Projections of National-Gridded Emissions of Hydrofluoroolefins (HFOs) in China.

Hydrofluoroolefins (HFOs) are being used as substitutes for potent greenhouse gas hydrofluorocarbons (HFCs). However, the use and environmental impacts of HFOs are of great concern due to the rapid degradation of HFOs to produce persistent and phytotoxic trifluoroacetic acid (TFA). Here, we provide a comprehensive projection of HFO emissions in China during 2024-2060 for the first time. Under the Kigali Amendment to the Montreal Protocol, China's HFO emissions are estimated to increase from 1.7 (1.3-2.3) to 148.8 (111.4-185.4) kt in 2024-2060 with cumulative emissions of 2.8 (2.0-3.5) Gt, and cumulative reduced HFCs emissions are evaluated to be 5.4 Gt CO2-equivalent. High HFO emissions would be distributed mainly in the North China Plain and the eastern and coastal areas. HFO-1234yf (2,3,3,3-tetrafluoropropene) contributes most of HFO emissions with a cumulative emission of 1.7 Gt in 2024-2060, while the cumulative increment of TFA deposition from HFO-1234yf emissions would reach 0.4-1.0 Gt. The long-term national-gridded HFO emission inventories can provide scientific support for evaluating the environmental risks of HFOs and developing HFC phase-out pathways for addressing climate change.

Heterogeneous Reaction of Peroxyacetyl Nitrate on Real-World PM2.5 Aerosols: Kinetics, Influencing Factors, and Atmospheric Implications.

The formation and decomposition of peroxyacetyl nitrate (PAN), an important atmospheric nitrogen oxide reservoir, can impact the level and cycling of free radicals and nitrogen compounds in the atmosphere. PAN sinks are poorly understood, highlighting the importance of elucidating the heterogeneous reaction of PAN on aerosol surfaces. Here, we report for the first time the uptake behavior, kinetics, and potential mechanism of PAN uptake on real-world aerosol PM2.5 using a flow tube system. The uptake coefficients (γ) of PAN increased non-linearly from (1.5 ± 0.7) × 10-5 at 0% relative humidity (RH) to (9.3 ± 2.0) × 10-5 at 80% RH. The γ decrease with increasing initial PAN concentration is consistent with the Langmuir-Hinshelwood mechanism. Organic components of aerosols may promote heterogeneous loss of PAN through redox reactions. Higher γ occurs with higher water content, lower pH, and lower ionic strength in the aqueous phase of aerosols. The present study suggests that heterogeneous reaction of PAN on ambient aerosols plays a non-negligible role in the atmospheric PAN budget and provides new insights into the role of PAN in promoting atmospheric oxidation capacity during hazy periods with cold and wet weather conditions.

Real-world emission characteristics of carbonyl compounds from on-road vehicles in Beijing and Zhengzhou, China.

Carbonyl compounds have a profound role in atmospheric chemistry, which can cause the formation of ozone and secondary organic aerosols. On-road vehicle emissions are an important source of carbonyl compounds, but systematic knowledge of real-world emission characteristics is still scarce. In this study, a total of 181 on-road vehicles of 16 types in Beijing and Zhengzhou, China, were tested using portable emission measurement system under real-world driving conditions. The total carbonyl compound emission factors of gasoline vehicles, diesel vehicles, motorcycles, and agricultural transport vehicles were 24.9 ± 11.4 mg/km, 42.5 ± 21.5 mg/km, 20.4 ± 6.8 mg/km, and 78.3 ± 34.3 mg/km, respectively. Vehicles fueled with E10 ethanol gasoline had significantly higher carbonyl compound emission factors compared to E0 gasoline vehicles. It was observed that the continuous tightening of emission standards has effectively reduced the emissions of carbonyl compounds from on-road vehicles. The carbonyl compound emission factors on highways were 1.3-1.9 times lower than those on general roads. The total carbonyl compound emissions from on-road vehicles in Beijing and Zhengzhou in 2019 were estimated to be 3.5 kt and 3.1 kt, with corresponding ozone formation potentials of 24.4 kt and 21.4 kt, respectively. Formaldehyde, acetaldehyde, propionaldehyde and acetone were the most significant contributors to total carbonyl compound emissions, and among them, formaldehyde, acetaldehyde and propionaldehyde were the main contributors to total ozone formation potential. Our results provide updated and supplementary information on on-road vehicle emission factors for carbonyl compounds and can facilitate the improvement of emission inventories and help in the development of control strategies to improve air quality.

