As acid snow floated down from hazy Beijing skies, I sent my daughters off to school on Tuesday morning with a grimly cheerful parting - "Try not to breathe too much" - and then went online to catch up on the microblog battle for the hearts, minds and lungs of the capital.

It was a contest over smog that was being fought across two social networks in two completely different languages between two contenders separated by the world's biggest firewall. At stake was the authority to define "unhealthy air" and, as a result, to shape public perceptions and expectations.

On one side was an automated air quality monitoring station set up by the US embassy in Beijing that issues hourly updates via Twitter on the @beijingair account. It states the date, time, pollutions readings for ozone and PM2.5 and a terse English summary of the health implications. At 8am, it read "very unhealthy" – an improvement on the "hazardous" level of the previous day and the alarming "beyond index" of last Friday.

On the other side was the personal microblog of Du Shaozhong, the deputy director of the Beijing Municipal Environmental Protection Administration, who has taken his passionate defence of the city's policies onto China's most influential website, Sina Weibo. One of his most recent posts read: "It is understandable if people hate bad weather, but venting your emotions is not helpful."

This was a response to a famous Chinese writer who claimed the pollution was getting worse. Du was on the defensive, a sharp change from his position at the start of the year, when he gave an ebulliant interview with the Guardian after the best run of clear skies in more than a decade.

Back then, he was talking of the improvement of official air quality statistics as a "positive, long-term story" due to the retrofitting of power stations, the relocation of dirty factories and measures to curb traffic emissions and industrial pollution.

In the months since, the government has ramped up its efforts to reduce airborne toxins by adding nitrogen oxide and carbon dioxide to the pollution index.

Yet public expectations have moved even faster, thanks in no small part to the US embassy's Twitter account. As I have mentioned in previous posts, the data on @beijingair is more comprehensive and timely than the pollution statistics released by the Chinese government. Though blocked by the Chinese censors' "great firewall", the tweets are quickly translated and spread around the Chinese internet, particularly when - as in recent weeks - the smog gets so thick that it swallows skyscrapers.

The smog persists because factories in neighbouring provinces release pollutants, construction sites fail to manage dust, traffic grows on the roads and power stations burn ever greater mountains of coal.

But this does not necessarily mean the pollution is getting worse. In terms of transparency and awareness, at least, the situation may even be better.

When I first arrived in this city in 2003, the smog was at least as horrendous and probably more frequent, though locals were then more inclined to call it "fog" and few domestic newspaper stories connected it, as they do now, to lung cancer and other respiratory diseases.

Today, however, thanks to @beijingair tweets seeping through the firewall, residents have more data to confirm what they have long been able to see with their own eyes: that official "blue-sky days" are nothing of the sort when the city is shrouded in a sulphurous haze.

It has also pushed the government into catch-up mode. Last month the environment ministry said it would acclerate plans to disclose hidden or missing data about the two forms of pollution most harmful to humans: ozone and tiny particulate matter, known as PM2.5 (both of which are included in the US embassy figures). As the World Resources Institute notes, the US did not start using this standard until 2007. Exactly when China will take the plunge remains unclear.

When it does, the data will no doubt be horrifying. According to this story in China Dialogue, PM2.5 levels may even have got worse in the past 10 years, contradicting official claims of improvement.

No matter how murky the baselines, the sooner the numbers come out the better. As the recent gloom shows, there is still a long way to go before Beijing's air is consistently healthy to breathe.