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Nitrogen dioxide (NO₂) and formaldehyde (HCHO) play vital roles in atmospheric photochemical processes. Their tropospheric vertical column density (TVCD) distributions have been monitored by satellite instruments. Evaluation of these observations is essential for applying these observations to study photochemistry. Assessing satellite products using observations at rural sites, where local emissions are minimal, is particularly useful due in part to the spatial homogeneity of trace gases. In this study, we evaluate OMI and TROPOMI NO₂and HCHO TVCDs using multi‐axis differential optical absorption spectroscopy (MAX‐DOAS) measurements at a rural site in the east coast of the Shandong province, China in spring 2018 during the Ozone Photochemistry and Export from China Experiment (OPECE) measurement campaign. On days not affected by local burning, we found generally good agreement of NO₂data after using consistent a priori profiles in satellite and MAX‐DOAS retrievals and accounting for low biases in scattering weights in one of the OMI products. In comparison, satellite HCHO products exhibited weaker correlations with MAX‐DOAS data, in contrast to satellite NO₂products. However, TROPOMI HCHO products showed significantly better agreement with MAX‐DOAS measurements compared to OMI data. Furthermore, case studies of the vertical profiles measured by MAX‐DOAS on burning days revealed large enhancements of nitrous acid (HONO), NO₂, and HCHO in the upper boundary layer, accompanied with considerable variability, particularly in HONO enhancements.

To understand diurnal variations in PM_2.5composition and aerosol extract absorption, PM_2.5samples were collected at intervals of 2 hr from 8:00 to 20:00 and 6 hr from 20:00 to 8:00 (the next day) in northern Nanjing, China, during the winter and summer of 2019–2020 and analyzed for bulk components, organic tracers, and light absorption of water and methanol extracts—a proxy measure of brown carbon (BrC). Diurnal patterns of measured species reflected the influences of primary emissions and atmospheric processes. Light absorption coefficients of water (Abs_365,w) and methanol extracts (Abs_365,m) at 365 nm shared a similar diurnal profile peaking at 18:00–20:00, generally following changes in biomass burning tracers. However, Abs_365,w, Abs_365,m, and their normalizations to organic aerosols increased at 14:00–16:00, earlier than that of levoglucosan in the late afternoon, which was attributed to secondarily formed BrC. The methanol extracts showed a less drastic decrease in light absorption at night than the water extracts and elevated absorption efficiency during 2:00–8:00. This is due to the fact that the water‐insoluble OC has a longer lifetime and stronger light absorption than the water‐soluble OC. According to the source apportionment results solved by positive matrix factorization (PMF), biomass burning and secondary formation were the major BrC sources in northern Nanjing, with an average total relative contribution of about 90%. Compared to previous studies, diurnal source cycles were added to the PMF simulations in this work by using time‐resolved speciation data, which avoided misclassification of BrC sources.

An extensive set of primary and secondary pollutants was measured at a ground site in a remote location in the Yellow River Delta, China during the Ozone Photochemistry and Export from China Experiment (OPECE) from March to April 2018. The measurements include volatile organic compounds (VOCs), peroxyacyl nitrates (PANs), ozone (O₃), particulate species, nitrogen oxides (NO_x), and SO₂. Observed VOC mixing ratios were comparable to those measured in heavily polluted cities in the U.S. and China. The VOC source signatures suggest a strong influence from Oil and Natural Gas (O&NG) emissions with potentially large contributions from Liquified Petroleum Gas (LPG) sources as well. Consistently elevated concentrations of O₃, PAN, and its rarely measured homologs peroxybenzoylic nitric anhydride (PBzN) and peroxyacrylic nitric anhydride (APAN) at the OPECE site indicate complex photochemistry in a heterogeneous VOC environment. Diagnostic 0‐D box model simulations are used to investigate the budgets of RO_x(OH + HO₂ + RO₂), and the rate and efficiency of O₃production. Model sensitivity calculations indicate that O₃production at OPECE site is VOC limited in spring. This suggests that reduction in VOCs should be a priority for reducing O₃, where production and fugitive emissions from O&NG provide an attractive target. While initial reductions in NO_xmight increase O₃production, reduction of NO_xalong with VOCs will be a necessary step to achieve long‐term ozone reduction.