Atmospheric fate and impacts of HFO-1234yf from mobile air conditioners in East Asia.

HFO-1234yf (2,3,3,3-tetrafluoropropene) is being used as refrigerant to replace HFC-134a (1,1,1,2-tetrafluoroethane), a potent greenhouse gas, in mobile air conditioners. However, the environmental impacts of HFO-1234yf, which is quickly and almost completely transformed to the persistent and phytotoxic trifluoroacetic acid (TFA), is of great concern. Here, we used the nested-grid chemical transport model, GEOS-Chem, to assess the fate and environmental impacts of HFO-1234yf emissions from mobile air conditioners in East Asia. With total emissions of 30.3 Gg yr-1, the annual mean concentrations of HFO-1234yf in China, Japan, and South Korea were 4.00, 3.23, and 5.54 pptv (parts per trillion volume), respectively, and the annual deposition fluxes (dry plus wet) of TFA in these regions were 0.35, 0.48, and 0.53 kg km-2 yr-1, dominated by wet deposition. About 14 %, 13 % and 11 % of HFO-1234yf emissions were deposited as TFA in China, Japan and South Korea, respectively, i.e. a large portion of TFA was deposited in areas outside of the emission boundary regions. The TFA characteristics in Japan and South Korea was significantly influenced by emission from China, which contributions ranged from 43 % to 94 % for the TFA concentrations and 44 % to 98 % for the TFA depositions across the four seasons. This suggests that the influence of neighboring emission sources cannot be ignored when assessing the impact of HFO-1234yf emissions in individual countries.

A WRF-CMAQ modeling of atmospheric peroxyacetyl nitrate and source apportionment in Central China.

Atmospheric peroxyacetyl nitrate (PAN), as an essential constituent in the photochemical smog, is formed from photochemical reactions between volatile organic compounds (VOCs) and NOx. However, limited regional studies on distribution, formation and sources of PAN restrict the further understanding of the atmospheric behavior and environmental significance of PAN. In this study, the variation characteristics of PAN and the influencing factors to PAN concentrations were investigated using the WRF-CMAQ model simulation in the central China during July 2019. The results showed that the monthly mean concentration of PAN in the near-surface layer was 0.4 ppbv and increased with the height rising, accompanied by strong intra-day variation. The process analysis suggested that the removal was mainly controlled by dry deposition (57 %), followed by the gas-phase chemistry (43 %) which was mainly attributed to the thermal decomposition. Based on the sensitivity simulation, PAN concentrations decreased effectively in most of the simulated regions when precursors of VOCs and NOx emissions were reduced, and PAN concentrations were more sensitive to VOCs emissions than NOx emissions. The reduction of NOx and VOCs could lead to enhanced atmospheric oxidation in east-central region, which in turn hindered the decrease of PAN concentrations. During the simulation period, we found that emissions from industry and transportation sectors had the greatest impact on PAN concentrations in the central China, with contributions of 39 %-49 % and 33 %-41 %, respectively.

Strong relations of peroxyacetyl nitrate (PAN) formation to alkene and nitrous acid during various episodes.

Peroxyacetyl nitrate (PAN) is one of the critical secondary pollutants in photochemical smog. This study investigated the relationship between PAN and PAN precursors with the Regional Atmospheric Chemical Mechanism version 2 model in six episodes recorded in Zhengzhou. In all episodes, peroxyacetyl radical (PA) was primarily produced by acetaldehyde oxidation, with more than 70% contributions. In photochemical episodes and photochemical-haze co-occurring episodes (combined episodes), methylglyoxal secondarily contributes 8.1%-10.6% to PA while in haze pollution, the propagation of other radicals to PA is the second most important source (12.0%-19.1%). Among anthropogenic non-methane hydrocarbons, alkene restricted PAN formation as first-generation precursors, with the relative incremental reactivity of PAN (RIRPAN) more than 0.6 during three-type episodes. Nitrous acid (HONO) also played important role in PAN formation. Especially during photochemical episodes, RIRPAN(HONO) reached 0.79, which was comparable to the RIRPAN value of alkene. Through sensitivity analysis of the relative formation of PAN to O3 (the amount of PAN generated when 100 ppb O3 formed), HONO was identified as the key precursor of PAN in haze pollution by promoting the oxidation of NMHC, while alkene predominated the relative formation of PAN to O3 in photochemical and combined pollution through producing acetaldehyde. The sensitivity of PAN to HONO is obviously enhanced with higher NOx/VOC ratios during photochemical and combined pollution.