Empty streets have shown how clean our air can be, but what will happen when we go back to work?

In recent days, as much of the world has adopted to working from home, social distancing, cocooning and a general slow down in transport intensity, there has been a marked uptick in air cleanliness in many urban environments. This post will look at the air pollution related to road traffic, and cover some of the wider systematic issues of measurement, sampling, sources and arbitration. We will also take a look at some of the available scientific evidence on health implications of air quality and road traffic, as well as some specific examples of air quality changes in selected large cities.

Fig. 1 – Brake Emissions from a hot brake disc

First, we should set some context, specifically around braking. In previous posts, we’d discussed the contribution brakes make to overall vehicle emissions (here), and talked in detail about how regenerative braking works to mitigate the use of friction braking and therefore mitigate emissions (here and here). The short version is that brakes can contribute over 50% of non-exhaust particulate emissions, in urban environments. Of course, while regenerative braking can help mitigate using the friction brakes, there is still some friction involved. And there’s yet to be a (modern) electric vehicle sold without friction brakes.

Fig. 2 – Particulate Matter and Humans – source UK Clean Air Report 2019

And while we will concern ourselves with brake emissions primarily, once we talk about pollution outside laboratory conditions, it makes more sense to discuss particulate size, content and prevalence rather than a specific source on the vehicle (as we will discuss when we review measurement techniques). When we discuss emissions, those smaller than 2.5 microns (referred to as PM2.5) are of real concern, as they can enter the blood stream via the lungs, and collect in vital human organs. This in turn lowers heart and lung capacity, damaging overall immunity.

Air Quality and Roads

It probably stands to reason that if you stood next to a road for long periods of a day, you would expect higher levels of exposure to transport related emissions. But it is worth delving a little deeper into this, to understand the picture. The first thing to understand is how air quality is established.

There are three major elements that we need to discuss. The first is sampling the source of pollution. For vehicles, this means laboratory testing of sources of pollution, and getting a baseline for the fleet average in a given area. But as we found out in Dieselgate, laboratory testing can only tell you so much, and real world performance can vary widely. If we consider a given roadside, then the traffic density and makeup will play an important role.

Even in the world of brake emissions, there is still no universal method for measuring particulate emissions. Work towards legislation in this area is being led by the EU’s Joint Research Centre, and there is a view forming on what a test cycle should look like, as well as a testing dyno setup. The detail is still being worked out on things like airflow, soak times, temperature measurement and climatic controls. In agreeing a universal sampling method, many compromises must be found, to accommodate the wide variety of vehicle types with a single, easily reproducible laboratory standard. So, as with exhaust emissions, what we see in the real world may not immediately correlate with lab findings.

It is worth comparing brake particulate emissions with those from the exhaust. In a recent study, it was estimated that brake particulate mass emissions can account for between 0.1 and 15 mg/km, compared to a hard limit of 4.5 mg/km for exhaust emissions, as per Bharat Stage VI (also Euro VI, China 5). And if we consider brake particulate numbers, they can range between 4 billion/km and 7 trillion/km. The equivalent limits for exhaust gases are 600 Billion/km. So we can consider brakes as at best a significant minority of overall vehicle emissions, but probably a major component in urban areas, due to the more frequent use of brakes.

industry
Fig. 3 – Smoke Stack – image used under CC licence

The next topic we need to discuss is background pollution. Any roadside environment already has a level of particulate matter before the first vehicle rolls along. This may be from a variety of sources, including naturally occurring ones. In air quality, a distinction is made between fixed and mobile sources. A fixed source might be a smoke stack from a local industrial plant, and a mobile source would be a vehicle. When considering a single roadside, it is necessary to consider nearby sources which can make up the existing pollution load, before any traffic turns up. This is achieved by establishing a local air quality inventory – an attempt to collate all directly measured (like our smoke stack) and foreseeable local sources (eg. domestic chimneys or nearby traffic), and give some consideration for local climatic conditions.

