The influence of aerosols on the NO2 photolysis rate in a suburban site in North China

The photolysis of NO₂ is an important driving force of tropospheric ozone. The intensity of this photolysis reaction affects atmospheric oxidation and photochemical pollution process. Photolysis rate of nitrogen dioxide (JNO₂) is affected by aerosols, temperature, solar zenith angle (SZA), clouds, and so on. Among them, aerosol is an important influencing factor because of its complicated and irregular change; aerosol quantitative effect on JNO₂ is constructive for the coordinated control of O₃ and particulate matter. In order to quantitatively assess the impact of aerosols on JNO₂ in the long-term, the reconstructed JNO₂ data in a suburban site in North China from 2005 to 2019 are used. We found that JNO₂ and aerosol optical depth (AOD) presented logarithmic relations under different solar zenith angle (SZA) levels, the aerosol attenuation effect on JNO₂ decreased as AOD increased. Two main influencing factors of JNO₂, SZA, and AOD, were fitted into a quadratic polynomial to quantify the AOD effect on JNO₂. The results showed that the average annual AOD effect on JNO₂ in Xianghe from 2005 to 2019 was -28.6% compared to an aerosol free atmosphere; the seasonal mean AOD effect in spring, summer, autumn, and winter was -27.1% and -35.1%, -25.5% and -26.3%, respectively. During the study period, JNO₂ increased with an average of 5 × 10^-5 s^-1 per year, while the annual average aerosol optical depth (AOD) was 0.80 ± 0.10, showing an overall downward trend. Annual mean AOD attenuation effect on JNO₂ decreased over time; the decreases were larger in spring and summer, and smaller in autumn and winter.

The treatment of uncertainties in reactive pollution dispersion models at urban scales

The ability to predict NO₂ concentrations ([NO₂]) within urban street networks is important for the evaluation of strategies to reduce exposure to NO₂. However, models aiming to make such predictions involve the coupling of several complex processes: traffic emissions under different levels of congestion; dispersion via turbulent mixing; chemical processes of relevance at the street-scale. Parameterisations of these processes are challenging to quantify with precision. Predictions are therefore subject to uncertainties which should be taken into account when using models within decision making. This paper presents an analysis of mean [NO₂] predictions from such a complex modelling system applied to a street canyon within the city of York, UK including the treatment of model uncertainties and their causes. The model system consists of a micro-scale traffic simulation and emissions model, and a Reynolds averaged turbulent flow model coupled to a reactive Lagrangian particle dispersion model. The analysis focuses on the sensitivity of predicted in-street increments of [NO₂] at different locations in the street to uncertainties in the model inputs. These include physical characteristics such as background wind direction, temperature and background ozone concentrations; traffic parameters such as overall demand and primary NO₂ fraction; as well as model parameterisations such as roughness lengths, turbulent time- and length-scales and chemical reaction rate coefficients. Predicted [NO₂] is shown to be relatively robust with respect to model parameterisations, although there are significant sensitivities to the activation energy for the reaction NO + O₃ as well as the canyon wall roughness length. Under off-peak traffic conditions, demand is the key traffic parameter. Under peak conditions where the network saturates, road-side [NO₂] is relatively insensitive to changes in demand and more sensitive to the primary NO₂ fraction. The most important physical parameter was found to be the background wind direction. The study highlights the key parameters required for reliable [NO₂] estimations suggesting that accurate reference measurements for wind direction should be a critical part of air quality assessments for in-street locations. It also highlights the importance of street scale chemical processes in forming road-side [NO₂], particularly for regions of high NO_x emissions such as close to traffic queues.

Solar actinic flux spectroradiometry: a technique for measuring photolysis frequencies in the atmosphere

A spectroradiometer has been developed for direct measurement of the solar actinic UV flux (scalar intensity) and determination of photolysis frequencies in the atmosphere. The instrument is based on a scanning double monochromator with an entrance optic that exhibits an isotropic angular response over a solid angle of 2pi sr. Actinic flux spectra are measured at a resolution of 1 nm across a range of 280-420 nm, which is relevant for most tropospheric photolysis processes. The photolysis frequencies are derived from the measured radiation spectra by use of published absorption cross sections and quantum yields. The advantage of this technique compared with the traditional chemical actinometry is its versatility. It is possible to determine the photolysis frequency for any photochemical reaction of interest provided that the respective molecular photodissociation parameters are known and the absorption cross section falls within a wavelength range that is accessible by the spectroradiometer. The instrument and the calibration procedures are described in detail, and problems specific to measurement of the actinic radiation are discussed. An error analysis is presented together with a discussion of the spectral requirements of the instrument for accurate measurements of important tropospheric photolysis frequencies (J(O(1))(D), J(NO(2)), J(HCHO)). An example of measurements from previous atmospheric chemistry field campaigns are presented and discussed.

Changes in tropospheric composition and air quality

Reductions in stratospheric ozone (O3) cause increased penetration of ultraviolet-B (UV-B) radiation to the troposphere, and therefore increases in the chemical activity in the lower atmosphere (the troposphere). Tropospheric ozone levels are sensitive to local concentrations of nitrogen oxides (NOx) and hydrocarbons. Model studies suggest that additional UV-B radiation reduces tropospheric ozone in clean environments (low NOx), and increases tropospheric ozone in polluted areas (high NOx). Assuming other factors remain constant, additional UV-B will increase the rate at which primary pollutants are removed from the troposphere. Increased UV-B is expected to increase the concentration of hydroxyl radicals (OH) and result in faster removal of pollutants such as carbon monoxide (CO), methane (CH4), non-methane hydrocarbons (NMHCs), sulfur and nitrogen oxides, hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs). Concentrations of peroxy radicals (both inorganic and organic) are expected to increase, leading to higher atmospheric levels of hydrogen peroxide (H2O2) and organic peroxides. The effects of UV-B increases on tropospheric O3, OH, methane, CO, and possibly other tropospheric constituents, while not negligible, will be difficult to detect because the concentrations of these species are also influenced by many other variable factors (e.g., emissions). Trifluoroacetic acid (TFA, CF3COOH) is produced in the atmosphere by the degradation of HCFC-123 (CF3CHCl2), HCFC-124 (CF3CHFCl), and HFC-134a (CF3CH2F), which are used as substitutes for ozone-depleting substances. The atmospheric oxidation mechanisms of these replacement compounds are well established. Reported measurements of TFA in rain, rivers, lakes, and oceans show it to be a ubiquitous component of the hydrosphere, present at levels much higher than can be explained by reported sources. The levels of TFA produced by the atmospheric degradation of HFCs and HCFCs emitted up to the year 2020 are estimated to be orders of magnitude below those of concern, and to make only a minor contribution to the current environmental burden of TFA. No significant effects on humans or the environment have been identified from TFA produced by atmospheric degradation of HCFCs and HFCs. Numerous standard short-term studies have shown that TFA has, at most, moderate toxicity.

The impact of aerosols on solar ultraviolet radiation and photochemical smog

Photochemical smog, or ground-level ozone, has been the most recalcitrant of air pollution problems, but reductions in emissions of sulfur and hydrocarbons may yield unanticipated benefits in air quality. While sulfate and some organic aerosol particles scatter solar radiation back into space and can cool Earth's surface, they also change the actinic flux of ultraviolet (UV) radiation. Observations and numerical models show that UV-scattering particles in the boundary layer accelerate photochemical reactions and smog production, but UV-absorbing aerosols such as mineral dust and soot inhibit smog production. Results could have major implications for the control of air pollution.