms of the roles of different vapor precursors. We also find that a ceiling exists for the growth of new particles from 10 nm to larger sizes in most NPF events. We thereby introduce a term called the maximum geometric median diameter of new particles (D_pgmax) and correlate it with the growth rate of new particles (GR). A moderately good linear correlation is also obtained between the D_pgmax and GR, and only GR values larger than 7.9 nm h⁻¹ can lead to new particles growing with a D_pgmax beyond 50 nm based on the equation. By combining simultaneous measurements of the particle number size distributions and cloud condensation nuclei (CCN) at different super saturations (SS), we observed a clear increase in CCN when the D_pg of new particles exceeded 50 nm at SS=0.4 %. However, this case did not occur for SS=0.2 %. Consistent with the results of previous studies in the continental atmosphere, our results imply that particles smaller than 50 nm are unlikely activated as CCN at SS=0.4 % in the marine atmosphere. Moreover, κ decrease from 0.4 to 0.1 during the growth period of new particles, implying that organics likely overwhelm the growth of new particles to CCN size. The chemical analysis of nano-Micro-Orifice Uniform Deposit Impactor (nano-MOUDI) samples reveals that trimethylamine (TMA) and oxalic acid might appreciably contribute to the growth of new particles in some cases.

To better understand the responses of particle number concentrations (PNCs) and new particle formation (NPF) to the largely reduced air pollutant emissions in urban atmospheres, we investigated the particle number size distributions in Qingdao, a coastal megacity in northern China, during two separate periods, 2010–2012 and 2016–2018. The results show only an average of 5% decrease in the total PNCs in 2016–2018 relative to 2010–2012, although the PM_2.5 mass concentration decreased by over 40%. Nucleation‐mode PNCs decreased by 20%, which is attributable to reductions in primary emissions from on‐road vehicles and secondary sources. Unexpectedly, the accumulation‐mode PNCs increased by 11% in 2016–2018 relative to 2010–2012. The SO₂ concentrations strongly decreased by ∼60%, and the NPF frequencies slightly decreased from 34% in 2010–2012 to 25% in 2016–2018. However, there were no significant changes in the apparent new particle formation rate (FR) or net maximum increase in nucleation‐mode PNCs (NMINP). The increased concentrations of other precursors may substantially negate the effect of SO₂ reduction, leading to the invariant FR and NMINP. The maximum sizes of grown new particles decreased by 50%; combined with the decreased NPF frequency, the contribution of grown new particles to accumulation‐mode PNCs expectedly decreased in 2016–2018 relative to 2010–2012. Alternatively, the increased accumulation‐mode PNCs in 2016–2018 mainly attribute to the enhanced primary combustion emissions excluding on‐road vehicle emissions. The increase in accumulation‐mode PNCs in 2016–2018 provides insights on the inter‐annual variation in satellite‐based cloud properties, which appeared insensitive to the large decrease in satellite‐based aerosol optical depth.

Atmospheric deposition of long-range transport of anthropogenic reactive nitrogen (N_r, mainly comprised of NH_x, NO_y and water-soluble organic nitrogen, WSON) from continents may have profound impact on marine biogeochemistry. In addition, surface ocean dissolved organic nitrogen (DON) may also contribute to aerosol WSON in the overlying atmosphere. Despite the importance of off-continent dispersion and N_r interactions at the atmosphere–ocean boundary, our knowledge of the sources of various nitrogen species in the atmosphere over the open ocean remains limited due to insufficient observations. We conducted two cruises in the spring of 2014 and 2015 from the coast of China through the East China seas (ECSs, i.e. the Yellow Sea and East China Sea) to the open ocean (i.e. the Northwest Pacific Ocean, NWPO). Concentrations of water-soluble total nitrogen (WSTN), NO₃⁻ and NH₄⁺, as well as the δ¹⁵N of WSTN and NO₃⁻ in marine aerosol, were measured during both cruises. In the spring of 2015, we also analysed the concentrations and δ¹⁵N of NO₃⁻ and the DON of surface seawater (SSW; at a depth of 5 m) along the cruise track. Aerosol NO₃⁻, NH₄⁺ and WSON decreased logarithmically (1–2 orders of magnitude) with distance from the shore, reflecting strong anthropogenic emission sources of NO₃⁻, NH₄⁺ and WSON in China. Average aerosol NO₃⁻ and NH₄⁺ concentrations were significantly higher in 2014 (even in the remote NWOP) than in 2015 due to the stronger wind field in 2014, underscoring the role of the Asian winter monsoon in the seaward transport of anthropogenic NO₃⁻ and NH₄⁺. However, the background aerosol WSON over the NWPO in 2015 (13.3 ± 8.5 nmol m⁻³) was similar to that in 2014 (12.2 ± 6.3 nmol m⁻³), suggesting an additional non-anthropogenic WSON source in the open ocean. Obviously, marine DON emissions should be considered in model and field assessments of net atmospheric WSON deposition in the open ocean. This study contributes information on parallel isotopic marine DON composition and aerosol N_r datasets, but more research is required to explore complex N_r sources and deposition processes in order to advance our understanding of anthropogenic influences on the marine nitrogen cycle and nitrogen exchange at land–ocean and atmosphere–ocean interfaces.