Air Quality Monitoring Station
Fig. 4 – Air Quality measurement station – image used under CC licence

The final concept we need to consider with respect to air quality is interpolation. While we know how we can directly quantify certain pollution sources, either in terms of traffic characteristics or direct measurement of local sources, it is not possible to sample the air quality at a wide range of locations simultaneously. So, in order to create a street by street picture, available data is fed into a model, and that picture can start to emerge. Approaches similar to Finite Element Analysis are used to create a grid, and nodes within the grid can be solved, based on the known data inputs (given sufficient processor capacity). This can then be visualised as a street map.

An important element that can be understood from such discrete data levels is how far from a busy road the particulate matter will travel. A study modelling an urban area in Los Angeles suggested that up to 60% of particulate will remain within 200m of the road centreline, but as far away as 1km 20% of the particles can still be expected. Another study on roadside particles and toxicity suggested that samples from 20m and 275m away from the roadside showed similar chemical makeup and toxicity for both sampling sites. The effects of these samples on mice showed detrimental effects on lungs and heart function. When we consider the impact of brake dust, the relevance extends far beyond the individual wheel, the vehicle and the road, and reaches across cities.

Observations of traffic-free cities

Events at the beginning of 2020 have meant that many cities have had a reduction in people moving, due to Government lock-downs. As a result, significant improvements in air quality have been observed, which correlates with a reduction in local traffic. Let’s look at a few examples.

Wuhan, China

First, let’s examine the city of Wuhan. The latest population estimate for Wuhan city is over 8.8 million, with the metro area covering 19 million people. Wuhan is the 9th most populous city in China. And while recent events have brought the city to the attention of many around the world, it is also the home of one of the worlds largest automobile companies, Dongfeng Motor Corp. Also in the city are a number of Joint Ventures, featuring Renault, PSA and GM. Automotive companies dominate the city’s economy.

Clip 1 – Nitrogen Dioxide levels during lockdown – used under CC licence

Wuhan was the centre of the Covid 19 outbreak, and as such, subject to prolonged containment measures during the early months of 2020. Observations from ESA’s Copernicus Sentinal 5P instrument tracked Nitrogen Oxide levels during January to March, and show a marked reduction in levels. At the same time, Copernicus data was used by the ESA to deduce between 20% and 30% less PM2.5 throughout China, as seen in their tweet from early march.

Tweet 1 – Satellite observation based changes in particulate matter over China. Source: Copernicus Atmosphere Monitoring Service (CAMS)

Traffic flows in Wuhan effectively ceased once the government restrictions took effect, with over 99% reductions recorded using data from GPS aggregate devices (Baidu Migrate, TomTom). While there was some activity (such as intense construction work for medical facilities), overall public and private transport was heavily curtailed. Coal demand for electricity production also tracked about 1/3 lower during the period, as economic activity was mostly paused.

Fig. 5 – Wuhan Daily Average PM2.5. Lockdown implemented Jan 23 – source AQICN.org

When considering the clean air implications of traffic reductions, it is necessary to review the medium term air quality trend. For Wuhan, this trend already pointed to a city that was busily improving air quality for most of the previous decade, delivered in part through improved vehicle exhaust emissions standards (China 4 and China 5) and improvements in industrial emissions. Hubei province is also home to the Three Gorges Dam, the world’s largest power station, and has been fully operational since 2012. The trend can be seen clearly when reviewing Air Quality Index historic data, where the last 5 years of the previous decade saw marked reductions in terms of average and peak PM2.5 measurements (see Fig. 5 above).

The Dam
Fig. 6 – Three Gorges Dam – image used under CC licence

Pune, India

Pune is claimed to be the ninth most populous city in India, with an urban population of 3.1 Million, and the metro area is home to 7.3 Million people. Pune is one of the hubs of Automotive Industry in India, and home to a large number of factories, as well as many R&D centres for Indian companies. It is also home to the majority of foreign brands addressing the Indian market.

Pune is also one of the most congested cities in the world, ranking at no. 5 on the TomTom international list (and 3rd most congested in India, behind Mumbai and Bengaluru – Wuhan sits at 164 on the list). However, in terms of air quality, Pune ranks in the middle in terms of Indian cities, albeit India has the unwelcome accolade that 6 out of 10 of the most polluted cities are Indian.

