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<p>Online instruments have been widely applied for the measurement of <span class="inline-formula">PM_2.5</span> and its chemical components; however, these instruments have a major shortcoming in terms of the lack or limited number of species in field measurements. To this end, a new mass closure <span class="inline-formula">PM_2.5</span> online integrated system was developed and applied in this work to develop more comprehensive information on chemical species in <span class="inline-formula">PM_2.5</span>. For the new system, one isokinetic sampling system for <span class="inline-formula">PM_2.5</span> was coupled with an aerosol chemical speciation monitor (Aerodyne, ACSM), an aethalometer (Magee, AE-31), an automated multi-metals monitor (Cooper Corporation, Xact-625) and a hybrid synchronized ambient particulate real-time analyzer monitor (Thermo Scientific, SHARP-5030i) to enable high-resolution temporal (1&thinsp;<span class="inline-formula">h</span>) measurements of organic matter, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="40da026c69d6bb7b362f8aefb7758b92"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-13-5407-2020-ie00001.svg" width="29pt" height="17pt" src="amt-13-5407-2020-ie00001.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="b3512ed4eb493ff037a5c39221523c47"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-13-5407-2020-ie00002.svg" width="25pt" height="16pt" src="amt-13-5407-2020-ie00002.png"/></svg:svg></span></span>, <span class="inline-formula">Cl⁻</span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="8cff18dc7544e09830abea500d71300b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-13-5407-2020-ie00003.svg" width="24pt" height="15pt" src="amt-13-5407-2020-ie00003.png"/></svg:svg></span></span>, black carbon, important elements and <span class="inline-formula">PM_2.5</span> mass concentrations. The new online integrated system was first deployed in Shenzhen, China, to measure the <span class="inline-formula">PM_2.5</span> composition from 25 September to 30 October 2019. Our results showed that the average <span class="inline-formula">PM_2.5</span> concentration in this work was 33&thinsp;<span class="inline-formula">µg m⁻³</span>, and the measured species reconstructed the <span class="inline-formula">PM_2.5</span> well and almost formed a mass closure (94&thinsp;%). The multi-linear engine (ME-2) model was employed for the comprehensive online <span class="inline-formula">PM_2.5</span> chemical dataset to apportion the sources with predetermined constraints, in which the organic ion fragment <i>m/z</i> 44 in ACSM data was used as the tracer for secondary organic aerosol (SOA). Nine sources were determined and obtained reasonable time series and diurnal variations in this study, including identified SOA (23&thinsp;%), secondary sulfate (22&thinsp;%), vehicle emissions (18&thinsp;%), biomass burning (11&thinsp;%), coal burning (8.0&thinsp;%), secondary nitrate (5.3&thinsp;%), fugitive dust (3.8&thinsp;%), ship emissions (3.7&thinsp;%) and industrial emissions (2.1&thinsp;%). The potential source contribution function (PSCF) analysis indicated that the major source area could be the region north of the sampling site. This is the first system in the world that can perform online measurements of <span class="inline-formula">PM_2.5</span> components with a mass closure, thus providing a new powerful tool for <span class="inline-formula">PM_2.5</span> long-term daily measurement and source apportionment.</p>