Lung function testing revealed the problem
The Californian study adds to the body of evidence pointing to the damage caused by traffic and other air pollution in cities.
Other German research - published at the same time - has found that traffic-related air pollution is linked to coughs in babies and toddlers.
The number of children diagnosed with asthma has increased substantially in many developed countries in recent years.
Some campaigners blame this on air pollution - even though, in general, improvements in car technology mean that emissions are less damaging than in previous decades.
Other experts say that the rise in centrally heated, double-glazed houses may be sensitising children to a multitude of different allergens.
Gases link
The US team looked at 846 asthmatic children living in eight urban areas over a single summer.
They compared their reports of asthma symptoms with levels of air pollutants such as nitrogen dioxide and particles given off by combustion engines.
Rises in nitrogen dioxide could lead to almost a 50% increase in morning symptoms.
Rises in ozone were linked to declines in "peak flow readings" from asthmatic children - which measure lung function.
And rises in sulphur dioxide increased the incidence of asthma symptoms by a third.
The researchers wrote: "Summertime air pollution is associated with increased asthma morbidity and decreased pulmonary function among inner-city children with asthma.
"Adverse respiratory effects were observed in all cities, at levels below current USA air quality standards."
The study comes only weeks after another study suggested a similar link between pollution and asthma symptoms.
Coughing at night
The German research, also published in the European Respiratory Journal, looked at children aged two years or less, living in Munich.
Parents were asked to report if they suffered from coughs without having a chest infection.
The researchers found that as particulate air pollution increased, the chances of these dry coughs increased by approximately a third.
The researchers said that while this showed a link between traffic air pollution and cough, it was not enough to show that pollutants might actually be the root cause of asthma in children.