Where Indian traffic differs from Chinese traffic is the prevalence of 2-Wheelers. Motorcycles typically outsell cars by 6 or 7 to 1, meaning motorcycles are the dominant form of private transport. In China, car sales outpace motorcycle sales, by about 20%. When we consider ways to mitigate vehicle emissions, it is worth reflecting that electric 2-wheelers are a long way behind their 4-wheel counterparts, both in terms of market penetration and regenerative braking capabilities (something that we’ll dive into in a future blog).

Rush Hour - Pune, India
Fig. 7 – Rush hour in Pune – image used under CC licence

While it is early days for the Indian lockdown, initial reports suggest that road traffic has been almost entirely curtailed, with highest daily peaks below 10% (compared to over 100% in 2019) by TomTom’s journey time inflation index. There are also reports of drop in electricity consumption nationally of between 10% and 20%. While most thermal power plants in the state of Maharashtra are far from Pune, India’s largest hydroelectric plant at Koyna is 200km south of Pune.

Fig. 8 – Pune Daily Average PM2.5 data. Lockdown begun Mar 24, curfew begun Mar 22. – Source AQICN.org

As can be seen from the AQI data, Pune has also been busily improving its air quality over the past five years. And while the absolute numbers are significantly lower than Wuhan, the trend is markedly similar. This is in line with national policy in India, with air quality becoming an increasingly pressing political topic in recent years. In 2019, Pune created an Air Quality Improvement plan, with the aim to improve air quality by 20%-30% over five years.

However, it is relatively early in the lockdown to make significant comparison, as air quality is affected by weather, temperature and agricultural trends in the short term. But it can be expected that Nitrogen Oxide levels, as well as particulate matter counts will decrease in line with an absence of local traffic, leading to a better air quality in many Indian cities during the lockdown.

The drive towards cleaner air

In both cases, the lockdowns at the beginning of 2020 will mean that the third decade of this millennium will most likely see a new record in terms of urban air quality. (In the case of Pune, it is likely that the medium term goal of 20%-30% air quality improvement may be achieved this year.) This comes off the back of a decade of rapid social and economic development, and a legislative context that has been pushing to clean up both transport and energy production. (While energy policy is a bit outside the realms of a brakes blog, the later part of the last decade saw China and India head the international list of solar investment, at a time when solar displaced coal as the go-to technology for cheap power). Social development in the two most populous nations have seen an increased focus on cleaning urban air, with recent projects in Xi’an and Dehli to install urban air purifier pilot projects.

Video 2 – An Urban Air Purifier in Xi’an – source CCTV

Vehicle exhaust limits in both countries are in line with the most stringent standards on the planet. When China 6 regulations enter into force in July, these will be the most wide-ranging and strictest tailpipe standards anywhere, shaving 1/3rd off the permitted particulate matter allowances. Another important characteristic for transport and pollution is average fleet age, and in both countries the markets are expanding, meaning a fleet age roughly half that typical in European countries. Combining this with the increasingly stringent regulation, exhaust emissions will dramatically fall over the medium term.

The lockdown legacy

The lockdown has shown a correlation between clean air and an absence of exhaust emissions. While the science of clean air is far more complicated (road transport contributes between 15% to 50% of PM2.5), the politics is unlikely to be. While by the second half of 2020, the majority of economic activity should have restarted, the memory of empty streets and clean air will live long in collective memories.

It’s in this context that non-exhaust emissions will come into sharp focus during this decade. There is a mounting body of evidence that brakes play a significant role in urban air quality, and converging view on how brake emissions could be measured. And while there are known solutions for cleaning the combustion process (and even replacing it for most transport applications), for braking, there is no ready alternative, no easy fix. We know that brake material selection can improve the picture, and brake dust collectors have been trialed. However, so far, vehicle manufacturers have not had a financial incentive to innovate in this area. It would seem a fair bet that such an incentive will soon be available…