Kulmala and co-authors provide a fascinating insight into the consequences of the huge concentration of population living in eastern China.  The demographic statistics are remarkable: 10% of the world’s population living in a 1Mkm² area of eastern China, and more than 50% of global population concentrated in an area of South-east Asia covering only a small proportion of the global landmass.  The paper makes a strong case that the “gigacity” in eastern China greatly affects local weather patterns and may influence global climate through the Asian monsoon.  Such influences appear inevitable, and a strong case is made for further research.

In formulating such a research programme many factors need to be considered.  While urban Beijing is clearly very densely populated, travelling only a few kilometres beyond the boundaries of the city reveals open countryside with evidently low population density.  The extent to which this rural hinterland moderates the impact of the built-up areas upon the weather and climate is a key question, as is the consequence of further development of this rural space, which will surely fill gradually if population expands further. 

The paper shows the impact of haze pollution on mixing heights and explains how this serves to exacerbate ground-level pollution by reducing vertical mixing.  It appears to imply that the haze is an inevitable consequence of the gigacity, a point on which my opinion would differ.  An analysis of urban/rural gradients in pollutant concentrations in Beijing (Harrison et al., 2021) demonstrates that for many pollutants, the high concentrations are a regional phenomenon and little influenced by emissions within Beijing itself.  However, this is not the case for all pollutants, and most notably for nitrogen dioxide which shows marked urban and roadside increments above the rural concentrations.  This demonstrates that local measures can improve air quality for some pollutants.  Additionally, a careful analysis of air quality trends in Beijing which corrects for the influences of changing weather (Vu et al., 2019) shows the success of the 2013-2017 five-year pollution control plan in reducing all measured pollutants except ozone.  This was achieved largely by enhanced controls on road traffic and domestic combustion, but much can still be done to reduce emissions from rural biomass burning (Wu et al., 2020) and from industry, thereby reducing concentrations of long-lived primary pollutants such as carbon monoxide, and secondary particles formed from emissions of SO₂, NO_x and VOC.  Such measures, by reducing the haze phenomenon, will increase mixing depths and have huge benefits for air quality.  The haze is not unique to China (consider Delhi which has a worse haze problem than Beijing), and nor is it an inevitable consequence of the gigacity, although such extensive urbanisation and industrialisation makes the control challenge inevitably greater.

Another point to consider is the point at which population size and density creates an entity which has properties similar to those of Eastern China.  Although the population is far lower, the north-eastern United States (referred to as the BosWash megalopolis, with a population of over 50M), and western Europe (197M population) show a degree of connectivity in which air pollution phenomena cannot be solved by one municipality, or in Europe, one country acting alone.  There is inevitably a continuum of urbanised land masses ranging from individual major megacities to the Chinese gigacity, and research

needs to study the impacts of population size and density, and geographic extent upon weather, climate and air quality in the wider context of impacts upon human health and ecosystems.

References

Harrison, R.M., Vu, T.V., Jafar, H., Shi, Z., 2021.  More mileage in reducing urban air pollution from road traffic, Environ. Intl., 149, 106329.

Vu, T.V.,  Shi, Z.,  Cheng, J.,  Zhang, Q., He, K.,  Wang, S., Harrison, R.M., 2019.  Assessing the impact of clean air action on air quality trends in Beijing using a machine learning technique, Atmos. Chem. Phys., 19, 11303-11314.

Wu, X., Chen, C., Vu, T.V., Liu, D., Baldo, C., Shen, X., Zhang, Q., Cen, K., Zheng, M., He, K., Shi, Z., Harrison R.M., 2020.  Source apportionment of fine organic carbon (OC) using receptor modelling at a rural site of Beijing: Insight into seasonal and diurnal variation of source contributions, Environ. Pollut., 266, 115078.

The authors proposed a novel concept of gigacity, explains its great importance on regional air quality and global climate, and propose a large-scale research and innovation program to investigate its impact and the underlying mechanism. This is an important and very timely initiative and the answer to this question could be crucial for future planning of urbanization towards sustainable development.

Gigacity is a nice concept and the authors have taken the eastern part of China as an example. It would be helpful if the authors could also explicitly define what is a gigacity in analogy to a megacity. Then people can tell if there is any other region falls into this category, e.g., eastern US, Europe, Indo-Gangetic Plain?

The authors have emphasized the importance of high-resolution modeling (10 meters) in forecasting air quality and suggests expanding this into a gigacity scale (100 km to more than 1000 km). But is 10 meters really necessary? According to the citations of Huang et al. (20202a and 2020b), where spatial resolution of a few tens of km has been adopted, I guess the authors mean 10 km here? If yes, I don’t think that the resolution is really the limiting factor here in order to study the interactions and links in a gigacity scale. But on the other hand, to study the impact of gigacity within itself and on global climate, a direct application of the Earth System modeling would be the future. There how to increase the spatial resolution of Earth System models and reduce its dependence on sub-grid parameterization can be very challenging.

For the challenges specific for the gigacity, the authors may want to point out what questions have already been solved and what are the remaining open questions. For example, the current regional and global models have already the capacity to simulate the Urban Heat Island effects in the gigacity scale. Maybe not perfect, but what would be processes and mechanisms that need to be improved or added there for the applications in a gigacity scale? Similar questions also apply to the aerosol-radiation-cloud interactions and other links.

For the proposed research program, the authors may want to further explain how it would differ from previous/current research programs on air quality studies in megacity or at a regional scale. For example, for the observation network, does it only differ by the number/density of stations?

“Since this gigacity has roughly 50 times more people and 60 times larger surface area than Beijing – a typical megacity – its future is crucial not only for local people but also globally. The area is a huge emitter of greenhouse gases and air pollution as well as a potential source for local, regional and global epidemiological diseases.” Due to the high population density, the region or rather the cities there might become potential hotspot, but not necessarily the source for local, regional and global epidemiological diseases. In view of the few cases and efficient control of COVID-19 in the region, as well as the significant improvement of the air quality there in recent years, I would like to second and add on to the statement from the other reviewer that the haze and/or the spread of an epidemic are not an inevitable consequence of the gigacity, although such extensive urbanisation and industrialisation makes the control challenge inevitably greater.

Minor:

“with increasing risks of cardiovascular diseases and diabetes, violence and injuries, outbreaks of infectious disease, like the current pandemic of COVID-19 (Huang et al., 2020a; Tian et al., 2020), and inequity between people living in urban areas.” The reference Huang et al., 2020a is not supporting the statement about increasing risks of diseases.

“It is already well known that reduced concentrations of nitrogen oxides will, under otherwise polluted conditions, increase ozone production and increase secondary aerosol particle concentrations (Ding et al., 2013, Liu and Tang, 2020).” Many studies show that reducing NOx will lead to a reduction of PM.

“If suitable observations are lacking, the option is to run the models using reanalysis data.” Observations are also needed to produce the reanalysis data. The authors may want to explain a bit more on this.

“We need to continuously harvest data (Kulmala, 2018), even if we do not yet know for sure that we need it.” This sentence should be reformulated. It sounds like we still need to do some measurements without a motivation, but I guess the authors does not mean that.