High ozone concentrations have become the major summertime air quality problem in China. Extensive in situ observations are deployed for developing strategies to effectively control the emissions of ozone precursors, that is, nitrogen oxides (NO_X = NO + NO₂) and volatile organic compounds (VOCs). The modeling analysis of in situ observations often makes uses of the dependence of ozone peak concentration on NO_Xand VOC emissions, because ozone observations are among the most widely available air quality measurements. To extract more information from regulatory ozone observations, we extend the ozone‐precursor relationship to ozone peak time in this study. We find that the sensitivities of ozone peak time and concentration are complementary for regions with large anthropogenic emissions such as China. The ozone peak time is sensitive to both VOC and NO_Xemissions, and the sensitivity is nearly linear in the transition regime of ozone production compared to the changing ozone peak concentration sensitivity in this regime, making the diagnostics of ozone peak time particularly valuable. The extended ozone‐precursor relationships can be readily applied to understand the effects on ozone by emission changes of NO_Xand VOC and to assess potential biases of NO_Xand VOC emission inventories. These observation constraints based on regulatory ozone observations can complement the other measurement and modeling analysis methods nicely. Furthermore, we suggest that the ozone peak time sensitivity we discussed here to be used as a model evaluation measure before the empirical kinetic modeling approach (EKMA) diagram is applied to understand the effectiveness of emission control on ozone concentrations.

Ammonia control has received increasing attention as a measure to decrease particulate concentrations. Modeling analysis of observation data from central China over the period of September 2015 to August 2016 shows clear asymmetric responses of particulate pH and mass to ammonia emissions. With a change of ±80% of NH_x(NH₃+ NH₄⁺), the corresponding ΔpH are +0.5 and −3.0, respectively, and the corresponding particulate NH₄⁺changes are +2.62% and −61.8%, respectively. This asymmetry implies that there is a Critical Total Ammonia Concentration, above which particulate pH and mass are insensitive to ammonia control. Analysis of the observation data suggests that the Critical Total Ammonia Concentration is −25%. The estimated cost for an NH_xreduction of 25% is $140 – 320 million for Hubei province, which is the initial cost barrier before ammonia control can effectively affect particulate pH and mass in central China. This cost barrier will increase as NO_xand SO₂emissions decrease.

Abstract. New particle formation (NPF) and subsequent particle growth are importantsources of condensation nuclei (CN) and cloud condensation nuclei (CCN).While many observations have shown positive contributions of NPF to CCN atlow supersaturation, negative NPF contributions were often simulated inpolluted environments. Using the observations in a coastal city of Qingdao,Beijing, and Gucheng in north China, we thoroughly evaluate the simulatednumber concentrations of CN and CCN using an NPF-explicit parameterizationembedded in the WRF-Chem model. For CN, the initial simulation shows largebiases of particle number concentrations at 10–40 and 40–100 nm. Byadjusting the process of gas–particle partitioning, including the massaccommodation coefficient (MAC) of sulfuric acid, the phase changes in primary organic aerosol emissions, and the condensational amount of nitric acid, the improvement of the particle growth process yields substantially reduced overestimation of CN. Regarding CCN, secondary organic aerosol (SOA) formed from the oxidation of semi-volatile and intermediate-volatility organic compounds (S/IVOCs) is called SI-SOA, the yield of which is an important contributor. At default settings, the SI-SOA yield is too high without considering the differences in precursor oxidation rates. Lowering the SI-SOA yield under linear H2SO4 nucleation scheme results in much-improved CCN simulations compared to observations. On the basis of the bias-corrected model, we find substantially positive contributions of NPF to CCN at low supersaturation (∼ 0.2 %) over broad areas of China, primarily due to competing effects of increasing particle hygroscopicity, a result of reductions in SI-SOA amount, surpassing that of particle size decreases. The bias-corrected model is robustly applicable to other schemes, such as the quadratic H2SO4 nucleation scheme, in terms of CN and CCN, though the dependence of CCN on SI-SOA yield is diminished likely due to changes in particle composition. This study highlights potentially much larger NPF contributions to CCN on a regional and even global basis.