During the walk back to the Rosewood,I see no belching trucks,no people burning rubbish in the streets. Perhaps the city isn't as pristine as,say,Tokyo,but it isn't so actively squalid as the likes of New Delhi,either. Why,then,does it rival the latter as the world's most polluted city?

The answer lies further afield,an unfortunate combination of industrial coal-burning and an enveloping mountain range leaving the emissions nowhere to go. During the depths of the freezing winters and sweltering summers,this conspires to turn the air into a noxious soup.

The following morning I decide to get out of the city,to head to the Great Wall of China to see if things are better in the countryside. It's a good time to be leaving:the morning reading is 180,"Unhealthy"so"everyone may begin to experience health effects;members of sensitive groups may experience more serious health effects". It looks like a kind of mist over the city,as though the firmament has a cataract. The sun appears as an impotent peach disk,neutered by the haze.

I'm picked up by Peter Cao,a guide with Great Wall Hiking. As we edge our way through the city's rush hour traffic,moped drivers with dust masks push past us. Above,the buildings appear greyer. The atmosphere has,quite literally,turned sour.

I drift off to sleep and wake about an hour later,now north of the city. The smog has followed us all the way. Fortunately,it doesn't affect my lungs but that means I have few excuses for how difficult I find following Peter up to the top of the mighty Great Wall. Here at Jiankou,the Wall has not been preserved or restored. It's technically off-limits to the public but,Peter tells me,"there aren't enough police,so it's OK".

The view from the top should be incredible,but everything beyond a few hundred metres is obscured by the pollution. I imagine that the Ming dynasty troops,who built this section of the Wall 400 years ago,would have looked on this weird weather as some kind of omen or curse. It's frustrating,having to guess at what might be beyond this unnaturally close horizon,especially when considering that the Chinese know how to fix this problem,as they did in 2008 for their Olympic Games,and then again for the 2015 World Athletics Championships. Then,they shut down many of the factories and power stations at the edge of the city,and introduced stringent congestion checks to cut down on traffic emissions. It worked – the air improved. Why not have it like that all the time? Why wilfully harm your people? Peter doesn't think it's quite as simple or sinister as that."It works in a cycle,"he says,a little unconvincingly."Wind will come tomorrow,the smog will go."

Mercifully,the guide is right. We spend the night at a local guesthouse in the village of Gubeikou,then the next morning head back to the Wall,making it to the top of the ancient bricks just in time for sunrise,dawn emerging through a more natural morning mist. Half an hour later,a breeze starts as suddenly as a hairdryer and soon the sky seems almost unnaturally blue,as though by redoubling its efforts today,it'll improve its average for the week. Off the sides of the Wall apricot blossom shimmers in the sun. It had all seemed so lifeless yesterday.

The next day is my last in Beijing and the air is a crisp 50,which is comparable with a worse-than-average day in Melbourne or Sydney. It's also the tomb sweeping holiday,or Qingming Festival,when the living seek to placate the dead,paying tribute to their forebears and making sure they haven't risen from the grave. How much actual sweeping of actual tombs goes on is unclear,but the sky couldn't be more so. No one has to work and it feels like a great time to be in Beijing. I decide to spend this glorious day hitting up the main tourist sites:Tiananmen Square,the Forbidden City,Jingshan Park.

While foreigners are abundant in the first two,the park is predominantly filled with locals. Many choose to make the steep climb to a three-storey pagoda,the centrepiece of the grounds,from which views across the whole city are today possible.

At the bottom of the hill,I find what looks like a semi-organised group of pensioners,singing as part of a ramshackle choir led by a lone harmonica. Those who can,stand to sing,and as the crowds pass by,some stop to join in,adding more and more quivering voices to the group. I've no idea what they're singing about and,this being China,no one volunteers to tell me. Instead I confer my own meaning on these sad sounds – perhaps they're singing about the dead whose graves they will later visit;perhaps they're singing about the bygone days of communist rule. In any case,they seem to be singing about loss and longing,and I can't help but wonder if one day they'll instead be singing a lament to mourn the loss of their beautiful blue sky.

TRIP NOTES

MORE INFORMATION

english.visitbeijing.com.cn

GETTING THERE

Air China are the only airline to offer a direct service to Beijing from either Melbourne or Sydney. Other routes include changes,longer travel times and greater expense. Seeairchina.com

STAYING THERE

The Rosewood Beijing is one of the most refined hotels in the city,offering tranquility and,when the weather allows,wonderful views across Beijing. The staff speak perfect English and their hot-pot restaurant,Red Bowl,offers a chance to enjoy traditional Chinese food with an accurately translated menu. Double start from around $370 per night. Seerosewoodhotels.com

THE GREAT WALL

Great Wall Hiking offers some of the most adventurous trips to the famous frontier. From day trips up to strength-sapping nine-day treks,they can design itineraries to all fitness and interest levels. Pick up and drop off from Beijing,with accommodation at local guest houses in between. Seegreatwallhiking.com

John Golder was a guest of Great Wall Hiking and the Rosewood Hotel.